Excretory system
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About This Presentation
for B.Sc students
Slide Content
Excretory system notes 2017
1 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Lecture Notes
On
Excretory system
By,
K. P. KOMAL
ASSISTANT PROFESSOR
DEPARTMENT OF BIOCHEMIS TRY
GOVERNMENT SCIENCE COLLEGE, CHITRADURGA. 577501
KARNATAKA STATE.
1 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Lecture Notes
On
Excretory system
By,
K. P. KOMAL
ASSISTANT PROFESSOR
DEPARTMENT OF BIOCHEMIS TRY
GOVERNMENT SCIENCE COLLEGE, CHITRADURGA. 577501
KARNATAKA STATE.
Excretory system notes 2017
2 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Excretory System
When our cells perform their functions, certain waste products are released in to the
blood stream. These are toxic and hence need to be removed from the body.
The process of removal of wastes produced in the cells of the living organisms is called
excretion.
Waste removal is done by the blood capillaries in the kidneys.
When the blood reaches the two kidneys, it contains both useful and harmful substances.
The useful substances are absorbed back into the blood. The wastes are removed as urine.
From the kidneys, the urine goes into the urinary bladder through tube-likeureters. It is
stored in the bladder and is passed out through the urinary opening at the end of a
muscular tube called urethra. The kidneys, ureters, bladder and urethra form the
excretory system.
An adult human being normally passes about 1–1.8 L of urine in 24 hours, and the
urine consists of 95% water, 2.5 % urea and 2.5% other waste products .
The excretory system is a passive biological system that removes excess, unnecessary
materials from the body fluids of an organism, so as to help maintain internal chemical
homeostasis and prevent damage to the body.
The dual function of excretory systems is the elimination of the waste
products of metabolism and to drain the body of used up and broken down components
in a liquid and gaseous state.
Excretory Products and their Elimination
Animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions like Na+,
K+, Cl
–
, phosphate, sulphate,, either by metabolic activities or by other means like excess
ingestion. These substances have to be removed totally or partially.
Ammonia, urea and uric acid are the major forms of nitrogenous wastes excreted by the
animals.
The way in which waste chemicals are removed from the body of the animal depends on
the availability of water.
2 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Excretory System
When our cells perform their functions, certain waste products are released in to the
blood stream. These are toxic and hence need to be removed from the body.
The process of removal of wastes produced in the cells of the living organisms is called
excretion.
Waste removal is done by the blood capillaries in the kidneys.
When the blood reaches the two kidneys, it contains both useful and harmful substances.
The useful substances are absorbed back into the blood. The wastes are removed as urine.
From the kidneys, the urine goes into the urinary bladder through tube-likeureters. It is
stored in the bladder and is passed out through the urinary opening at the end of a
muscular tube called urethra. The kidneys, ureters, bladder and urethra form the
excretory system.
An adult human being normally passes about 1–1.8 L of urine in 24 hours, and the
urine consists of 95% water, 2.5 % urea and 2.5% other waste products .
The excretory system is a passive biological system that removes excess, unnecessary
materials from the body fluids of an organism, so as to help maintain internal chemical
homeostasis and prevent damage to the body.
The dual function of excretory systems is the elimination of the waste
products of metabolism and to drain the body of used up and broken down components
in a liquid and gaseous state.
Excretory Products and their Elimination
Animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions like Na+,
K+, Cl
–
, phosphate, sulphate,, either by metabolic activities or by other means like excess
ingestion. These substances have to be removed totally or partially.
Ammonia, urea and uric acid are the major forms of nitrogenous wastes excreted by the
animals.
The way in which waste chemicals are removed from the body of the animal depends on
the availability of water.
Excretory system notes 2017
3 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Ammonia is the most toxic form and requires large amount of water for its elimination,
whereas uric acid, being the least toxic, can be removed with a minimum loss of water.
Aquatic animals like fishes, excrete cell waste in gaseous form (ammonia) which directly
dissolves in water.
Some land animals like birds, lizards, snakes excrete a semi -solid, white coloured
compound (uric acid).
The major excretory product in humans is urea which is excreted through urine.
Sometimes a person’s kidneys may stop working due t o infection or injury. As a result of
kidney failure, waste products start accumulating in the blood. Such persons cannot
survive unless their blood is filtered periodically through an artificial kidney. This process is
called dialysis.
The process of excreting ammonia is Many bony fishes, aquatic amphibians and aquatic
insects are ammonotelic in nature.
Ammonia, as it is readily soluble, is generally excreted by diffusion across body surfaces or
through gill surfaces (in fish) as ammonium ions. Kidneys do not play any significant role
in its removal.
Terrestrial adaptation necessitated the production of lesser toxic nitrogenous wastes
like urea and uric acid for conservation of water.
Mammals, many terrestrial amphibians and marine fishes mainly excrete ure a and are
called ureotelic animals. Ammonia produced by metabolism is converted into urea in
the liver of these animals and released into the blood which is filtered and excreted out by
the kidneys.
Some amount of urea may be retained in the kidney matrix of some of these animals to
maintain a desired osmolarity [the concentration of a solution expressed as the total
number of solute particles per litre].
Reptiles, birds, land snails and insects excrete nitrogenous wastes as uric acid in the form
of pellet or paste with a minimum loss of water and are called uricotelic animals.
Protonephridia or flame cells are the excretory structures in Platyhelminthes (Flatworms,
e.g., Planaria), rotifers, some annelids and the cephalochordate.
3 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Ammonia is the most toxic form and requires large amount of water for its elimination,
whereas uric acid, being the least toxic, can be removed with a minimum loss of water.
Aquatic animals like fishes, excrete cell waste in gaseous form (ammonia) which directly
dissolves in water.
Some land animals like birds, lizards, snakes excrete a semi -solid, white coloured
compound (uric acid).
The major excretory product in humans is urea which is excreted through urine.
Sometimes a person’s kidneys may stop working due t o infection or injury. As a result of
kidney failure, waste products start accumulating in the blood. Such persons cannot
survive unless their blood is filtered periodically through an artificial kidney. This process is
called dialysis.
The process of excreting ammonia is Many bony fishes, aquatic amphibians and aquatic
insects are ammonotelic in nature.
Ammonia, as it is readily soluble, is generally excreted by diffusion across body surfaces or
through gill surfaces (in fish) as ammonium ions. Kidneys do not play any significant role
in its removal.
Terrestrial adaptation necessitated the production of lesser toxic nitrogenous wastes
like urea and uric acid for conservation of water.
Mammals, many terrestrial amphibians and marine fishes mainly excrete ure a and are
called ureotelic animals. Ammonia produced by metabolism is converted into urea in
the liver of these animals and released into the blood which is filtered and excreted out by
the kidneys.
Some amount of urea may be retained in the kidney matrix of some of these animals to
maintain a desired osmolarity [the concentration of a solution expressed as the total
number of solute particles per litre].
Reptiles, birds, land snails and insects excrete nitrogenous wastes as uric acid in the form
of pellet or paste with a minimum loss of water and are called uricotelic animals.
Protonephridia or flame cells are the excretory structures in Platyhelminthes (Flatworms,
e.g., Planaria), rotifers, some annelids and the cephalochordate.
Excretory system notes 2017
4 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Protonephridia are primarily concerned with ionic and fluid volume regulation, i.e.,
osmoregulation. Nephridia are the tubular excretory structures of earthworms and other
annelids. Nephridia help to remove nitrogenous wastes and maintain a fluid and ionic
balance.
Malpighian tubules are the excretory structures of most of the insects including
cockroaches. Malpighian tubules help in the removal of nitrogenous wastes and
osmoregulation.
Antennal glands or green glands perform the excretory function in crustaceans like
prawns.
Human Excretory System
In humans, the excretory system consists of a pair of kidneys, one pair of ureters, a
urinary bladder and a urethra.
Kidneys
Kidneys are reddish brown, bean shaped structures situated between the levels of last
thoracic and third lumbar vertebra close to the dorsal inner wall of the abdominal cavity.
Each kidney of an adult human measures 10 -12 cm in length, 5-7 cm in width, 2-3
cm in thickness with an average weight of 120-170 g.
Towards the center of the inner concave surface of the kidney is a notch
calledhilum through which ureter, blood vessels and nerves enter.
4 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Protonephridia are primarily concerned with ionic and fluid volume regulation, i.e.,
osmoregulation. Nephridia are the tubular excretory structures of earthworms and other
annelids. Nephridia help to remove nitrogenous wastes and maintain a fluid and ionic
balance.
Malpighian tubules are the excretory structures of most of the insects including
cockroaches. Malpighian tubules help in the removal of nitrogenous wastes and
osmoregulation.
Antennal glands or green glands perform the excretory function in crustaceans like
prawns.
Human Excretory System
In humans, the excretory system consists of a pair of kidneys, one pair of ureters, a
urinary bladder and a urethra.
Kidneys
Kidneys are reddish brown, bean shaped structures situated between the levels of last
thoracic and third lumbar vertebra close to the dorsal inner wall of the abdominal cavity.
Each kidney of an adult human measures 10 -12 cm in length, 5-7 cm in width, 2-3
cm in thickness with an average weight of 120-170 g.
Towards the center of the inner concave surface of the kidney is a notch
calledhilum through which ureter, blood vessels and nerves enter.
Excretory system notes 2017
5 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Inner to the hilum is a broad funnel shaped space called the renal pelvis with projections
called calyces.
Inside the kidney, there are two zones, an outer cortex and an inner medulla. The
medulla is divided into a few conical masses (medullary pyr amids) projecting into the
calyces (singularity: calyx).
Each kidney has nearly one million complex tubular structures called nephrons, which are
the functional units.
Each nephron has two parts – the glomerulus and the renal tubule.
Glomerulus is a tuft of capillaries formed by the afferent arteriole – a fine branch
of renal artery. Blood from the glomerulus is carried away by an efferent arteriole.
The renal tubule begins with a double walled cup-like structure called Bowman’s capsule,
which encloses the glomerulus.
5 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Inner to the hilum is a broad funnel shaped space called the renal pelvis with projections
called calyces.
Inside the kidney, there are two zones, an outer cortex and an inner medulla. The
medulla is divided into a few conical masses (medullary pyr amids) projecting into the
calyces (singularity: calyx).
Each kidney has nearly one million complex tubular structures called nephrons, which are
the functional units.
Each nephron has two parts – the glomerulus and the renal tubule.
Glomerulus is a tuft of capillaries formed by the afferent arteriole – a fine branch
of renal artery. Blood from the glomerulus is carried away by an efferent arteriole.
The renal tubule begins with a double walled cup-like structure called Bowman’s capsule,
which encloses the glomerulus.
