13_Fluid_and_Electrolyte_Balance copy copy.pdf

Lecture about fluid balance and electrolytes

1. © 2011 Pearson Education, Inc.
•Fluid, Electrolyte, and Acid-
Base Balance

2. © 2011 Pearson Education, Inc.
Section 1: Fluid and Electrolyte Balance
•Fluids constitute ~50%–60% of total body
composition
•Minerals (inorganic substances) are dissolved
within and form ions called electrolytes
•Fluid compartments
•Intracellular fluid (ICF)
•Water content varies most here due to variation in:
•Tissue types (muscle vs. fat)
•Distinct from ECF due to plasma membrane transport
•Extracellular fluid (ECF)
•Interstitial fluid volume varies
•Volume of blood (women < men)

3. © 2011 Pearson Education, Inc.
Figure 24 Section 1 1
Total body composition of adult males
Total body composition of adult males and females
Total body composition of adult females
Intracellular
fluid 33%
Interstitial
fluid 21.5%
Plasma 4.5%
Solids 40%
(organic and inorganic materials)
Other
body
fluids
(≤1%)
Adult males
ICF
ECF
Other
body
fluids
(≤1%)
Interstitial
fluid 18%
Intracellular
fluid 27%
Plasma 4.5%
Solids 50%
(organic and inorganic materials)
ICF ECF
Adult females

4. © 2011 Pearson Education, Inc.
Module 24.1: Fluid balance
•Fluid balance
•Water content stable over time
•Gains
•Primarily absorption along digestive tract
•As nutrients and ions are absorbed, osmotic gradient created
causing passive absorption of water
•Losses
•Mainly through urination (over 50%) but other routes as well
•Digestive secretions are reabsorbed similarly to ingested
fluids

5. © 2011 Pearson Education, Inc.
Figure 24.1 1

6. © 2011 Pearson Education, Inc.
Figure 24.1 2
Dietary Input Digestive Secretions
Water Reabsorption
Food and drink 2200 mL
The digestive tract sites of water gain
through ingestion or secretion, or water
reabsorption, and of water loss
Small intestine
reabsorbs 8000 mL
Colon reabsorbs 1250 mL
150 mL lost
in feces
1400
mL
1200 mL
9200 mL
5200 mL
Colonic mucous secretions
200 mL
Intestinal secretions 2000 mL
Liver (bile) 1000 mL
Pancreas (pancreatic
juice) 1000 mL
Gastric secretions 1500 mL
Saliva 1500 mL

7. © 2011 Pearson Education, Inc.
Module 24.1: Fluid balance
•ICF and ECF compartments balance
•Very different composition
•Are at osmotic equilibrium
•Loss of water from ECF is replaced by water in ICF
•= Fluid shift
•Occurs in minutes to hours and restores osmotic equilibrium
•Dehydration
•Results in long-term transfer that cannot replace ECF water
loss
•Homeostatic mechanisms to increase ECF fluid volume will
be employed

8. © 2011 Pearson Education, Inc.
Figure 24.1 3
The major factors that affect ECF volume
ICF ECF
Water absorbed across
digestive epithelium
(2000 mL)
Metabolic
water
(300 mL)
Water vapor lost
in respiration and
evaporation from
moist surfaces
(1150 mL)
Water lost in
feces (150 mL)
Water secreted
by sweat glands
(variable)
Water lost in urine
(1000 mL)
Plasma membranes

9. © 2011 Pearson Education, Inc.
Figure 24.1 4
Changes to the ICF and ECF when water losses outpace water gains
Intracellular
fluid (ICF)
Extracellular
fluid (ECF)
The ECF and ICF are in
balance, with the two
solutions isotonic.
ECF water loss Water loss from ECF
reduces volume and
makes this solution
hypertonic with respect
to the ICF.
Increased
ECF volume
Decreased ICF volume
An osmotic water shift
from the ICF into the
ECF restores osmotic
equilibrium but
reduces the ICF
volume.

