Nephron structure & Urine formation

cross section of nephron and its function
urine formation mechamism

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ASSIGNMENT

ROLL NO : 1610818
NAME : H.KALIMANI
CLASS : 3
rd
B.Sc., ZOOLOGY
SUBJECT : physiology
TOPIC : nephron structure & urine formation
SUBMITTED TO: Dr. mohaideen batcha m.sc., m.phill., p.hd
DATE : 01.02.2019
RE FERENCE : Edited and Revised by Lindsay M. Biga, Sierra Dawson,
Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern,
Katie Morrison-Graham, Devon Quick, Jon Runyeon Art edited and created by
Leeah Whittier (OPEN TEXTBOOK SITE, ANATOMY& PHYSIOLOGY)

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Nephron – Structural anatomy


Histologically, each kidney is composed of approximately 1 million of Uniferous
tubules or nephron. Nephron is the structural and functional unit of the kidney.

Each nepron has two major portions:

1. A Renal corpuscle (Malpighian body)
2. A Renal tubule

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1. Renal corpuscles (Malpighian body):
A renal corpuscle consists of a glomerulus surrounded by a glomerular capsule
(Bowman’s capsule).
The glomerulus is a capillary network that arises from an afferent arteriole and
empties into an efferent arteriole. The diameter of the efferent arteriole is smaller
than that of the afferent arteriole, which helps maintain a fairly high blood pressure
in the glomerulus.
Bowman’s capsule is double walled cup like structure and it encloses the
glomerulus. The wall of glomerulus and the Bowman’s capsule consists of a single
layer of flattened epithelial cells.

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Glomerular capsule consists of three layers
i) Outer parietal layer consists of squamous epithelium cells with minute
pore of 12nm diameter called fenestrations.
ii) Middle basement membrane which is selectively permeable.
iii) The inner visceral layer of large nucleated cell called podocytes.
Podocytes bears finger like projections known as podocels. The areas
between the two podocels is filtration slit underlying basement membrane.

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2. Renal tubules:
The renal tubules continues from Bowman’s capsule and consists of the following
parts:

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Proximal convoluted tubule (in the real cortex), loop of Henle (in the renal
medulla), and distal convoluted tubule (in the renal cotex).
i) Proximal convoluted tubules (PCT): It is proximal part of renal tubules
next to Bowman’s capsule. It is lined with microvilli. Maximum
reabsorption of water, glucose, amino acids and electrolytes takes place here.
ii) Loop of Henle: It is U shaped middle portion of renal tubules. It is
composed of ascending and descending loop. Ascending loop is thick walled
and impermeable to water while descending loop is thin walled and
permeable to water. Counter current mechanism is crucial role of loop of
Henle.
iii) Distal convoluted tubules (DCT): It is the distal part of renal tubules that
leads to collecting ducts. It is similar in structure and function with PCT.
iv) Collecting tubules: It is not apart of nephron rather it is a part of kidney.
The distal convoluted tubules from several nephrons empty into a collecting
tubule. Several collecting tubules then unite into to form a papillary duct
that empties urine into a minor calyx and then into major calyx and finally
into renal pelvis.


Types of nephron
1. Cortical nephron: 80% of the nephrons are short and located within the
cortex.
2. Juxtamedulary nephron: 20% of nephron have long loops of Henle that
extend into the medulla.

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Urine Formation

Different parts of the nephron utilize three specific processes to produce urine:
filtration, reabsorption, and secretion. You will learn how each of these processes
works and where they occur along the nephron and collecting ducts. The
physiologic goal is to modify the composition of the plasma and, in doing so,
produce the waste product urine.

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Glomerular Filtration
Glomerular filtration occurs as blood passes into the glomerulus producing a
plasma-like filtrate (minus proteins) that gets captured by the Bowman’s
(glomerular) capsule and funneled into the renal tubule. This filtrate produced then
becomes highly modified along its route through the nephron by the following
processes, finally producing urine at the end of the collecting duct.
Tubular Reabsorption
As the filtrate travels along the length of the nephron, the cells lining the tubule
selectively, and often actively, take substances from the filtrate and move them out
of the tubule into the blood. Recall that the glomerulus is simply a filter and
anything suspended in the plasma that can fit through the holes in the filtration
membrane can end up in the filtrate. This includes very physiologically important
molecules such as water, sodium, chloride, and bicarbonate (along with many
others) as well as molecules that the digestive system used a lot of energy to
absorb, such as glucose and amino acids. These molecules would be lost in the
urine if not reclaimed by the tubule cells. These cells are so efficient that they can
reclaim all of the glucose and amino acids and up to 99% of the water and

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important ions lost due to glomerular filtration. The filtrate that is not reasbsorbed
becomes urine at the base of the collecting duct.
Tubular Secretion
Tubular secretion occurs mostly in the PCT and DCT where unfiltered substances
are moved from the peritubular capillary into the lumen of the tubule. Secretion
usually removes substances from the blood that are too large to be filtered (ex:
antibiotics, toxins) or those that are in excess in the blood (ex: H+, K+). These
substances secreted into the tubule are destined to leave the body as components of
urine.

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Substances Secreted or Reabsorbed in the Nephron and Their Locations (Table 5)
Substance PCT Loop of Henle DCT
Collecting
ducts
Glucose
Almost 100
percent
reabsorbed;
secondary
active
transport with
Na
+

Oligopeptides,
proteins,
amino acids
Almost 100
percent
reabsorbed;
symport with
Na
+

Vitamins Reabsorbed

Lactate Reabsorbed

Creatinine Secreted

Urea
50 percent
reabsorbed by
diffusion; also
secreted
Secretion,
diffusion in
descending
limb

Sodium
65 percent
actively
reabsorbed
25 percent
reabsorbed in
thick
ascending
limb; active
transport
5 percent
reabsorbed;
active
5 percent
reabsorbed,
stimulated by
aldosterone;
active
Chloride
Reabsorbed,
symport with
Na
+, diffusion
Reabsorbed in
thin and thick
ascending
limb;
diffusion in
ascending
limb
Reabsorbed;
diffusion
Reabsorbed;
symport
Water
67 percent
reabsorbed
osmotically
with solutes
15 percent
reabsorbed in
descending
limb; osmosis
8 percent
reabsorbed if
ADH;
osmosis
Variable
amounts
reabsorbed,
controlled by
ADH,
osmosis

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Substances Secreted or Reabsorbed in the Nephron and Their Locations (Table 5)
Substance PCT Loop of Henle DCT
Collecting
ducts
Bicarbonate
80–90
percent
symport
reabsorption
with Na
+
Reabsorbed,
symport with
Na
+ and
antiport with
Cl
–; in
ascending
limb

H
+
Secreted;
diffusion

NH4
+
Secreted;
diffusion

HCO3
–
Reabsorbed;
diffusion
Reabsorbed;
diffusion in
ascending
limb
Reabsorbed;
diffusion
Reabsorbed;
antiport with
Na
+
Some drugs Secreted

Potassium
65 percent
reabsorbed;
diffusion
20 percent
reabsorbed in
thick
ascending
limb; symport
Secreted;
active
Secretion
controlled by
aldosterone;
active
Calcium
Reabsorbed;
diffusion
Reabsorbed in
thick
ascending
limb;
diffusion

Magnesium
Reabsorbed;
diffusion
Reabsorbed in
thick
ascending
limb;
diffusion
Reabsorbed

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Substances Secreted or Reabsorbed in the Nephron and Their Locations (Table 5)
Substance PCT Loop of Henle DCT
Collecting
ducts
Phosphate
85 percent
reabsorbed,
inhibited by
parathyroid
hormone,
diffusion