Excretory system notes 2017
6 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Glomerulus along with Bowman’s capsule, is called the malpighian body or renal
corpuscle.
The tubule continues further to form a highly coiled network – proximal convoluted
tubule (PCT).
A hairpin shaped Henle’s loop is the next part of the tubule which has a descending and
an ascending limb.
The ascending limb continues as another highly coiled tubul ar region called distal
convoluted tubule (DCT).
The DCTs of many nephrons open into a straight tube called collecting duct, many of
which converge and open into the renal pelvis through medullary pyramids in the calyces.
The Malpighian corpuscle, PCT and DCT of the nephron are situated in the cortical region
of the kidney whereas the loop of Henle dips into the medulla.
In majority of nephrons, the loop of Henle is too short and extends only very little into
the medulla. Such nephrons are called cortical nephrons.
In some of the nephrons, the loop of Henle is very long and runs deep into the medulla.
These nephrons are called juxta medullary nephrons.
The efferent arteriole emerging from the glomerulus forms a fine capillary network
around the renal tubule called the peritubular capillaries.
A minute vessel of this network runs parallel to the Henle’s loop forming a ‘U’ shaped
vasa recta. Vasa recta is absent or highly reduced in cortical nephrons.
Functions of the Kidney
Regulation of blood ionic composition: kidneys help regulate the blood levels of several
ions, most importantly Na
+
, K
+
, Ca
2+
, Cl
-
and phosphate ions(HPO 4
2-
).
Maintainance of blood osmolarity: by separately regulating loss of water and loss of
solutes in urine, kidneys maintain constant blood osmolarity.
Regulation of blood volume:
Regulates blood pressure
6 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Glomerulus along with Bowman’s capsule, is called the malpighian body or renal
corpuscle.
The tubule continues further to form a highly coiled network – proximal convoluted
tubule (PCT).
A hairpin shaped Henle’s loop is the next part of the tubule which has a descending and
an ascending limb.
The ascending limb continues as another highly coiled tubul ar region called distal
convoluted tubule (DCT).
The DCTs of many nephrons open into a straight tube called collecting duct, many of
which converge and open into the renal pelvis through medullary pyramids in the calyces.
The Malpighian corpuscle, PCT and DCT of the nephron are situated in the cortical region
of the kidney whereas the loop of Henle dips into the medulla.
In majority of nephrons, the loop of Henle is too short and extends only very little into
the medulla. Such nephrons are called cortical nephrons.
In some of the nephrons, the loop of Henle is very long and runs deep into the medulla.
These nephrons are called juxta medullary nephrons.
The efferent arteriole emerging from the glomerulus forms a fine capillary network
around the renal tubule called the peritubular capillaries.
A minute vessel of this network runs parallel to the Henle’s loop forming a ‘U’ shaped
vasa recta. Vasa recta is absent or highly reduced in cortical nephrons.
Functions of the Kidney
Regulation of blood ionic composition: kidneys help regulate the blood levels of several
ions, most importantly Na
+
, K
+
, Ca
2+
, Cl
-
and phosphate ions(HPO 4
2-
).
Maintainance of blood osmolarity: by separately regulating loss of water and loss of
solutes in urine, kidneys maintain constant blood osmolarity.
Regulation of blood volume:
Regulates blood pressure
Excretory system notes 2017
7 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Urine Formation or Renal excretory mechanism
Urine formation involves three main processes namely, glomerular filtration,
reabsorption and secretion, that takes place in different parts of the nephron.
The first step in urine formation is the filtration of blood, which is carried out by the
glomerulus and is called glomerular filtration.
On an average, 1100-1200 ml of blood is filtered by the kidneys per minute.
7 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Urine Formation or Renal excretory mechanism
Urine formation involves three main processes namely, glomerular filtration,
reabsorption and secretion, that takes place in different parts of the nephron.
The first step in urine formation is the filtration of blood, which is carried out by the
glomerulus and is called glomerular filtration.
On an average, 1100-1200 ml of blood is filtered by the kidneys per minute.
Excretory system notes 2017
8 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
The glomerular capillary blood pressure causes filtration of blood through 3 layers, i.e.,
the endothelium of glomerular blood vessels, the epithelium of Bowman’s capsule and
a basement membrane between these two layers.
The epithelial cells of Bowman’s capsule called podocytes are arranged in an intricate
manner so as to leave some minute spaces called filtration slits or slit pores. Blood is
filtered so finely through these membranes, that almost all the constituents of the
plasma except the proteins pass onto the lumen of the Bowman’s capsule. Therefore, it is
considered as a process of ultra-filtration.
The amount of the filtrate formed by the kidneys per minute is called glomerular
filtration rate (GFR). GFR in a healthy individual is approximately 125 ml/minute,
i.e., 180 liters per day!
The kidneys have built-in mechanisms for the regulation of glomerular filtration rate.
One such efficient mechanism is carried out by juxta glomerular apparatus (JGA).
A comparison of the volume of the filtrate formed per day (180 liters per day) with that
of the urine released (1.5 litres), suggest that nearly 99 per cent of the filtrate has to be
reabsorbed by the renal tubules. This process is called reabsorption.
8 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
The glomerular capillary blood pressure causes filtration of blood through 3 layers, i.e.,
the endothelium of glomerular blood vessels, the epithelium of Bowman’s capsule and
a basement membrane between these two layers.
The epithelial cells of Bowman’s capsule called podocytes are arranged in an intricate
manner so as to leave some minute spaces called filtration slits or slit pores. Blood is
filtered so finely through these membranes, that almost all the constituents of the
plasma except the proteins pass onto the lumen of the Bowman’s capsule. Therefore, it is
considered as a process of ultra-filtration.
The amount of the filtrate formed by the kidneys per minute is called glomerular
filtration rate (GFR). GFR in a healthy individual is approximately 125 ml/minute,
i.e., 180 liters per day!
The kidneys have built-in mechanisms for the regulation of glomerular filtration rate.
One such efficient mechanism is carried out by juxta glomerular apparatus (JGA).
A comparison of the volume of the filtrate formed per day (180 liters per day) with that
of the urine released (1.5 litres), suggest that nearly 99 per cent of the filtrate has to be
reabsorbed by the renal tubules. This process is called reabsorption.
Excretory system notes 2017
9 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
The tubular epithelial cells in different segments of nephron perform this either by active
or passive mechanisms. For example, substances like glucose, amino acids, Na+, etc., in the
filtrate are reabsorbed actively whereas the nitrogenous wastes are absorbed by passive
transport. Reabsorption of water also occurs passively in the initial segments of the
nephron.
During urine formation, the tubular cells secrete substances like H
+
, K
+
and ammonia into
the filtrate. Tubular secretion is also an important step in urine formation as it helps in
the maintenance of ionic and acid base balance of body fluids.
Function of the Tubules: Proximal Convoluted Tubule (PCT)
PCT is lined by simple cuboidal epithelium which increases the surface area for
reabsorption. Nearly all of the essential nutrients, and 70-80 per cent of electrolytes and
water are reabsorbed by this segment.
PCT also helps to maintain the pH and ionic balance of the body fluids by selective
secretion of hydrogen ions, ammonia and p otassium ions into the filtrate and by
absorption of HCO
3-
from it.
Henle’s Loop
Reabsorption is minimum in its ascending limb. However, this region plays a significant
role in the maintenance of high osmolarity of medullary interstitial fluid.
The descending limb of loop of Henle is permeable to water but almost impermeable to
electrolytes. This concentrates the filtrate as it moves down.
9 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
The tubular epithelial cells in different segments of nephron perform this either by active
or passive mechanisms. For example, substances like glucose, amino acids, Na+, etc., in the
filtrate are reabsorbed actively whereas the nitrogenous wastes are absorbed by passive
transport. Reabsorption of water also occurs passively in the initial segments of the
nephron.
During urine formation, the tubular cells secrete substances like H
+
, K
+
and ammonia into
the filtrate. Tubular secretion is also an important step in urine formation as it helps in
the maintenance of ionic and acid base balance of body fluids.
Function of the Tubules: Proximal Convoluted Tubule (PCT)
PCT is lined by simple cuboidal epithelium which increases the surface area for
reabsorption. Nearly all of the essential nutrients, and 70-80 per cent of electrolytes and
water are reabsorbed by this segment.
PCT also helps to maintain the pH and ionic balance of the body fluids by selective
secretion of hydrogen ions, ammonia and p otassium ions into the filtrate and by
absorption of HCO
3-
from it.
Henle’s Loop
Reabsorption is minimum in its ascending limb. However, this region plays a significant
role in the maintenance of high osmolarity of medullary interstitial fluid.
The descending limb of loop of Henle is permeable to water but almost impermeable to
electrolytes. This concentrates the filtrate as it moves down.
Excretory system notes 2017
10 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
The ascending limb is impermeable to water but allows transport of electrolytes actively
or passively. Therefore, as the concentrated filtrate pass upward, it gets diluted due to
the passage of electrolytes to the medullary fluid.
Distal Convoluted Tubule (DCT)
Conditional reabsorption of Na+ and water takes place in this segment. DCT is also
capable of reabsorption of HCO
3-
and selective secretion of hydrogen and potassium ions
and NH3 to maintain the pH and sodium-potassium balance in blood.
Collecting Duct
This long duct extends from the cortex of the kidney to the inner parts of the medulla.
Large amounts of water could be reabsorbed from this region to produce a concentrated
urine.
This segment allows passage of small amounts of urea into the medullary interstitium to
keep up the osmolarity.
It also plays a role in the maintenance of pH and ionic balance of blood by the selective
secretion of H
+
and K
+
ions.
Mechanism of Concentration of the Filtrate
Mammals have the ability to produce a concentrated urine. The Henle’s loop and vasa
recta play a significant role in this.
The flow of filtrate in the two limbs of Henle’s loop is in opposite directions and thus
forms a counter current.
The flow of blood through the two limbs of vasa recta is also in a counter current
pattern.
The proximity between the Henle’s loop and vasa recta, as well as the counter current in
them help in maintaining an increasing osmolarity towards the inner medullary
interstitium. This gradient is mainly caused by NaCl and urea.
NaCl is transported by the ascending limb of Henle’s loop which is exchanged with the
descending limb of vasa recta. NaCl is returned to the interstitium by the ascending
portion of vasa recta.
10 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
The ascending limb is impermeable to water but allows transport of electrolytes actively
or passively. Therefore, as the concentrated filtrate pass upward, it gets diluted due to
the passage of electrolytes to the medullary fluid.