10. © 2011 Pearson Education, Inc.
Figure 24.2 1
Mineral balance, the balance between ion absorption (in the digestive tract) and ion excretion (primarily at the kidneys)
Ion Absorption Ion Excretion
ICFECF
Ion absorption occurs across the
epithelial lining of the small intestine
and colon.
Ion reserves (primarily
in the skeleton)
Ion pool in body fluids
Sweat gland
secretions
(secondary
site of ion loss)
Kidneys
(primary site
of ion loss)

11. © 2011 Pearson Education, Inc.
Module 24.3: Water and sodium balance
•Sodium balance(when sodium gains equal
losses)
•Relatively small changes in Na
+
are accommodated
by changes in ECF volume
•Homeostatic responses involve two parts
1.ADH control of water loss/retention by kidneys and thirst
2.Fluid exchange between ECF and ICF

12. © 2011 Pearson Education, Inc.
Module 24.3: Water and sodium balance
•Sodium balance(continued)
•Exchange changes in Na
+
are accommodated by
changes in blood pressure and volume
•Hyponatremia (natrium, sodium)
•Low ECF Na
+
concentration (<136 mEq/L)
•Can occur from overhydration or inadequate salt intake
•Hypernatremia
•High ECF Na
+
concentration (>145 mEq/L)
•Commonly from dehydration

13. © 2011 Pearson Education, Inc.
Module 24.3: Water and sodium balance
•Sodium balance(continued)
•Exchange changes in Na
+
are accommodated by
changes in blood pressure and volume (continued)
•Increased blood volume and pressure
•Natriuretic peptides released
•Increased Na
+
and water loss in urine
•Reduced thirst
•Inhibition of ADH, aldosterone, epinephrine, and
norepinephrine release
•Decreased blood volume and pressure
•Endocrine response
•Increased ADH, aldosterone, RAAS mechanism
•Opposite bodily responses to above

14. © 2011 Pearson Education, Inc.
Figure 24.3 2
Rising blood
pressure and
volume
HOMEOSTASIS
Normal ECF
volume
HOMEOSTASIS
RESTORED
Falling ECF
volume
HOMEOSTASIS
DISTURBED
Rising ECF volume by fluid
gain or fluid and Na

gain
Combined
Effects
Responses to Natriuretic Peptides
Increased blood
volume and
atrial distension
Natriuretic peptides
released by cardiac
muscle cells
The mechanisms that regulate water balance
when ECF volume changes
Increased Na

loss in urine
Increased water loss in urine
Reduced thirst
Inhibition of ADH, aldosterone,
epinephrine, and norepinephrine
release
Reduced
blood
volume
Reduced
blood
pressure
Start

15. © 2011 Pearson Education, Inc.
Figure 24.3 2
Falling blood
pressure and
volume
HOMEOSTASIS
Normal ECF
volume
Endocrine Responses
Increased renin secretion
and angiotensin II
activation
Combined Effects
Increased aldosterone
release
Increased ADH release
Increased urinary Na

retention
Decreased urinary water loss
Increased thirst
Increased water intake
Decreased blood
volume and
blood pressure
HOMEOSTASIS
DISTURBED
Falling ECF volume by fluid
loss or fluid and Na

loss
HOMEOSTASIS
RESTORED
Rising ECF
volume
Start
The mechanisms that regulate water balance
when ECF volume changes

16. © 2011 Pearson Education, Inc.
CLINICAL MODULE24.4: Potassium
imbalance
•Potassium balance (K
+
gain = loss)
•Major gain is through digestive tract absorption
•~100 mEq (1.9–5.8 g)/day
•Major loss is excretion by kidneys
•Primary ECF potassium regulation by kidneys since intake fairly
constant
•Controlled by aldosterone regulating Na
+
/K
+
exchange pumps in
DCT and collecting duct of nephron
•Low ECF pH can cause H
+
to be substituted for K
+
•Potassium is highest in ICF due to Na
+
/K
+
exchange pump
•~135 mEq/L in ICF vs. ~5 mEq/L in ECF

