1. 21.1: Introduction The term balance suggests a state of constancy
For water and electrolytes that means equal amounts enter and leave the body
Mechanisms that replace lost water and electrolytes and excrete excesses maintain this balance
This results in stability of the body at all times
Keep in mind water and electrolyte balance are interdependent

2. 21.2: Distribution of Body Fluids
Body fluids are not uniformly distributed
They occupy compartments of different volumes that contain varying compositions
Water and electrolyte movement between these compartments is regulated to stabilize their distribution and the composition of body fluids

3. Fluid Compartments
An average adult female is about 52% water by weight, and an average male about 63% water by weight
There are about 40 liters of water (with its dissolved electrolytes) in the body, distributed into two major compartments:
Intracellular fluid – 63% - fluid inside cells
Extracellular fluid – 37% - fluid outside cells
Interstitial fluid
Blood plasma
Lymph
Transcellular fluid – separated from other extracellular fluids by epithelial layers
Cerebrospinal fluid
Aqueous and vitreous humors
Synovial fluid
Serous fluid
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Extracellular
fluid
(37%) 32 30 28 26 24 22
Liters 20 18 16
Intracellular
fluid
(63%) 14 12 10 8 6 4 2

4. Total Body Water 40 liters (10.6 gallons)
63% Intracellular Fluid
d37% Extracellular Fluid
Interstitial Fluid
Plasma
Lymph
Transcellular Fluid
Female – 52%
Male – 63%

5. Intracellular fluid (63%) Extracellular fluid (37%)
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Total body water
Interstitial fluid
Plasma
Lymph
Intracellular fluid
(63%)
Membranes of
body cells
Transcellular fluid
Extracellular fluid
(37%)

6. Body Fluid Composition
Extracellular fluids are generally similar in composition including high concentrations of sodium, calcium, chloride and bicarbonate ions
Blood plasma has more proteins than interstitial fluid or lymph
Intracellular fluids have high concentrations of potassium, magnesium, phosphate, and sulfate ions
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Relative concentrations and ratios of ions in extracellular and intracellular fluids
150
140
Extracellular fluid
130
Intracellular fluid
120 1 10
100 90 80
Ion concentration (m Eq/L) 70 60 50 40 30 20 10
Na+ K+
Ca+2
Mg+2
Cl-
HCO3-
PO4-3
SO4-2
Ratio
14:1
1:28
5:1
1:19
26:1
3:1
1:19
1:2
(Extracellular: intracellular)

7. Movement of Fluid Between Compartments
Two major factors regulate the movement of water and electrolytes from one fluid compartment to another
Hydrostatic pressure
Osmotic pressure
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Fluid leaves plasma
at arteriolar end of
capillaries because
outward force of
hydrostatic pressure
predominates
Capillary wall
Plasma
Fluid returns to
plasma at venular
ends of capillaries
because inward force
of colloid osmotic
pressure predominates
Interstitial fluid
Lymph
vessel
Hydrostatic pressure
within interstitial
spaces forces fluid
into lymph capillaries
Lymph
Transcellular
fluid
Intracellular
fluid
Interstitial fluid is
in equilibrium with
transcellular and
intracellular fluids
Serous
membrane
Cell
membrane

8. 21.3: Water Balance
Water balance exists when water intake equals water output
Homeostasis requires control of both water intake and water output

9. Water Intake
The volume of water gained each day varies among individuals averaging about 2,500 milliliters daily for an adult:
60% from drinking
30% from moist foods
10% as a bi-product of
oxidative metabolism of
nutrients called water of
metabolism
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Average daily intake of water
Average daily output of water
Water lost in sweat
(150 mL or 6%)
Water of
metobolism
(250 mL or 10%)
Water lost in feces
(150 mL or 6%)
Water in
moist food
(750 mL or 30%)
Water lost through
skin and lungs
(700 mL or 28%)
Total intake
(2,500 mL)
Total output
(2,500 mL)
Water in
beverages
(1,500 mL or 60%)
Water lost in urine
(1,500 mL or 60%)
(a)
(b)

10. Regulation of Water Intake
The primary regulator of water intake is thirst.

11. Water Output
Water normally enters the body only through the mouth, but it can be lost by a variety of routes including:
Urine (60% loss)
Feces (6% loss)
Sweat (sensible perspiration) (6% loss)
Evaporation from the skin (insensible perspiration)
The lungs during breathing
(Evaporation from the skin and the lungs is a 28% loss)

12. Regulation of Water Output
The osmoreceptor-ADH mechanism in the hypothalamus regulates the concentration of urine produced in the kidney.