Distal Convoluted Tubule (DCT)
Conditional reabsorption of Na+ and water takes place in this segment. DCT is also
capable of reabsorption of HCO
3-
and selective secretion of hydrogen and potassium ions
and NH3 to maintain the pH and sodium-potassium balance in blood.
Collecting Duct
This long duct extends from the cortex of the kidney to the inner parts of the medulla.
Large amounts of water could be reabsorbed from this region to produce a concentrated
urine.
This segment allows passage of small amounts of urea into the medullary interstitium to
keep up the osmolarity.
It also plays a role in the maintenance of pH and ionic balance of blood by the selective
secretion of H
+
and K
+
ions.
Mechanism of Concentration of the Filtrate
Mammals have the ability to produce a concentrated urine. The Henle’s loop and vasa
recta play a significant role in this.
The flow of filtrate in the two limbs of Henle’s loop is in opposite directions and thus
forms a counter current.
The flow of blood through the two limbs of vasa recta is also in a counter current
pattern.
The proximity between the Henle’s loop and vasa recta, as well as the counter current in
them help in maintaining an increasing osmolarity towards the inner medullary
interstitium. This gradient is mainly caused by NaCl and urea.
NaCl is transported by the ascending limb of Henle’s loop which is exchanged with the
descending limb of vasa recta. NaCl is returned to the interstitium by the ascending
portion of vasa recta.
Excretory system notes 2017
11 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle’s
loop which is transported back to the interstitium by the collecting tubule.
The above described transport of substances facilitated by the special arrangement of
Henle’s loop and vasa recta is called the counter current mechanism. This mechanism
helps to maintain a concentration gradient in the medullary interstitium.
Presence of such interstitial gradient helps in an easy passage of water from the
collecting tubule thereby concentrating the filtrate (urine). Human kidneys can produce
urine nearly four times concentrated than the initial filtrate formed.
Regulation of Kidney Function
The functioning of the kidneys is efficiently monitored and regulated by hormonal
feedback mechanisms involving the hypothalamus, JGA and to a certain extent, the heart.
Osmoreceptors in the body are activated by changes in blood volume, body fluid volume
and ionic concentration. An excessive loss of fluid from the body can activate these
receptors which stimulate the hypothalamus to release antidiuretic hormone
(ADH) or vasopressin from the neurohypophysis.
ADH facilitates water reabsorption from latter parts of the tubule, thereby
preventing diuresis [increased or excessive production of urine].
An increase in body fluid volume can switch off the osmoreceptors and suppress the ADH
release to complete the feedback.
ADH can also affect the kidney function by its constrictory effects on blood vessels. This
causes an increase in blood pressure. An increase in blood pressure can increase the
glomerular blood flow and thereby the GFR.
The JGA plays a complex regulatory role. A fall in glomerular blood flow/glomerular
blood pressure/GFR can activate the JG cells to release renin which converts
angiotensinogen in blood to angiotensin I and further to angiotensin II.
Angiotensin II, being a powerful vasoconstrictor, increases the glomerular blood
pressure and thereby GFR.
Angiotensin II also activates the adrenal cortex to release Aldosterone. Aldosterone causes
reabsorption of Na+ and water from the distal parts of the tubule. This also leads to an
11 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle’s
loop which is transported back to the interstitium by the collecting tubule.
The above described transport of substances facilitated by the special arrangement of
Henle’s loop and vasa recta is called the counter current mechanism. This mechanism
helps to maintain a concentration gradient in the medullary interstitium.
Presence of such interstitial gradient helps in an easy passage of water from the
collecting tubule thereby concentrating the filtrate (urine). Human kidneys can produce
urine nearly four times concentrated than the initial filtrate formed.
Regulation of Kidney Function
The functioning of the kidneys is efficiently monitored and regulated by hormonal
feedback mechanisms involving the hypothalamus, JGA and to a certain extent, the heart.
Osmoreceptors in the body are activated by changes in blood volume, body fluid volume
and ionic concentration. An excessive loss of fluid from the body can activate these
receptors which stimulate the hypothalamus to release antidiuretic hormone
(ADH) or vasopressin from the neurohypophysis.
ADH facilitates water reabsorption from latter parts of the tubule, thereby
preventing diuresis [increased or excessive production of urine].
An increase in body fluid volume can switch off the osmoreceptors and suppress the ADH
release to complete the feedback.
ADH can also affect the kidney function by its constrictory effects on blood vessels. This
causes an increase in blood pressure. An increase in blood pressure can increase the
glomerular blood flow and thereby the GFR.
The JGA plays a complex regulatory role. A fall in glomerular blood flow/glomerular
blood pressure/GFR can activate the JG cells to release renin which converts
angiotensinogen in blood to angiotensin I and further to angiotensin II.
Angiotensin II, being a powerful vasoconstrictor, increases the glomerular blood
pressure and thereby GFR.
Angiotensin II also activates the adrenal cortex to release Aldosterone. Aldosterone causes
reabsorption of Na+ and water from the distal parts of the tubule. This also leads to an
Excretory system notes 2017
12 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
increase in blood pressure and GFR. This complex mechanism is generally known as
the Renin-Angiotensin mechanism.
An increase in blood flow to the atria of the heart can cause the release of Atrial
Natriuretic Factor (ANF). ANF can cause vasodilation (dilation of blood vessels) and
thereby decrease the blood pressure. ANF mechanism, therefore , acts as a check on the
renin-angiotensin mechanism.
Micturition
Urine formed by the nephrons is ultimately carried to the urinary bladder where it is
stored till a voluntary signal is given by the central nervous system (CNS). This signal is
initiated by the stretching of the urinary bladder as it gets filled with urine. In response,
the stretch receptors on the walls of the bladder send signals to the CNS. The CNS passes
on motor messages to initiate the contraction of smooth muscles of the bladder and
simultaneous relaxation of the urethral sphincter causing the release of urine. The process
of release of urine is called micturition and the neural mechanisms causing it is called
the micturition reflex.
An adult human excretes, on an average, 1 to 1.5 litres of urine per day. The urine
formed is a light yellow coloured watery fluid which is slightly acidic (pH-6.0) and has a
characterestic odour.
On an average, 25-30 gm of urea is excreted out per day. Various conditions can affect
the characteristics of urine.
Analysis of urine helps in clinical diagnosis of many metabolic discorders as well as
malfunctioning of the kidney. For example, presence of glucose (Glycosuria) and ketone
bodies (Ketonuria) in urine are indicative of diabetes mellitus.
Role of other Organs in Excretion
Other than the kidneys, lungs, liver and skin also help in the elimination of excretory
wastes.
Our lungs remove large amounts of CO
2
(approximately 200mL/ minute) and also
significant quantities of water every day.
12 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
increase in blood pressure and GFR. This complex mechanism is generally known as
the Renin-Angiotensin mechanism.
An increase in blood flow to the atria of the heart can cause the release of Atrial
Natriuretic Factor (ANF). ANF can cause vasodilation (dilation of blood vessels) and
thereby decrease the blood pressure. ANF mechanism, therefore , acts as a check on the
renin-angiotensin mechanism.
Micturition
Urine formed by the nephrons is ultimately carried to the urinary bladder where it is
stored till a voluntary signal is given by the central nervous system (CNS). This signal is
initiated by the stretching of the urinary bladder as it gets filled with urine. In response,
the stretch receptors on the walls of the bladder send signals to the CNS. The CNS passes
on motor messages to initiate the contraction of smooth muscles of the bladder and
simultaneous relaxation of the urethral sphincter causing the release of urine. The process
of release of urine is called micturition and the neural mechanisms causing it is called
the micturition reflex.
An adult human excretes, on an average, 1 to 1.5 litres of urine per day. The urine
formed is a light yellow coloured watery fluid which is slightly acidic (pH-6.0) and has a
characterestic odour.
On an average, 25-30 gm of urea is excreted out per day. Various conditions can affect
the characteristics of urine.
Analysis of urine helps in clinical diagnosis of many metabolic discorders as well as
malfunctioning of the kidney. For example, presence of glucose (Glycosuria) and ketone
bodies (Ketonuria) in urine are indicative of diabetes mellitus.
Role of other Organs in Excretion
Other than the kidneys, lungs, liver and skin also help in the elimination of excretory
wastes.
Our lungs remove large amounts of CO
2
(approximately 200mL/ minute) and also
significant quantities of water every day.
Excretory system notes 2017
13 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Liver, the largest gland in our body, secretes bile-containing substances like bilirubin,
biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs . Most of these
substances ultimately pass out alongwith digestive wastes.
The sweat and sebaceous glands in the s kin can eliminate certain substances through
their secretions. Sweat produced by the sweat glands is a watery fluid containing NaCl,
small amounts of urea, lactic acid, etc.
Though the primary function of sweat is to facilitate a cooling effect on the body surface,
it also helps in the removal of some of the wastes mentioned above.
Sebaceous glands eliminate certain substances like sterols, hydrocarbons and waxes
through sebum. This secretion provides a protective oily covering for the skin. Small
amounts of nitrogenous wastes could be eliminated through saliva too.
Disorders of the Excretory System
Malfunctioning of kidneys can lead to accumulation of urea in blood, a condition called
uremia, which is highly harmful and may lead to kidney failure. In such patients, urea can
be removed by a process called hemodialysis.
Blood drained from a convenient artery is pumped into a dialyzing unit after adding an
anticoagulant like heparin. The unit contains a coiled cellophane tube surrounded by a
fluid (dialyzing fluid) having the same composition as that of plasma except the
nitrogenous wastes.
The porous cellophane membrane of the tube allows the passage of molecules based on
concentration gradient. As nitrogenous wastes are absent in the dialyzing fluid, these
substances freely move out, thereby clearing the blood.
The cleared blood is pumped back to the body through a vein after adding anti -heparin
to it. This method is a boon for thousands of uremic patients all over the world.
Kidney transplantation is the ultimate method in the correction of acute renal failures
(kidney failure). A functioning kidney is used in transplantation from a donor, preferably
a close relative, to minimise its chances of rejection by the immune system of the host.
Modern clinical procedures have increased the success rate of such a complicated
technique.
13 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Liver, the largest gland in our body, secretes bile-containing substances like bilirubin,
biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs . Most of these
substances ultimately pass out alongwith digestive wastes.
The sweat and sebaceous glands in the s kin can eliminate certain substances through
their secretions. Sweat produced by the sweat glands is a watery fluid containing NaCl,
small amounts of urea, lactic acid, etc.
Though the primary function of sweat is to facilitate a cooling effect on the body surface,
it also helps in the removal of some of the wastes mentioned above.
Sebaceous glands eliminate certain substances like sterols, hydrocarbons and waxes
through sebum. This secretion provides a protective oily covering for the skin. Small
amounts of nitrogenous wastes could be eliminated through saliva too.