17. © 2011 Pearson Education, Inc.
Figure 24.4 1
The major factors involved in potassium balance
Factors Controlling Potassium Balance
Approximately 100
mEq (1.9–5.8 g) of
potassium ions are
absorbed by the
digestive tract each
day.
Roughly
98 percent of the
potassium
content of the
human body is in
the ICF, rather
than the ECF.
The K

concentration in the
ECF is relatively low. The rate
of K

entry from the ICF
through leak channels is
balanced by the rate of K

recovery by the Na

/K

exchange pump.
When potassium
balance exists,
the rate of urinary
K

excretion
matches the rate
of digestive tract
absorption.
The potassium ion
concentration in the
ECF is approximately
5 mEq/L.
KEY
Absorption
Secretion
Diffusion through
leak channels
The potassium ion
concentration of the
ICF is approximately
135 mEq/L.
Renal K

losses
are approximately
100 mEq per day

18. © 2011 Pearson Education, Inc.
Figure 24.4 2
The role of aldosterone-sensitive exchange pumps
in the kidneys in determining the potassium
concentration in the ECF
The primary mechanism of
potassium secretion involves
an exchange pump that
ejects potassium ions while
reabsorbing sodium ions.
Tubular
fluid
Sodium-potassium
exchange pump
Aldosterone-
sensitive
exchange
pump
The sodium ions are then pumped out
of the cell in exchange for potassium
ions in the ECF. This is the same pump
that ejects sodium ions entering the
cytosol through leak channels.
KEY
ECF

19. © 2011 Pearson Education, Inc.
Figure 24.4 3
Events in the kidneys that affect potassium balance
Under normal conditions, the
aldosterone-sensitive pumps
exchange K

in the ECF for
Na

in the tubular fluid. The
net result is a rise in plasma
sodium levels and increased
K

loss in the urine.
When the pH falls in the ECF
and the concentration of H

is
relatively high, the exchange
pumps bind H

instead of K

.
This helps to stabilize the pH
of the ECF, but at the cost of
rising K

levels in the ECF.
Distal
convoluted
tubule
Collecting
duct

20. © 2011 Pearson Education, Inc.
•Disturbances of potassium balance
•Hypokalemia (kalium, potassium)
•Below 2 mEq/L in plasma
•Can be caused by:
•Diuretics
•Aldosteronism (excessive aldosterone secretion)
•Symptoms
•Muscular weakness, followed by paralysis
•Potentially lethal when affecting heart
CLINICAL MODULE24.4: Potassium
imbalance

21. © 2011 Pearson Education, Inc.
•Disturbances of potassium balance (continued)
•Hyperkalemia
•Above 8 mEq/L in plasma
•Can be caused by:
•Chronically low pH
•Kidney failure
•Drugs promoting diuresis by blocking Na
+
/K
+
pumps
•Symptoms
•Muscular spasm including heart arrhythmias
CLINICAL MODULE24.4: Potassium
imbalance

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•Fluid, Electrolyte, and Acid-
Base Balance

31. © 2011 Pearson Education, Inc.
Section 2: Acid-Base Balance
•Acid-base balance (H
+
production = loss)
•Normal plasma pH: 7.35–7.45
•H
+
gains: many metabolic activities produce
acids
•CO
2(to carbonic acid) from aerobic respiration
•Lactic acid from glycolysis
•H
+
losses and storage
•Respiratory system eliminates CO
2
•H
+
excretion from kidneys
•Buffers temporarily store H
+

32. © 2011 Pearson Education, Inc.
Figure 24 Section 2 1
The major factors involved in the maintenance
of acid-base balance
Active tissues
continuously generate
carbon dioxide, which in
solution forms carbonic
acid. Additional acids,
such as lactic acid, are
produced in the course of
normal metabolic
operations.
Tissue cells
Buffer Systems
Normal
plasma pH
(7.35–7.45)
Buffer systems can
temporarily store H

and thereby provide
short-term pH
stability.
The respiratory system
plays a key role by
eliminating
carbon dioxide.
The kidneys play a major
role by secreting
hydrogen ions into the
urine and generating
buffers that enter the
bloodstream. The rate of
excretion rises and falls
as needed to maintain
normal plasma pH. As a
result, the normal pH of
urine varies widely but
averages 6.0—slightly
acidic.