13. 21.4: Electrolyte Balance
An electrolyte balance exists when the quantities of electrolytes the body gains equals those lost
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Metabolic
reactions
Foods
Fluids
Electrolyte intake
Electrolyte output
Perspiration
Feces
Urine

14. Electrolyte Intake
The electrolytes of greatest importance to cellular functions release sodium, potassium, calcium, magnesium, chloride, sulfate, phosphate, bicarbonate, and hydrogen ions
These ions are primarily obtained from foods, but some are from water and other beverages, and some are by-products of metabolism

15. Regulation of Electrolyte Intake
Ordinarily, a person obtains sufficient electrolytes by responding to hunger and thirst
A severe electrolyte deficiency may cause salt craving

16. Electrolyte Output
The body loses some electrolytes by perspiring (more on warmer days and during strenuous exercise)
Some are lost in the feces
The greatest output is as a result of kidney function and urine output

17. Regulation of Electrolyte Output
The concentrations of positively charged ions, such as sodium (Na+), potassium (K+) and calcium (Ca+2) are of particular importance
These ions are vital for nerve impulse conduction, muscle fiber contraction, and maintenance of cell membrane permeability
Sodium ions account for nearly 90% of the positively charged ions in extracellular fluids

18. Regulation of Electrolyte Output
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Potassium ion
concentration increases
Calcium ion
Concentration decreases
Parathyroid glands
are stimulated
Adrenal cortex is signaled
Parathyroid hormone
is secreted
Aldosterone is secreted
Renal tubules conserve
calcium and increase
secretion of phosphate
Intestinal absorption
of calcium increases
Renal tubules
increase reabsorption of
sodium ions and increase
secretion of potassium ions
Activity of bone-resorbing
osteoclasts increases
Increased phosphate
excretion in urine
Addition of phosphate
to bloodstream
Sodium ions are
conserved and potassium
ions are excreted
Calcium ion concentration
returns toward normal
Normal phosphate
concentration is maintained

19. 21.5: Acid-Base Balance
Acids are electrolytes that ionize in water and release hydrogen ions
Bases are substances that combine with hydrogen ions
Acid-base balance entails regulation of the hydrogen ion concentrations of body fluids
This is important because slight changes in hydrogen ion concentrations can alter the rates of enzyme-controlled metabolic reactions, shift the distribution of other ions, or modify hormone actions
pH number indicates the degree to which a solution is acidic or basic (alkaline).
The more acid the solution, the lower its pH
The more alkaline the solution, the higher its pH

20. Sources of Hydrogen Ions
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Aerobic
respiration
of glucose
Anaerobic
respiration
of glucose
Incomplete
oxidation of
fatty acids
Oxidation of
sulfur-containing
amino acids
Hydrolysis of
phosphoproteins
and nucleic acids
Carbonic
acid
Lactic
acid
Acidic ketone
bodies
Sulfuric
acid
Phosphoric
acid H+
Internal environment

21. Regulation of Hydrogen Ion Concentration
Either an acid shift or an alkaline (basic) shift in the body fluids could threaten the internal environment
Normal metabolic reactions generally produce more acid than base
The reactions include cellular metabolism of glucose, fatty acids, and amino acids
Maintenance of acid-base balance usually eliminates acids in one of three ways:
Acid-base buffer systems
Respiratory excretion of carbon dioxide
Renal excretion of hydrogen ions