Disorders of the Excretory System
Malfunctioning of kidneys can lead to accumulation of urea in blood, a condition called
uremia, which is highly harmful and may lead to kidney failure. In such patients, urea can
be removed by a process called hemodialysis.
Blood drained from a convenient artery is pumped into a dialyzing unit after adding an
anticoagulant like heparin. The unit contains a coiled cellophane tube surrounded by a
fluid (dialyzing fluid) having the same composition as that of plasma except the
nitrogenous wastes.
The porous cellophane membrane of the tube allows the passage of molecules based on
concentration gradient. As nitrogenous wastes are absent in the dialyzing fluid, these
substances freely move out, thereby clearing the blood.
The cleared blood is pumped back to the body through a vein after adding anti -heparin
to it. This method is a boon for thousands of uremic patients all over the world.
Kidney transplantation is the ultimate method in the correction of acute renal failures
(kidney failure). A functioning kidney is used in transplantation from a donor, preferably
a close relative, to minimise its chances of rejection by the immune system of the host.
Modern clinical procedures have increased the success rate of such a complicated
technique.
Excretory system notes 2017
14 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Renal calculi: Stone or insoluble mass of crystallised salts (oxalates, etc.) formed within
the kidney.
Glomerulonephritis: Inflammation of glomeruli of kidney.
Composition of Urine:
A. Volume:
1. A normal adult excretes daily from 1000 ml to 1800 ml of urine. The average is
1500 ml containing 60 gm. of solids.
2. The quantity depends on the water intake, external temperature, the diet and the in -
dividual’s mental and physical condition.
3. A high protein diet increases excretion be cause the urea formed as a result of
catabolism of protein has a diuretic action.
4. The diuretic action of tea, coffee and cocoa is due to caffeine.
5. The decreased volume of urine in hot weather is due to an increased loss of water by
perspiration.
6. Nervousness or excitement causes increased urinary volume.
7. Increased urine volumes are observed in diabetes insipidus, diabetes mellitus and
certain types of kidney diseases and decreased volumes are found in acute nephritis,
fevers, diseases of the heart, diarrhea and vomiting.
B. Specific Gravity:
1. The specific gravity of urine in 24 hours lies between 1.003 and 1.030 and varies
according to concentration of solutes in the urine. The figures in the second and third
decimal places, multiplied by 2.66 (Long’s coefficient) give roughly the total solids in the
urine in gm./L. 50 gm. of solids are the average normal for the day.
2. The specific gravity of the urine varies with the food, water intake, and the activity of
the individual.
3. In chronic interstitial nephritis, the specific gravity is lowered.
4. The specific gravity is increased in the excretion of abnormal substances such as
albumin or glucose (e.g., diabetes mellitus).
C. Colour:
14 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Renal calculi: Stone or insoluble mass of crystallised salts (oxalates, etc.) formed within
the kidney.
Glomerulonephritis: Inflammation of glomeruli of kidney.
Composition of Urine:
A. Volume:
1. A normal adult excretes daily from 1000 ml to 1800 ml of urine. The average is
1500 ml containing 60 gm. of solids.
2. The quantity depends on the water intake, external temperature, the diet and the in -
dividual’s mental and physical condition.
3. A high protein diet increases excretion be cause the urea formed as a result of
catabolism of protein has a diuretic action.
4. The diuretic action of tea, coffee and cocoa is due to caffeine.
5. The decreased volume of urine in hot weather is due to an increased loss of water by
perspiration.
6. Nervousness or excitement causes increased urinary volume.
7. Increased urine volumes are observed in diabetes insipidus, diabetes mellitus and
certain types of kidney diseases and decreased volumes are found in acute nephritis,
fevers, diseases of the heart, diarrhea and vomiting.
B. Specific Gravity:
1. The specific gravity of urine in 24 hours lies between 1.003 and 1.030 and varies
according to concentration of solutes in the urine. The figures in the second and third
decimal places, multiplied by 2.66 (Long’s coefficient) give roughly the total solids in the
urine in gm./L. 50 gm. of solids are the average normal for the day.
2. The specific gravity of the urine varies with the food, water intake, and the activity of
the individual.
3. In chronic interstitial nephritis, the specific gravity is lowered.
4. The specific gravity is increased in the excretion of abnormal substances such as
albumin or glucose (e.g., diabetes mellitus).
C. Colour:
Excretory system notes 2017
15 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
1. Normal urine is pale yellow or amber. The colour is roughly proportional to the specific
gravity. Very dilute urine is colourless.
2. Teroehrome, composed of a polypeptide and urobilin, is the chief pigment of urine.
Traces of coproporphyrtn, urobilinogen and litroerytbrin are also found in urine.
3. Reddish urine is due to the ingestion of naturally coloured foods (e.g., beetroot,
blackberries). In fever, the urine may be dark yellow or brownish because of con -
centration. In liver disease, the urine may be green, brown, or deep yellow due to bile
pigments. Blood or hemoglobin develops smoky to red colour. The urine is dark brown
due to methemoglobin and homogentisic acid. Methylene blue gives the urine a green
appearance.
4. The urine is transparent. A turbidity is developed in alkaline urine by precipitation of
calcium phosphate. Strongly acid urine is pink due to the precipitation of uric acid salts.
D. Odour:
1. Fresh urine is normally aromatic.
2. The odour is modified by the ingestion of certain foods or drugs. This is noticed after
eating asparagus; the odour is due to methyl mercaptan.
3. The ammoniacal smell of urine is due to the action of bacteria on urea.
4. In ketosis, the odour of excreted acetone is detected.
E. pH:
1. The mixed sample of normal urine in 24 hours has a pH 6.0. Individual samples vary
from 4.6 to 8.0.
2. The urine is acid in high protein intake because excess phosphate and sulfate are
formed in the catabolism of protein. Acidity is also increased in acidosis and in fever.
3. The urine becomes alkaline on standing due to the conversion of urea to ammonia and
loss of CO2 to air. It may be alkaline in alkalosis such as after excessive vomiting and after
meals due to H
+
secretion in the stomach (the “alkaline tide”).
4. The acidity of urine is increased after strenuous muscular exercise (elimination of lactic
acid), by ingestion of ammonium salts of strong acids. An alkaline urine may be produced
by ingestion of sufficient NaHCO3. Ammonium carbonate does not produce an alkaline
urine because ammonia is rapidly converted into urea.
15 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
1. Normal urine is pale yellow or amber. The colour is roughly proportional to the specific
gravity. Very dilute urine is colourless.
2. Teroehrome, composed of a polypeptide and urobilin, is the chief pigment of urine.
Traces of coproporphyrtn, urobilinogen and litroerytbrin are also found in urine.
3. Reddish urine is due to the ingestion of naturally coloured foods (e.g., beetroot,
blackberries). In fever, the urine may be dark yellow or brownish because of con -
centration. In liver disease, the urine may be green, brown, or deep yellow due to bile
pigments. Blood or hemoglobin develops smoky to red colour. The urine is dark brown
due to methemoglobin and homogentisic acid. Methylene blue gives the urine a green
appearance.
4. The urine is transparent. A turbidity is developed in alkaline urine by precipitation of
calcium phosphate. Strongly acid urine is pink due to the precipitation of uric acid salts.
D. Odour:
1. Fresh urine is normally aromatic.
2. The odour is modified by the ingestion of certain foods or drugs. This is noticed after
eating asparagus; the odour is due to methyl mercaptan.
3. The ammoniacal smell of urine is due to the action of bacteria on urea.
4. In ketosis, the odour of excreted acetone is detected.
E. pH:
1. The mixed sample of normal urine in 24 hours has a pH 6.0. Individual samples vary
from 4.6 to 8.0.
2. The urine is acid in high protein intake because excess phosphate and sulfate are
formed in the catabolism of protein. Acidity is also increased in acidosis and in fever.
3. The urine becomes alkaline on standing due to the conversion of urea to ammonia and
loss of CO2 to air. It may be alkaline in alkalosis such as after excessive vomiting and after
meals due to H
+
secretion in the stomach (the “alkaline tide”).
4. The acidity of urine is increased after strenuous muscular exercise (elimination of lactic
acid), by ingestion of ammonium salts of strong acids. An alkaline urine may be produced
by ingestion of sufficient NaHCO3. Ammonium carbonate does not produce an alkaline
urine because ammonia is rapidly converted into urea.
Excretory system notes 2017
16 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Constituents of Urine:
Normal Constituents of Urine:
A. Urea: Nitrogenous Constituents:
1. Urea is the main end product of catabo lism of protein in mammals. Its excretion is
directly proportional to the protein intake. It consists of 80-90% of the total urinary
nitrogen.
2. In fever, diabetes, or excess adrenocortical activity, urea excretion is increased due to
increased protein catabolism.
3. Decreased urea excretion is due to decreased urea production in the last stages of fatal
liver disease.
4. In acidosis, there is decreased urea excretion.
B. Ammonia:
1. Ammonia is formed by the kidney from glutamine or amino acids in acidosis.
2. There is a high ammonia output in the urine in uncontrolled diabetes mellitus in which
renal function is unimpaired.
C. Creatinine and Creatine:
1. Creatine is excreted by children and pregnant women and much smaller amounts in
men. The excretion in men is 6% of the total excretion of creatinine.
2. Creatinine is formed from creatine. It is excreted in relatively constant amounts
regardless of diet.
3. The creatinine coefficient is the ratio between the amount of creatinine excreted in 24
hours and the body weight in kg. It is usually 20-26 mg/kg/day in normal men and 14 -
22 mg/kg/day in normal women.
4. Creatinine excretion is decreased in many pathological conditions.
5. Creatine excretion is also found in patho logical states such as starvation, hyper-
thyroidism, impaired carbohydrate metabolism and infections.
6. Creatine excretion is decreased in hypothyroidism.
D. Uric Acid:
16 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Constituents of Urine:
Normal Constituents of Urine:
A. Urea: Nitrogenous Constituents:
1. Urea is the main end product of catabo lism of protein in mammals. Its excretion is
directly proportional to the protein intake. It consists of 80-90% of the total urinary
nitrogen.
2. In fever, diabetes, or excess adrenocortical activity, urea excretion is increased due to
increased protein catabolism.
3. Decreased urea excretion is due to decreased urea production in the last stages of fatal
liver disease.
4. In acidosis, there is decreased urea excretion.
B. Ammonia:
1. Ammonia is formed by the kidney from glutamine or amino acids in acidosis.
2. There is a high ammonia output in the urine in uncontrolled diabetes mellitus in which
renal function is unimpaired.