33. © 2011 Pearson Education, Inc.
Section 2: Acid-Base Balance
•Classes of acids
•Fixed acids
•Do not leave solution
•Remain in body fluids until kidney excretion
•Examples: sulfuric and phosphoric acid
•Generated during catabolism of amino acids, phospholipids,
and nucleic acids
•Organic acids
•Part of cellular metabolism
•Examples: lactic acid and ketones
•Most metabolized rapidly so no accumulation

34. © 2011 Pearson Education, Inc.
Section 2: Acid-Base Balance
•Classes of acids (continued)
•Volatile acids
•Can leave body by external respiration
•Example: carbonic acid (H
2CO
3)

35. © 2011 Pearson Education, Inc.
Module 24.5: Buffer systems
•pH imbalance
•ECH pH normally between 7.35 and 7.45
•Acidemia (plasma pH <7.35): acidosis (physiological
state)
•More common due to acid-producing metabolic activities
•Effects
•CNS function deteriorates, may cause coma
•Cardiac contractions grow weak and irregular
•Peripheral vasodilation causes BP drop
•Alkalemia (plasma pH >7.45): alkalosis (physiological
state)
•Can be dangerous but relatively rare

36. © 2011 Pearson Education, Inc.
Figure 24.5 1

37. © 2011 Pearson Education, Inc.
Figure 24.5 2
The narrow range of normal pH of the ECF, and the conditions that result from pH shifts outside the normal range
The pH of the ECF
(extracellular fluid)
normally ranges from
7.35 to 7.45.
pH
When the pH of plasma falls below
7.5, acidemia exists. The
physiological state that results is
called acidosis.
When the pH of plasma rises
above 7.45, alkalemia exists.
The physiological state that
results is called alkalosis.
Severe acidosis (pH below 7.0) can be deadly
because (1) central nervous system function
deteriorates, and the individual may become
comatose; (2) cardiac contractions grow weak and
irregular, and signs and symptoms of heart failure
may develop; and (3) peripheral vasodilation
produces a dramatic drop in blood pressure,
potentially producing circulatory collapse.
Severe alkalosis is also
dangerous, but serious cases
are relatively rare.
Extremely
acidic
Extremely
basic

38. © 2011 Pearson Education, Inc.
Module 24.5: Buffer systems
•CO
2partial pressure effects on pH
•Most important factor affecting body pH
•H
2O + CO
2H
2CO
3H
+
+ HCO
3
–
•Reversible reaction that can buffer body pH
•Adjustments in respiratory rate can affect body pH

39. © 2011 Pearson Education, Inc.
Figure 24.5 3
When carbon dioxide levels rise, more carbonic acid
forms, additional hydrogen ions and bicarbonate ions
are released, and the pH goes down.
When the P
CO
2
falls, the reaction runs in reverse, and
carbonic acid dissociates into carbon dioxide and
water. This removes H

ions from solution and
increases the pH.
If P
CO
2
rises If P
CO
2
falls
P
CO
2
40–45
mm Hg
pH
7.35–7.45
The inverse relationship between the P
CO
2
and pH
HOMEOSTASIS
H
2O CO
2
H
2CO
3 H

HCO
3

H

HCO
3
 H
2CO
3 H
2O CO
2

40. © 2011 Pearson Education, Inc.
Module 24.5: Buffer systems
•Buffer
•Substance that opposes changes to pH by removing
or adding H
+
•Generally consists of:
•Weak acid (HY)
•Anion released by its dissociation (Y
–
)
•HY H
+
+ Y
–
and H
+
+ Y
–
HY

41. © 2011 Pearson Education, Inc.
Module 24.6: Major body buffer systems
•Three major body buffer systems
•All can only temporarily affect pH (H
+
not eliminated)
1.Phosphate buffer system
•Buffers pH of ICF and urine
2.Carbonic acid–bicarbonate buffer system
•Most important in ECF
•Fully reversible
•Bicarbonate reserves (from NaHCO
3in ECF) contribute