22. Chemical Buffer Systems
Chemical buffer systems are in all body fluids and are based on chemicals that combine with excess acids or bases.
Bicarbonate buffer system
The bicarbonate ion converts a strong acid to a weak acid
Carbonic acid converts a strong base to a weak base
H+ + HCO3-  H2CO3  H+ + HCO3-
Phosphate buffer system
The monohydrogen phosphate ion converts a strong acid to a weak acid
The dihydrogen phosphate ion converts a strong base to a weak base
H+ + HPO4-2  H2PO4-  H+ + HPO4-2
Protein buffer system
NH3+ group releases a hydrogen ion in the presence of excess base
COO- group accepts a hydrogen ion in the presence of excess acid

24. Respiratory Excretion of Carbon Dioxide
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The respiratory center in the brainstem helps regulate hydrogen ion concentrations in the body fluids by controlling the rate and depth of breathing
If body cells increase their production of CO2…
Cells increase production of CO2
CO2 reacts with H2O to produce H2CO3
H2CO3 releases H+
Respiratory center is stimulated
Rate and depth of breathing increase
More CO2 is eliminated through lungs

25. Renal Excretion of Hydrogen Ions Increased concentration
Nephrons help regulate the hydrogen ion concentration of body fluids by excreting hydrogen ions in the urine
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High intake of proteins
Concentration of H+
in body fluids returns
toward normal
Increased metabolism
of amino acids
Increased concentration
of H+ in urine
Increased formation
of sulfuric acid and
phosphoric acid
Increased secretion
of H+ into fluid of
renal tubules
Increased concentration
of H+ in body fluids

26. Time Course of pH Regulation
Various regulators of hydrogen ion concentration operate at different rates
Acid-base (chemical) buffers function rapidly
Respiratory and renal (physiological buffers) mechanisms function more slowly
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Bicarbonate
buffer system
First line of defense
against pH shift
Chemical
buffer system
Phosphate
buffer system
Protein
buffer system
Respiratory
mechanism
(CO2 excretion)
Second line of
defense against
pH shift
Physiological
buffers
Renal
mechanism
(H+ excretion)

27. 21.6: Acid-Base Imbalances
Chemical and physiological buffer systems ordinarily maintain the hydrogen ion concentration of body fluids within very narrow pH ranges
Abnormal conditions may disturb the acid-base balance
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Acidosis
Alkalosis
pH scale
6.8
7.0
7.8
8.0
Normal pH range
Survival range

28. Acidosis
Acidosis results from the accumulation of acids or loss of bases, both of which cause abnormal increases in the hydrogen ion concentrations of body fluids
Alkalosis results from a loss of acids or an accumulation of bases accompanied by a decrease in hydrogen ion concentrations
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Accumulation
of acids
Loss of
bases
Increased concentration of H+
Acidosis
pH drops
pH scale
7.4
pH rises
Alkalosis
Decreased concentration of H+
Loss of
acids
Accumulation
of bases

29. Acidosis
Two major types of acidosis are respiratory acidosis and metabolic acidosis
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Kidney failure
to excrete acids
Excessive production of acidic
ketones as in diabetes mellitus
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Decreased rate
and depth of
breathing
Obstruction of
air passages
Decreased
gas exchange
Accumulation of nonrespiratory acids
Metabolic acidosis
Accumulation of CO2
Excessive loss of bases
Respiratory
acidosis
Prolonged diarrhea
with loss of alkaline
intestinal secretions
Prolonged vomiting
with loss of intestinal
secretions

30. Net increase in alkaline substances
Alkalosis
Respiratory alkalosis develops as a result of hyperventilation
Metabolic alkalosis results from a great loss of hydrogen ions or from a gain in bases, both accompanied by a rise in the pH of blood
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• Anxiety
Gastric
drainage
Vomiting with loss
of gastric secretions
• Fever
• Poisoning
• High altitude
Hyperventilation
Loss of acids
Excessive loss of CO2
Decrease in concentration of H2CO3
Net increase in alkaline substances
Decrease in concentration of H+
Metabolic alkalosis
Respiratory alkalosis