C. Creatinine and Creatine:
1. Creatine is excreted by children and pregnant women and much smaller amounts in
men. The excretion in men is 6% of the total excretion of creatinine.
2. Creatinine is formed from creatine. It is excreted in relatively constant amounts
regardless of diet.
3. The creatinine coefficient is the ratio between the amount of creatinine excreted in 24
hours and the body weight in kg. It is usually 20-26 mg/kg/day in normal men and 14 -
22 mg/kg/day in normal women.
4. Creatinine excretion is decreased in many pathological conditions.
5. Creatine excretion is also found in patho logical states such as starvation, hyper-
thyroidism, impaired carbohydrate metabolism and infections.
6. Creatine excretion is decreased in hypothyroidism.
D. Uric Acid:
Excretory system notes 2017
17 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
1. It is the end product of the oxidation of purines in the body. It is not only formed
from dietary nucleoprotein but also from the breakdown of cellular nucleoprotein in the
body.
2. It is slightly soluble in water and precipitates readily from acid urine on standing.
3. Uric acid excretion is increased in leukemia, severe liver disease and various stages of
gout.
4. The concentrated urine on cooling forms a brick -red deposit which is mainly acid
urate.
5. Pure uric acid is colourless. Deposits of uric acid and urates are coloured by absorbed
urinary pigments, particularly the red uroervthrin.
6. The specificity of the analysis of uric acid is increased by treatment with uricase, the
enzyme (from hog kidney) which converts uric acid to allantoin.
E. Amino Acids:
1. About 150-200 mg of amino acid nitrogen is excreted in the urine of adults in 24
hours.
2. The infant at birth excretes about 3 mg amino acid nitrogen per pound of body
weight, and up to the age of 6 months the value reaches to 1 mg/pound which is
maintained throughout childhood. Prema ture infants excrete 10 times amino acid
nitrogen than that of full-term infant.
3. The low excretion of amino acid nitrogen is due to its high renal threshold value.
4. Increased amounts of amino acids are excreted in liver disease and in certain types of
poisoning.
5. In cystinuria, 4 amino acids-arginine, cystine, lysine and ornithine are excreted in
urine.
F. Allantoin:
1. It is the partial oxidative products of uric acid. Small quantities of the allantoin are
excreted in human urine.
2. In other sub-primate mammals, allantoin, the principal end product of purine me -
tabolism, is excreted.
G. Sulphates:
17 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
1. It is the end product of the oxidation of purines in the body. It is not only formed
from dietary nucleoprotein but also from the breakdown of cellular nucleoprotein in the
body.
2. It is slightly soluble in water and precipitates readily from acid urine on standing.
3. Uric acid excretion is increased in leukemia, severe liver disease and various stages of
gout.
4. The concentrated urine on cooling forms a brick -red deposit which is mainly acid
urate.
5. Pure uric acid is colourless. Deposits of uric acid and urates are coloured by absorbed
urinary pigments, particularly the red uroervthrin.
6. The specificity of the analysis of uric acid is increased by treatment with uricase, the
enzyme (from hog kidney) which converts uric acid to allantoin.
E. Amino Acids:
1. About 150-200 mg of amino acid nitrogen is excreted in the urine of adults in 24
hours.
2. The infant at birth excretes about 3 mg amino acid nitrogen per pound of body
weight, and up to the age of 6 months the value reaches to 1 mg/pound which is
maintained throughout childhood. Prema ture infants excrete 10 times amino acid
nitrogen than that of full-term infant.
3. The low excretion of amino acid nitrogen is due to its high renal threshold value.
4. Increased amounts of amino acids are excreted in liver disease and in certain types of
poisoning.
5. In cystinuria, 4 amino acids-arginine, cystine, lysine and ornithine are excreted in
urine.
F. Allantoin:
1. It is the partial oxidative products of uric acid. Small quantities of the allantoin are
excreted in human urine.
2. In other sub-primate mammals, allantoin, the principal end product of purine me -
tabolism, is excreted.
G. Sulphates:
Excretory system notes 2017
18 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
The urine sulphur is derived from sulphur containing amino acids such as methionine and
cystine and therefore, its output varies with protein intake.
The urine sulphur exists in 3 forms:
I. Inorganic (sulfate) sulfur:
1. This is the completely oxidized sulfur precipitated from urine.
2. It is proportionate to the ingested protein with a ratio of 5 : 1 between urine
nitrogen and inorganic sulfate.
II. Ethereal sulfur (conjugated sulfates):
1. It is about 10% of the total excreted sulfur.
This includes the organic combination of sulfur excreted in the urine.
2. It consists of the sulphuric esters of certain phenols.
3. It forms no precipitate on addition of acidified BaCl2. Some of the phenols are
derived from putrefaction of protein in the large intestine.
4. Clinically, the ethereal sulfate is that of indoxyl indican which is formed from
bacterial decomposition of tryptophan in the large intestine.
5. Normally, 5-20 mg of indican are excreted and the amount increases in
constipation. In cholera, typhus gangrene of lung, sufficient indican is excreted.
Indoxyl liberated from indican is oxidized to indigo blue on exposure to air.
III. Neutral sulfur:
1. These are un-oxidized sulfur and contained in cystine, taurine, thiocyanate or
sulfides.
2. They do not vary with the diet.
3. They are mainly the products of endogenous metabolism.
Other organic compounds:
H. Chlorides:
These are excreted as NaCl and output varies with intake.
I. Phosphates:
1. The urine phosphates consist of sodium and potassium phosphates as well as cal cium
and magnesium phosphates.
18 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
The urine sulphur is derived from sulphur containing amino acids such as methionine and
cystine and therefore, its output varies with protein intake.
The urine sulphur exists in 3 forms:
I. Inorganic (sulfate) sulfur:
1. This is the completely oxidized sulfur precipitated from urine.
2. It is proportionate to the ingested protein with a ratio of 5 : 1 between urine
nitrogen and inorganic sulfate.
II. Ethereal sulfur (conjugated sulfates):
1. It is about 10% of the total excreted sulfur.
This includes the organic combination of sulfur excreted in the urine.
2. It consists of the sulphuric esters of certain phenols.
3. It forms no precipitate on addition of acidified BaCl2. Some of the phenols are
derived from putrefaction of protein in the large intestine.
4. Clinically, the ethereal sulfate is that of indoxyl indican which is formed from
bacterial decomposition of tryptophan in the large intestine.
5. Normally, 5-20 mg of indican are excreted and the amount increases in
constipation. In cholera, typhus gangrene of lung, sufficient indican is excreted.
Indoxyl liberated from indican is oxidized to indigo blue on exposure to air.
III. Neutral sulfur:
1. These are un-oxidized sulfur and contained in cystine, taurine, thiocyanate or
sulfides.
2. They do not vary with the diet.
3. They are mainly the products of endogenous metabolism.
Other organic compounds:
H. Chlorides:
These are excreted as NaCl and output varies with intake.
I. Phosphates:
1. The urine phosphates consist of sodium and potassium phosphates as well as cal cium
and magnesium phosphates.
Excretory system notes 2017
19 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
2. The greater part of the excreted phos phates is derived from ingested food which
contains organic phosphates, e.g., nucleoprotein, phosphoprotein and phospholipids.
Phosphates of food are not completely absorbed. Some phosphate is also derived from
cellular breakdown.
3. Phosphate excretion is increased in certain bone diseases such as osteomalacia, wasting
diseases of the nervous system and in renal tubular rickets.
4. Marked increase of phosphate excretion is also observed in hyperparathyroidism a nd
decrease in hypoparathyroidism and in infectious diseases.
J. Oxalates:
1. The amount of oxalate in the urine is low (20 mg/day) and found as calcium oxalate
crystals in urinary deposits.
2. The excretion of oxalate is increased by ingestion of fruits and vegetables containing
high oxalates (spinach).
3. Large quantities of oxalate are excreted in urine in inherited metabolic diseases.
4. The oxalates present in urine are composed of partly unchanged ingested acid and
partly oxidative products of other compounds.
K. Minerals:
1. The 4 cations of the extracellular fluid— sodium, potassium, calcium and magne -
sium—are present in the urine.
2. Sodium content varies with intake. Urine potassium increases when the intake is
increased or in excessive tissue catabolism. The excretion of potassium is affected by
alkalosis. Sodium and potassium excretion are also controlled by the activity of the
adrenal cortex.
3. Calcium and magnesium are not com pletely absorbed and their presence in the urine
is low. But their presence in the urine varies in certain pathological states, particularly
those involving bone metabolism.
L. Enzymes:
1. Traces of many enzymes are excreted in urine including pancreatic amylase, pep sin,
trypsin and lipase.
2. The pancreatic amylase excretion is increased in pancreatic disease.
19 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
2. The greater part of the excreted phos phates is derived from ingested food which
contains organic phosphates, e.g., nucleoprotein, phosphoprotein and phospholipids.
Phosphates of food are not completely absorbed. Some phosphate is also derived from
cellular breakdown.
3. Phosphate excretion is increased in certain bone diseases such as osteomalacia, wasting
diseases of the nervous system and in renal tubular rickets.
4. Marked increase of phosphate excretion is also observed in hyperparathyroidism a nd
decrease in hypoparathyroidism and in infectious diseases.
J. Oxalates:
1. The amount of oxalate in the urine is low (20 mg/day) and found as calcium oxalate
crystals in urinary deposits.
2. The excretion of oxalate is increased by ingestion of fruits and vegetables containing
high oxalates (spinach).
3. Large quantities of oxalate are excreted in urine in inherited metabolic diseases.
4. The oxalates present in urine are composed of partly unchanged ingested acid and
partly oxidative products of other compounds.
K. Minerals:
1. The 4 cations of the extracellular fluid— sodium, potassium, calcium and magne -
sium—are present in the urine.
2. Sodium content varies with intake. Urine potassium increases when the intake is
increased or in excessive tissue catabolism. The excretion of potassium is affected by
alkalosis. Sodium and potassium excretion are also controlled by the activity of the
adrenal cortex.
3. Calcium and magnesium are not com pletely absorbed and their presence in the urine
is low. But their presence in the urine varies in certain pathological states, particularly
those involving bone metabolism.
L. Enzymes:
1. Traces of many enzymes are excreted in urine including pancreatic amylase, pep sin,
trypsin and lipase.
2. The pancreatic amylase excretion is increased in pancreatic disease.
Excretory system notes 2017
20 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
M. Hormones and vitamins:
1. Certain hormones (sex hormones) and vitamins (e.g., B, and C) are found in urine.
2. The vitamin needs are assessed by studying the urinary output after test doses. The
pregnancy test is also performed by the urinary sex hormones.