42. © 2011 Pearson Education, Inc.
Module 24.6: Major body buffer systems
•Three major body buffer systems (continued)
3.Protein buffer systems (in ICF and ECF)
•Usually operate under acid conditions (bind H
+
)
•Binding to carboxyl group (COOH
–
) and amino group
(—NH
2)
•Examples:
•Hemoglobin buffer system
•CO
2+ H
2O H
2CO
3HCO
3
–
+ Hb-H
+
•Only intracellular system with immediate effects
•Amino acid buffers (all proteins)
•Plasma proteins

43. © 2011 Pearson Education, Inc.
Figure 24.6 1
The body’s three major buffer systems
Buffer Systems
Intracellular fluid (ICF) Extracellular fluid (ECF)
occur in
Phosphate Buffer
System
Protein Buffer Systems Carbonic Acid–
Bicarbonate Buffer
System
Has an important
role in buffering the
pH of the ICF and
of urine
Contribute to the regulation of pH in the ECF and ICF;
interact extensively with the other two buffer systems
Is most important in the
ECF
Hemoglobin
buffer system
(RBCs only)
Amino acid
buffers
(All proteins)
Plasma
protein
buffers

44. © 2011 Pearson Education, Inc.
Module 24.6: Major body buffer systems
•Disorders
•Metabolic acid-base disorders
•Production or loss of excessive amounts of fixed or
organic acids
•Carbonic acid–bicarbonate system works to counter
•Respiratory acid-base disorders
•Imbalance of CO
2generation and elimination
•Must be corrected by depth and rate of respiration
changes

45. © 2011 Pearson Education, Inc.
CLINICAL MODULE24.8: Respiratory
acid-base disorders
•Respiratory acid-base disorders
•Respiratory acidosis
•CO
2generation outpaces rate of CO
2elimination at lungs
•Shifts bicarbonate buffer system toward generating more
carbonic acid
•H
2O + CO
2H
2CO
3H
+
+ HCO
3
–
•HCO
3
–
goes into bicarbonate reserve
•H
+
must be neutralized by any of the buffer systems
•Respiratory (increased respiratory rate)
•Renal (H
+
secreted and HCO
3
–
reabsorbed)
•Proteins (bind free H
+
)

46. © 2011 Pearson Education, Inc.
CLINICAL MODULE24.8: Respiratory
acid-base disorders
•Respiratory alkalosis
•CO
2elimination at lungs outpaces CO
2generation rate
•Shifts bicarbonate buffer system toward generating more
carbonic acid
•H
+
+ HCO
3
–
H
2CO
3H
2O + CO
2
•H
+
removed as CO
2 exhaled and water formed
•Buffer system responses
•Respiratory (decreased respiratory rate)
•Renal (HCO
3
–
secreted and H
+
reabsorbed)
•Proteins (release free H
+
)

47. © 2011 Pearson Education, Inc.
Module 24.7: Metabolic acid-base disorders
•Metabolic acid-base disorders
•Metabolic acidosis
•Develops when large numbers of H
+
are released by organic
or fixed acids
•Accommodated by respiratory and renal responses
•Respiratory response
•Increased respiratory rate lowers P
CO2
•H
+
+ HCO
3
–
H
2CO
3H
2O + CO
2
•Renal response
•Occurs in PCT, DCT, and collecting system
•H
2O + CO
2H
2CO
3H
+
+ HCO
3
–
H
+
secreted into urine
HCO
3
–
reabsorbed into ECF

48. © 2011 Pearson Education, Inc.
Figure 24.7 1
The responses to metabolic acidosis Addition
of H

Start
CO
2
CO
2H
2O H
2CO
3
(carbonic acid)
H

HCO
3

Lungs
(bicarbonate ion)
HCO
3

Na

NaHCO
3
(sodium bicarbonate)
Generation
of HCO
3

CARBONIC ACID–BICARBONATE BUFFER SYSTEM BICARBONATE RESERVE
Respiratory Response
to Acidosis
Renal Response to Acidosis
Other
buffer
systems
absorb H

KIDNEYS
Secretion
of H

Increased respiratory
rate lowers P
CO
2
,
effectively converting
carbonic acid molecules
to water.
Kidney tubules respond by (1) secreting H

ions, (2) removing CO
2, and (3) reabsorbing
HCO
3

to help replenish the bicarbonate
reserve.