Abnormal Constituents of the Urine:
A. Proteins:
Proteinuria (albuminuria) is the presence of albu min and globulin in the urine in
abnormal concentrations. The traces of protein (10-150 mg) present in normal urine
cannot be detected by the ordinary simple tests. Pathologically, several proteins, such as
serum albumin, serum globulin, hemoglobin, mucus, proteose, Bence -Jones proteins are
found in urine.
1. Physiologic proteinuria:
In this condition, less than 0.5% protein is present in urine which occurs after severe
exercise, after a high protein meal or as a result of some temporary impairment in renal
circulation when a person stands erect. In 30-35% of pregnancy, there is proteinuria.
2. Pathologic proteinuria:
Proteinuria is marked in glomerulonephritis. In neph rotic syndrome, a marked
proteinuria occurs. The proteinuria increases with the increasing severity of the renal
lesion. Proteinuria also results in poisoning of the renal tubules by heavy metals like mer-
cury, arsenic or bismuth.
3. Hemoglobin is also present as a result of hematuria due to hemorrhage from the
kidneys or urinary tract, clotting may occur due to sufficient fibrinogen on passing of
much blood.
4. Mucus is the term for an unidentified protein precipitated by acetic acid in the cold. It
is mucin. The mucus is increased in infection of the bladder.
5. Proteose may be found which is of little clinical significance.
6. Bence-Jones proteins found in the urine are the peculiar proteins which are light chain
fragments of globulins. Most commonly they occur in multiple myeloma and rarely in
leukemia. They are precipitated when the urine warmed to 50 -60°C and re-dissolved
almost completely at 100°C and precipitated again on cooling.
20 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
M. Hormones and vitamins:
1. Certain hormones (sex hormones) and vitamins (e.g., B, and C) are found in urine.
2. The vitamin needs are assessed by studying the urinary output after test doses. The
pregnancy test is also performed by the urinary sex hormones.
Abnormal Constituents of the Urine:
A. Proteins:
Proteinuria (albuminuria) is the presence of albu min and globulin in the urine in
abnormal concentrations. The traces of protein (10-150 mg) present in normal urine
cannot be detected by the ordinary simple tests. Pathologically, several proteins, such as
serum albumin, serum globulin, hemoglobin, mucus, proteose, Bence -Jones proteins are
found in urine.
1. Physiologic proteinuria:
In this condition, less than 0.5% protein is present in urine which occurs after severe
exercise, after a high protein meal or as a result of some temporary impairment in renal
circulation when a person stands erect. In 30-35% of pregnancy, there is proteinuria.
2. Pathologic proteinuria:
Proteinuria is marked in glomerulonephritis. In neph rotic syndrome, a marked
proteinuria occurs. The proteinuria increases with the increasing severity of the renal
lesion. Proteinuria also results in poisoning of the renal tubules by heavy metals like mer-
cury, arsenic or bismuth.
3. Hemoglobin is also present as a result of hematuria due to hemorrhage from the
kidneys or urinary tract, clotting may occur due to sufficient fibrinogen on passing of
much blood.
4. Mucus is the term for an unidentified protein precipitated by acetic acid in the cold. It
is mucin. The mucus is increased in infection of the bladder.
5. Proteose may be found which is of little clinical significance.
6. Bence-Jones proteins found in the urine are the peculiar proteins which are light chain
fragments of globulins. Most commonly they occur in multiple myeloma and rarely in
leukemia. They are precipitated when the urine warmed to 50 -60°C and re-dissolved
almost completely at 100°C and precipitated again on cooling.
Excretory system notes 2017
21 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
B. Glucose:
Normal individuals excrete not more than 16 -300 mg of sugar per day which is difficult to
detect by simple test. It is said to be glycosuria when more than this quantity is found in urine.
There are different causes of glycosuira.
Transient glycosuria is observed after emotional stress such as exciting atheletic contest. 15% of
cases of glycosuria are not due to diabetes glycosuria suggesting diabetes must be confirmed by
blood glucose studies to eliminate the probability of renal glycosuria.
The presence of glucose must be tested by Benedict’s test. But in case of pregnant women and
lactating mother, the Osazone test must be performed for urine glucose to eliminate the lactose
present in urine.
C. Other sugars:
1. Fructosuria:
Fructosuria is due to the disturbance in fructose metabolism but no other carbohydrates.
2. Galactosuria and lactosuria:
These may occur occasionally in infants, pregnant women and lactating mother. Galactosuria
may occur in inherited diseases due to the non-conversion of galactose to glucose.
3. Pentosuria:
This may occur transiently after intake of food containing large quantities of pentose’s, such as
grapes, cherries and plums. It may take place in inherited diseases in which pentose’s are not
metabolized. To detect these other sugars in urine it is wise to perform Osazone test.
D. Ketone bodies:
1. Only less than 1 mg of ketone bodies are excreted in urine normally in 24 hours.
2. Increased amount of ketone bodies are ex creted in urine in starvation, diabetes
mellitus, pregnancy, ether anesthesia, and some types of alkalosis.
3. Excess fat metabolism may induce a ketonuria in many animals.
4. Increased amount of ammonia is excreted in acidosis accompanying ketosis.
E. Bilirubin and Bile salts:
1. Bilirubin is found in the urine in cases of obstructive or hepatic jaundice.
2. Bilirubinuria is accompanied by the excretion of bile salts.
21 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
B. Glucose:
Normal individuals excrete not more than 16 -300 mg of sugar per day which is difficult to
detect by simple test. It is said to be glycosuria when more than this quantity is found in urine.
There are different causes of glycosuira.
Transient glycosuria is observed after emotional stress such as exciting atheletic contest. 15% of
cases of glycosuria are not due to diabetes glycosuria suggesting diabetes must be confirmed by
blood glucose studies to eliminate the probability of renal glycosuria.
The presence of glucose must be tested by Benedict’s test. But in case of pregnant women and
lactating mother, the Osazone test must be performed for urine glucose to eliminate the lactose
present in urine.
C. Other sugars:
1. Fructosuria:
Fructosuria is due to the disturbance in fructose metabolism but no other carbohydrates.
2. Galactosuria and lactosuria:
These may occur occasionally in infants, pregnant women and lactating mother. Galactosuria
may occur in inherited diseases due to the non-conversion of galactose to glucose.
3. Pentosuria:
This may occur transiently after intake of food containing large quantities of pentose’s, such as
grapes, cherries and plums. It may take place in inherited diseases in which pentose’s are not
metabolized. To detect these other sugars in urine it is wise to perform Osazone test.
D. Ketone bodies:
1. Only less than 1 mg of ketone bodies are excreted in urine normally in 24 hours.
2. Increased amount of ketone bodies are ex creted in urine in starvation, diabetes
mellitus, pregnancy, ether anesthesia, and some types of alkalosis.
3. Excess fat metabolism may induce a ketonuria in many animals.
4. Increased amount of ammonia is excreted in acidosis accompanying ketosis.
E. Bilirubin and Bile salts:
1. Bilirubin is found in the urine in cases of obstructive or hepatic jaundice.
2. Bilirubinuria is accompanied by the excretion of bile salts.
Excretory system notes 2017
22 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
3. Bile salts may be excreted in urine without bile pigment in certain stages in liver
disease.
4. In excessive hemolysis, traces of bilirubin without bile salts are excreted in urine.
F. Blood:
1. In the lesion of the kidney or urinary tract blood is excreted in the urine in addition to
its presence in nephritis.
2. Free hemoglobin is also found in urine after quick hemolysis, e.g., in black water fever
(a complication of malaria) or after severe burns.
G. Urobilinogen:
1. In excessive hemolysis, e.g., hemolytic jaundice or pernicious anemia, part of the bile
pigment formed by breakdown of hemoglobin is excreted in urine as urobilinogen.
2. Urobilin is formed from colourless urobilinogen when the urine is exposed to air. This
gives the urine an orange colour.
3. In liver disease or temporarily in constipation, large amounts of urobilin are found in
urine.
4. Urobilinogen excreted in urine normally are 1-3.5 mg/24 hrs.
H. Porphyrins:
1. Coprophyrins excreted in urine normally are 50-250 (µ gm./day.
2. Coproporphyria are excreted more in certain liver diseases.
3. The increased amount of coproporphyrins in the urine is a characteristic of the urine
of patients suffering from porphyria.
Urinary Deposits:
The commonest deposits are phosphates, oxalates and urates and are frequently seen in normal
urine.
Phosphates:
1. They are usually found in alkaline urines. The commonest is ammonium ma gnesium
phosphate which forms a characteristic crystal.
2. A less common form is calcium hydrogen phosphate which forms long prisms.
3. Amorphous calcium and magnesium phos phates may be deposited from alkaline urines.
The deposition of phosphates is due to a change in pH after the urine has been passed.
22 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
3. Bile salts may be excreted in urine without bile pigment in certain stages in liver
disease.
4. In excessive hemolysis, traces of bilirubin without bile salts are excreted in urine.
F. Blood:
1. In the lesion of the kidney or urinary tract blood is excreted in the urine in addition to
its presence in nephritis.
2. Free hemoglobin is also found in urine after quick hemolysis, e.g., in black water fever
(a complication of malaria) or after severe burns.
G. Urobilinogen:
1. In excessive hemolysis, e.g., hemolytic jaundice or pernicious anemia, part of the bile
pigment formed by breakdown of hemoglobin is excreted in urine as urobilinogen.
2. Urobilin is formed from colourless urobilinogen when the urine is exposed to air. This
gives the urine an orange colour.
3. In liver disease or temporarily in constipation, large amounts of urobilin are found in
urine.
4. Urobilinogen excreted in urine normally are 1-3.5 mg/24 hrs.
H. Porphyrins:
1. Coprophyrins excreted in urine normally are 50-250 (µ gm./day.
2. Coproporphyria are excreted more in certain liver diseases.
3. The increased amount of coproporphyrins in the urine is a characteristic of the urine
of patients suffering from porphyria.
Urinary Deposits:
The commonest deposits are phosphates, oxalates and urates and are frequently seen in normal
urine.
Phosphates:
1. They are usually found in alkaline urines. The commonest is ammonium ma gnesium
phosphate which forms a characteristic crystal.
2. A less common form is calcium hydrogen phosphate which forms long prisms.
3. Amorphous calcium and magnesium phos phates may be deposited from alkaline urines.
The deposition of phosphates is due to a change in pH after the urine has been passed.
Excretory system notes 2017
23 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Calcium Oxalate:
This is found in acid urine but may be found in alkaline urine. The crystals are of two
types—octahedra, dumb-bells. Calcium oxalate is insoluble in acetic acid.
Urates:
1. They are usually found in acid urines.
2. Uric acid separates into different forms including prisms, barrels, hexagons and nee-
dles which are always pigmented.
3. Urates are redissolved on warming the urine.
4. The cause of deposition of urates is the cooling of urine after it has been passed.
Feces:
Amount:
1. The quantity of feces varies from day to day and with the diet.
2. Vegetable food increases the bulk of feces but meat diet which is largely absorbed
diminishes the bulk.
3. An adult taking a mixed diet passes from 60 to 250 gm of moist feces containing 25 -
45 gm of solids per day.
4. The young children release high bulk.
Composition:
1. Normal adult feces have a water content of 65-80%.
2. One-third of the dry matter of feces is represented by bacteria. The rest are the
remaining’s of intestinal secretions, substances excreted by the large intestine (e.g., Ca,
Fe) and small amounts of food residues. The food residues are cellulose, fruit skins and
seeds.
3. The only substances in feces are fat, nitrogen and mineral elements.
Dry feces contain the following:
Colour:
1. The normal colour of feces is brown which is due to stercobilin (urobilin).
23 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Calcium Oxalate:
This is found in acid urine but may be found in alkaline urine. The crystals are of two
types—octahedra, dumb-bells. Calcium oxalate is insoluble in acetic acid.
Urates:
1. They are usually found in acid urines.
2. Uric acid separates into different forms including prisms, barrels, hexagons and nee-
dles which are always pigmented.
3. Urates are redissolved on warming the urine.
4. The cause of deposition of urates is the cooling of urine after it has been passed.
Feces:
Amount:
1. The quantity of feces varies from day to day and with the diet.
2. Vegetable food increases the bulk of feces but meat diet which is largely absorbed
diminishes the bulk.
3. An adult taking a mixed diet passes from 60 to 250 gm of moist feces containing 25 -
45 gm of solids per day.
4. The young children release high bulk.
Composition:
1. Normal adult feces have a water content of 65-80%.
2. One-third of the dry matter of feces is represented by bacteria. The rest are the
remaining’s of intestinal secretions, substances excreted by the large intestine (e.g., Ca,
Fe) and small amounts of food residues. The food residues are cellulose, fruit skins and
seeds.
3. The only substances in feces are fat, nitrogen and mineral elements.
Dry feces contain the following:
Colour:
1. The normal colour of feces is brown which is due to stercobilin (urobilin).
Excretory system notes 2017
24 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
2. Milk, cereals and meat, coffee, cocoa, black berries, etc. form darker stools.
3. Greenish colour due to excessive consumption of green vegetables.
4. The colour of the feces of newly -born infants is dark or blackish-green due to
biliverdin and porphyrin.
5. The stools of young infants on a milk diet is yellow due to bilirubin because of non-
development of intestinal bacterial flora.
6. The presence of blood in the stool gives the feces a red colour. Excessive hemolysis will
give a dark-brown feces.
7. Iron or bismuth gives a black stool.
Odour:
1. The normal odour is due to indole and skatole.
2. Mercaptans and H2S may produce odour.
3. A meat diet produces a more intense odour than a vegetable one and a milk diet least
of all.
pH:
1. Feces are normally slightly alkaline, pH 7.0-7.5.
2. They may be slightly acid with a large proportion of carbohydrate or fat.
Fat:
1. The total fat of feces is divided into two:
(a) Split fat (fatty acids).
(b) Unsplit fat (neutral fats, phospholipids, sterols, pigments).
2. The amount of fat is independent of the diet. The split fat diminishes in amount on
low-fat diets.
3. In diseases, fecal fat is increased when digestion or absorption of fat is impaired.
Nitrogen:
1. Fecal nitrogen is very little affected by the amount of protein ingested if the protein is
well masticated and well assimilated.
2. An adult of a mixed diet usually excretes about lgm of fecal nitrogen per day.
3. In diseases, fecal nitrogen is greatly increased by failure of digestion or absorption of
protein.
24 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
2. Milk, cereals and meat, coffee, cocoa, black berries, etc. form darker stools.
3. Greenish colour due to excessive consumption of green vegetables.
4. The colour of the feces of newly -born infants is dark or blackish-green due to
biliverdin and porphyrin.
5. The stools of young infants on a milk diet is yellow due to bilirubin because of non-
development of intestinal bacterial flora.
6. The presence of blood in the stool gives the feces a red colour. Excessive hemolysis will
give a dark-brown feces.
7. Iron or bismuth gives a black stool.
Odour:
1. The normal odour is due to indole and skatole.
2. Mercaptans and H2S may produce odour.
3. A meat diet produces a more intense odour than a vegetable one and a milk diet least
of all.
pH:
1. Feces are normally slightly alkaline, pH 7.0-7.5.
2. They may be slightly acid with a large proportion of carbohydrate or fat.
Fat:
1. The total fat of feces is divided into two:
(a) Split fat (fatty acids).
(b) Unsplit fat (neutral fats, phospholipids, sterols, pigments).
2. The amount of fat is independent of the diet. The split fat diminishes in amount on
low-fat diets.
3. In diseases, fecal fat is increased when digestion or absorption of fat is impaired.
Nitrogen:
1. Fecal nitrogen is very little affected by the amount of protein ingested if the protein is
well masticated and well assimilated.
2. An adult of a mixed diet usually excretes about lgm of fecal nitrogen per day.
3. In diseases, fecal nitrogen is greatly increased by failure of digestion or absorption of
protein.
Excretory system notes 2017
25 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Salts:
1. Moist feces contain about 2-3% of salts. Most abundant are calcium and phosphate.
2. Small amounts of magnesium, iron, sodium, potassium, chloride and sulphate are also
present in feces. The proportion of calcium is higher on a milk diet and that of
magnesium on a meat diet.
Sweat:
1. Sweat is a more dilute fluid and is always hypotonic.
2. The pH is about 4.5. But if the skin is washed and dried, sweat secreted is slightly
alkaline (pH 7.0-7.4).
3. It contains most of the diffusible constituents of plasma. The most abundant con -
stituent is NaCl.
4. The lactic acid content is more than normally found in blood.
Fluid and Electrolyte Balance
The kidneys are essential for regulating the volume and composition of bodily fluids. A most
critical concept is to understand is how water and sodium regulation are integrated to defend
the body against all possible disturbances in the volume and osmolarity of bodily fluids. Simple
examples of such disturbances include dehydration, blood loss, salt ingestion, and plain water
ingestion.
Water balance
Water balance is achieved in the body by ensuring that the amount of water consumed in
food and drink (and generated by metabolism) equals the amount of water excreted. The
consumption side is regulated by behavioral mechanisms, including thirst and salt cravings.
While almost a liter of water per day is lost through the skin, lungs, and feces, the kidneys are
the major site of regulated excretion of water.
One way the the kidneys can directly control the volume of bodily fluids is by the amount
of water excreted in the urine. Either the kidneys can conserve water by producing urine that is
concentrated relative to plasma, or they can rid the body of excess water by producing urine
that is dilute relative to plasma.
Direct control of water excretion in the kidneys is exercised by vasopressin, or anti -
diuretic hormone (ADH), a peptide hormone secre ted by the hypothalamus. ADH causes the
25 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Salts:
1. Moist feces contain about 2-3% of salts. Most abundant are calcium and phosphate.
2. Small amounts of magnesium, iron, sodium, potassium, chloride and sulphate are also
present in feces. The proportion of calcium is higher on a milk diet and that of
magnesium on a meat diet.
Sweat:
1. Sweat is a more dilute fluid and is always hypotonic.
2. The pH is about 4.5. But if the skin is washed and dried, sweat secreted is slightly
alkaline (pH 7.0-7.4).
3. It contains most of the diffusible constituents of plasma. The most abundant con -
stituent is NaCl.
4. The lactic acid content is more than normally found in blood.
Fluid and Electrolyte Balance
The kidneys are essential for regulating the volume and composition of bodily fluids. A most
critical concept is to understand is how water and sodium regulation are integrated to defend
the body against all possible disturbances in the volume and osmolarity of bodily fluids. Simple
examples of such disturbances include dehydration, blood loss, salt ingestion, and plain water
ingestion.
Water balance
Water balance is achieved in the body by ensuring that the amount of water consumed in
food and drink (and generated by metabolism) equals the amount of water excreted. The
consumption side is regulated by behavioral mechanisms, including thirst and salt cravings.
While almost a liter of water per day is lost through the skin, lungs, and feces, the kidneys are
the major site of regulated excretion of water.
One way the the kidneys can directly control the volume of bodily fluids is by the amount
of water excreted in the urine. Either the kidneys can conserve water by producing urine that is
concentrated relative to plasma, or they can rid the body of excess water by producing urine
that is dilute relative to plasma.
Direct control of water excretion in the kidneys is exercised by vasopressin, or anti -
diuretic hormone (ADH), a peptide hormone secre ted by the hypothalamus. ADH causes the
Excretory system notes 2017
26 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
insertion of water channels into the membranes of cells lining the collecting ducts, allowing
water reabsorption to occur. Without ADH, little water is reabsorbed in the collecting ducts and
dilute urine is excreted.
ADH secretion is influenced by several factors (note that anything that stimulates ADH
secretion also stimulates thirst):
1. By special receptors in the hypothalamus that are sensitive to increasing plasma
osmolarity (when the plasma gets too concentrated). These stimulate ADH secretion.
2. By stretch receptors in the atria of the heart, which are activated by a larger than
normal volume of blood returning to the heart from the veins. These inhibit ADH
secretion, because the body wants to rid itself of the excess fluid volume.
3. By stretch receptors in the aorta and carotid arteries, which are stimulated when
blood pressure falls. These stimulate ADH secretion, because the body wants to maintain
enough volume to generate the blood pressure necessary to deliver blood to the tissues.
Sodium balance
In addition to regulating total volume, the osmolarity (the amount of solute per unit
volume) of bodily fluids is also tightly regulated. Extreme variation in osmolarity causes cells to
shrink or swell, damaging or destroying cellular structure and disrupting normal cellular
function.
Regulation of osmolarity is achieved by balancing the intake and excretion of sodium with
that of water. (Sodium is by far the major solute in extracellular fluids, so it effectively
determines the osmolarity of extracellular fluids.)
An important concept is that regulation of osmolarity must be integrated with regulation
of volume, because changes in water volume alone have diluting or concentrating effects on the
bodily fluids. For example, when you become dehydrated you lose proportionately more water
than solute (sodium), so the osmolarity of your bodily fluids increases. In this situation the body
must conserve water but not sodium, thus stemming the rise in osmolarity. If you lose a large
amount of blood from trauma or surgery, however, your loses of sodium and water are
proportionate to the composition of bodily fluids. In this situation the body should conserve both
water and sodium.