49. © 2011 Pearson Education, Inc.
Figure 24.7 2
The activity of renal
tubule cells in CO
2
removal and HCO
3

production
Tubular
fluid
Renal tubule cells ECF
H

H

H

H

Na

Na

CO
2 CO
2
HCO
3

HCO
3

H
2CO
3

HCO
3

CO
2

H
2O
Cl

Cl

Carbonic
anhydrase
CO
2generated by the tubule
cell is added to the CO
2
diffusing into the cell from
the urine and from the ECF.
Steps in CO
2removal and
HCO
3

production
Carbonic anhydrase
converts CO
2 and water to
carbonic acid, which then
dissociates.
The chloride ions exchanged
for bicarbonate ions are
excreted in the tubular fluid.
Bicarbonate ions and
sodium ions are transported
into the ECF, adding to the
bicarbonate reserve.

50. © 2011 Pearson Education, Inc.

51. © 2011 Pearson Education, Inc.
Module 24.7: Metabolic acid-base disorders
•Metabolic alkalosis
•Develops when large numbers of H
+
are removed
from body fluids
•Rate of kidney H
+
secretion declines
•Tubular cells do not reclaim bicarbonate
•Collecting system transports bicarbonate into urine and
retains acid (HCl) in ECF

52. © 2011 Pearson Education, Inc.
Module 24.7: Metabolic acid-base disorders
•Metabolic alkalosis (continued)
•Accommodated by respiratory and renal
responses
•Respiratory response
•Decreased respiratory rate raises P
CO2
•H
2O + CO
2H
2CO
3H
+
+ HCO
3
–
•Renal response
•Occurs in PCT, DCT, and collecting system
•H
2O + CO
2H
2CO
3H
+
+ HCO
3
–
•HCO
3
–
secreted into urine (in exchange for Cl
–
)
•H
+
actively reabsorbed into ECF

53. © 2011 Pearson Education, Inc.
Figure 24.7 3
The responses to metabolic alkalosis
Start
Lungs
Removal
of H

CO
2H
2O H

HCO
3
H
2CO
3
(carbonic acid)
HCO
3

Na

NaHCO
3
(sodium bicarbonate)(bicarbonate ion)
CARBONIC ACID–BICARBONATE BUFFER SYSTEM BICARBONATE RESERVE
Generation
of H
 KIDNEYS
Secretion
of HCO
3

Other
buffer
systems
release H

Respiratory Response
to Alkalosis
Renal Response to Alkalosis
Decreased respiratory
rate elevates P
CO
2
,
effectively converting
CO
2molecules to
carbonic acid.
Kidney tubules respond by
conserving H

ions and
secreting HCO
3

.

54. © 2011 Pearson Education, Inc.
Figure 24.7 4
The events in the
secretion of bicarbonate
ions into the tubular
fluid along the PCT, DCT,
and collecting system
Tubular
fluid
Renal tubule cells ECF
H
2CO
3

CO
2

H
2O
Carbonic
anhydrase
H

CO
2
HCO
3
 H
HCO
3

CO
2
Cl
 Cl

CO
2generated by the tubule
cell is added to the CO
2
diffusing into the cell from the
tubular fluid and from the ECF.
Carbonic anyhydrase converts
CO
2and water to carbonic
acid, which then dissociates.
The hydrogen ions are actively
transported into the ECF,
accompanied by the diffusion
of chloride ions.
HCO
3

is pumped into the
tubular fluid in exchange for
chloride ions that will diffuse
into the ECF.

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