26 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
insertion of water channels into the membranes of cells lining the collecting ducts, allowing
water reabsorption to occur. Without ADH, little water is reabsorbed in the collecting ducts and
dilute urine is excreted.
ADH secretion is influenced by several factors (note that anything that stimulates ADH
secretion also stimulates thirst):
1. By special receptors in the hypothalamus that are sensitive to increasing plasma
osmolarity (when the plasma gets too concentrated). These stimulate ADH secretion.
2. By stretch receptors in the atria of the heart, which are activated by a larger than
normal volume of blood returning to the heart from the veins. These inhibit ADH
secretion, because the body wants to rid itself of the excess fluid volume.
3. By stretch receptors in the aorta and carotid arteries, which are stimulated when
blood pressure falls. These stimulate ADH secretion, because the body wants to maintain
enough volume to generate the blood pressure necessary to deliver blood to the tissues.
Sodium balance
In addition to regulating total volume, the osmolarity (the amount of solute per unit
volume) of bodily fluids is also tightly regulated. Extreme variation in osmolarity causes cells to
shrink or swell, damaging or destroying cellular structure and disrupting normal cellular
function.
Regulation of osmolarity is achieved by balancing the intake and excretion of sodium with
that of water. (Sodium is by far the major solute in extracellular fluids, so it effectively
determines the osmolarity of extracellular fluids.)
An important concept is that regulation of osmolarity must be integrated with regulation
of volume, because changes in water volume alone have diluting or concentrating effects on the
bodily fluids. For example, when you become dehydrated you lose proportionately more water
than solute (sodium), so the osmolarity of your bodily fluids increases. In this situation the body
must conserve water but not sodium, thus stemming the rise in osmolarity. If you lose a large
amount of blood from trauma or surgery, however, your loses of sodium and water are
proportionate to the composition of bodily fluids. In this situation the body should conserve both
water and sodium.
Excretory system notes 2017
27 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
As noted above, ADH plays a role in lowering osmolarity (reducing sodium concentration)
by increasing water reabsorption in the kidneys, thus helping to dilute bodily fluids. To prevent
osmolarity from decreasing below normal, the kidneys also have a regulated mechanism for
reabsorbing sodium in the distal nephron. This mechanism is controlled by aldosterone, a steroid
hormone produced by the adrenal cortex. Aldosterone secretion is controlled two ways:
1.The adrenal cortex directly senses plasma osmolarity. When the osmolarity increases
above normal, aldosterone secretion is inhibited. The lack of aldosterone causes less
sodium to be reabsorbed in the distal tubule. Remember that in this setting ADH
secretion will increase to conserve water, thus complementing the effect of low
aldosterone levels to decrease the osmolarity of bodily fluids. The net effect on urine
excretion is a decrease in the amount of urine excreted, with an increase in the
osmolarity of the urine.
2. The kidneys sense low blood pressure (which results in lower filtration rates and lower
flow through the tubule). This triggers a complex response to raise blood pressure
and conserve volume. Specialized cells (juxtaglomerular cells) in the afferent and efferent
arterioles produce renin, a peptide hormone that initiates a hormonal cascade that
ultimately produces angiotensin II. Angiotensin II stimulates the adrenal cortex to produce
aldosterone.
*Note that in this setting, where the body is attempting to conserve volume, ADH
secretion is also stimulated and water reabsorption increases. Because aldosterone is also
acting to increase sodium reabsorption, the net effect is retention of fluid that is roughly
the same osmolarity as bodily fluids. The net effect on urine excretion is a decrease in the
amount of urine excreted, with lower osmolarity than in the previous example.
Summary
Many nitrogen containing substances, ions, CO2, water, etc., that accumulate in the body
have to be eliminated.
Nature of nitrogenous wastes formed and their excretion vary among animals, mainly
depending on the habitat (availability of water).
Ammonia, urea and uric acid are the major nitrogenous wastes excreted.
27 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
As noted above, ADH plays a role in lowering osmolarity (reducing sodium concentration)
by increasing water reabsorption in the kidneys, thus helping to dilute bodily fluids. To prevent
osmolarity from decreasing below normal, the kidneys also have a regulated mechanism for
reabsorbing sodium in the distal nephron. This mechanism is controlled by aldosterone, a steroid
hormone produced by the adrenal cortex. Aldosterone secretion is controlled two ways:
1.The adrenal cortex directly senses plasma osmolarity. When the osmolarity increases
above normal, aldosterone secretion is inhibited. The lack of aldosterone causes less
sodium to be reabsorbed in the distal tubule. Remember that in this setting ADH
secretion will increase to conserve water, thus complementing the effect of low
aldosterone levels to decrease the osmolarity of bodily fluids. The net effect on urine
excretion is a decrease in the amount of urine excreted, with an increase in the
osmolarity of the urine.
2. The kidneys sense low blood pressure (which results in lower filtration rates and lower
flow through the tubule). This triggers a complex response to raise blood pressure
and conserve volume. Specialized cells (juxtaglomerular cells) in the afferent and efferent
arterioles produce renin, a peptide hormone that initiates a hormonal cascade that
ultimately produces angiotensin II. Angiotensin II stimulates the adrenal cortex to produce
aldosterone.
*Note that in this setting, where the body is attempting to conserve volume, ADH
secretion is also stimulated and water reabsorption increases. Because aldosterone is also
acting to increase sodium reabsorption, the net effect is retention of fluid that is roughly
the same osmolarity as bodily fluids. The net effect on urine excretion is a decrease in the
amount of urine excreted, with lower osmolarity than in the previous example.
Summary
Many nitrogen containing substances, ions, CO2, water, etc., that accumulate in the body
have to be eliminated.
Nature of nitrogenous wastes formed and their excretion vary among animals, mainly
depending on the habitat (availability of water).
Ammonia, urea and uric acid are the major nitrogenous wastes excreted.
Excretory system notes 2017
28 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Protonephridia, nephridia, malpighian tubules, green glands and the kidneys are the
common excretory organs in animals. They not only eliminate nitrogeno us wastes but also
help in the maintenance of ionic and acid-base balance of body fluids.
In humans, the excretory system consists of one pair of kidneys, a pair of ureters, a
urinary bladder and a urethra.
Each kidney has over a million tubular structures called nephrons. Nephron is the
functional unit of kidney and has two portions – glomerulus and renal tubule.
Glomerulus is a tuft of capillaries formed from afferent arterioles, fine branches of renal
artery.
The renal tubule starts with a double walled Bowman’s capsule and is further
differentiated into a proximal convoluted tubule (PCT), Henle’s loop (HL) and distal
convoluted tubule (DCT).
The DCTs of many nephrons join to a common collecting duct many of which ultimately
open into the renal pelvis through the medullary pyramids. The Bowman’s capsule
encloses the glomerulus to form Malpighian or renal corpuscle.
Urine formation involves three main processes, i.e., filtration, reabsorption andsecretion.
Filtration is a non-selective process performed by the glomerulus using the glomerular
capillary blood pressure. About 1200 ml of blood is filtered by the glomerulus per minute
to form 125 ml of filtrate in the Bowman’s capsule per minute (GFR).
JGA, a specialised portion of the nephrons, plays a significant role in the regulation of
GFR.
Nearly 99 per cent reabsorption of the filtrate takes place through different parts of the
nephrons.
PCT is the major site of reabsorption and selective secretion. HL [Henle’s Loop] primarily
helps to maintain osmolar gradient within the kidney interstitium.
DCT and collecting duct allow extensive reabsorption of water and certain electrolytes,
which help in osmoregulation: H+, K+ and NH3 could be secreted into the filtrate by the
tubules to maintain the ionic balance and pH of body fluids.
A counter current mechanism operates between the two limbs of the loop of Henle and
those of vasa recta (capillary parallel to Henle’s loop). The filtrate gets concentrated as it
28 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
Protonephridia, nephridia, malpighian tubules, green glands and the kidneys are the
common excretory organs in animals. They not only eliminate nitrogeno us wastes but also
help in the maintenance of ionic and acid-base balance of body fluids.
In humans, the excretory system consists of one pair of kidneys, a pair of ureters, a
urinary bladder and a urethra.
Each kidney has over a million tubular structures called nephrons. Nephron is the
functional unit of kidney and has two portions – glomerulus and renal tubule.
Glomerulus is a tuft of capillaries formed from afferent arterioles, fine branches of renal
artery.
The renal tubule starts with a double walled Bowman’s capsule and is further
differentiated into a proximal convoluted tubule (PCT), Henle’s loop (HL) and distal
convoluted tubule (DCT).
The DCTs of many nephrons join to a common collecting duct many of which ultimately
open into the renal pelvis through the medullary pyramids. The Bowman’s capsule
encloses the glomerulus to form Malpighian or renal corpuscle.
Urine formation involves three main processes, i.e., filtration, reabsorption andsecretion.
Filtration is a non-selective process performed by the glomerulus using the glomerular
capillary blood pressure. About 1200 ml of blood is filtered by the glomerulus per minute
to form 125 ml of filtrate in the Bowman’s capsule per minute (GFR).
JGA, a specialised portion of the nephrons, plays a significant role in the regulation of
GFR.
Nearly 99 per cent reabsorption of the filtrate takes place through different parts of the
nephrons.
PCT is the major site of reabsorption and selective secretion. HL [Henle’s Loop] primarily
helps to maintain osmolar gradient within the kidney interstitium.
DCT and collecting duct allow extensive reabsorption of water and certain electrolytes,
which help in osmoregulation: H+, K+ and NH3 could be secreted into the filtrate by the
tubules to maintain the ionic balance and pH of body fluids.
A counter current mechanism operates between the two limbs of the loop of Henle and
those of vasa recta (capillary parallel to Henle’s loop). The filtrate gets concentrated as it
Excretory system notes 2017
29 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
moves down the descending limb but is diluted by the ascendi ng limb. Electrolytes and
urea are retained in the interstitium by this arrangement.
DCT and collecting duct concentrate the filtrate about four times, an excellent
mechanism of conservation of water.
Urine is stored in the urinary bladder till a voluntary signal from CNS carries out its
release through urethra, i.e., micturition. Skin, lungs and liver also assist in excretion.
29 By, K. P. Komal, Asst. Professor of Biochemistry, chitradurga.
moves down the descending limb but is diluted by the ascendi ng limb. Electrolytes and
urea are retained in the interstitium by this arrangement.
DCT and collecting duct concentrate the filtrate about four times, an excellent
mechanism of conservation of water.
Urine is stored in the urinary bladder till a voluntary signal from CNS carries out its
release through urethra, i.e., micturition. Skin, lungs and liver also assist in excretion.
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