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Hole’s Human Anatomy
and Physiology
Twelfth Edition
Shier w Butler w Lewis
Chapter
21
Water, Electrolyte, and
Acid-Base Balance
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21.1: Introduction
• The term balance suggests a state of equilibrium
• 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 (Called what?)
• This results in stability of the body at all times
• Keep in mind water and electrolyte balance are
interdependent
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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
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Fluid Compartments
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40
38
36
Extracellular
fluid
(37%)
34
32
30
28
26
24
22
20
Liters
• Of the 40 liters of water in the
body of an average adult, about
two-thirds is intracellular fluid and
one-third is extracellular fluid
• An average adult female is about
52% water by weight, and an
average male about 63% water by
weight (Why do men have more
water than women?)
18
16
14
12
Intracellular
fluid
(63%)
10
8
6
4
2
0
4
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Total body water
Interstitial fluid
Plasma
Membranes of
body cells
Intracellular fluid
(63%)
Lymph
Transcellular fluid
Extracellular fluid
(37%)
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Body Fluid Composition
• Extracellular fluids are generally
similar in composition including
high concentrations of sodium,
calcium, chloride and bicarbonate
ions
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Relative concentrations and ratios of ions in extracellular and intracellular fluids
150
140
Extracellular fluid
130
Intracellular fluid
120
110
100
Ion concentration (m Eq/L)
• Intracellular fluids have high
concentrations of potassium,
magnesium, phosphate, and sulfate
ions
90
80
70
60
50
40
30
20
10
0
Ratio
Na+
K+
Ca+2
Mg+2
Cl-
HCO3-
PO4-3
SO4-2
14:1
1:28
5:1
1:19
26:1
3:1
1:19
1:2
(Extracellular: intracellular)
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Movement of Fluid
Between Compartments
• Two major factors regulate the movement of water and
electrolytes from one fluid compartment to another
• Hydrostatic pressure
Fluid leaves plasma
• Osmotic pressure
at arteriolar end of
Capillary wall
capillaries because
(Define each)
outward force of
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Plasma
•Water balance exists when
water intake equals water
output
• Homeostasis requires
control of both water intake
and water output
Interstitial fluid
hydrostatic pressure
predominates
Fluid returns to
plasma at venular
ends of capillaries
because inward force
Lymph of colloid osmotic
vessel pressure predominates
Lymph
Transcellular
fluid
Serous
membrane
Intracellular
fluid
Cell
membrane
Hydrostatic pressure
within interstitial
spaces forces fluid
into lymph capillaries
Interstitial fluid is
in equilibrium with
transcellular and
intracellular fluids
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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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Water of
metobolism
(250 mL or 10%)
Average daily output of water
Water lost in sweat
(150 mL or 6%)
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%)
Average daily intake of water
Total intake
(2,500 mL)
Total output
(2,500 mL)
Water in
beverages
(1,500 mL or 60%)
(a)
Water lost in urine
(1,500 mL or 60%)
(b)
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Regulation of Water Intake
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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 totals a 28%
loss)
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Regulation of Water Output
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21.4: Electrolyte Balance
• An electrolyte balance exists when the quantities of
electrolytes the body gains equals those lost
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Foods
Fluids
Metabolic
reactions
Electrolyte intake
Electrolyte output
Perspiration
Feces
Urine
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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
•Ordinarily, a person obtains sufficient electrolytes by
responding to hunger and thirst
• A severe electrolyte deficiency may cause salt craving
13
Electrolyte Output
• The body loses some electrolytes by perspiring typically 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
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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
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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
Sodium ions are
conserved and potassium
ions are excreted
Activity of bone-resorbing
osteoclasts increases
Increased phosphate
excretion in urine
Addition of phosphate
to bloodstream
Calcium ion concentration
returns toward normal
Normal phosphate
concentration is maintained
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21.5: Acid-Base Balance
• Electrolytes that ionize in water and release hydrogen ions
are acids
• Substances that combine with hydrogen ions are bases
• 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
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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
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Strengths of Acids and Bases
• Acids:
• Strong acids ionize more completely and release more H+
• Weak acids ionize less completely and release fewer H+
• Bases:
• Strong bases ionize more completely and release more OH• Weak bases ionize less completely and release fewer OH-
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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
• These 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
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Acid-Base Buffer Systems
• 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
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Respiratory Secretion of Carbon Dioxide
• 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…
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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
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Renal Excretion of Hydrogen Ions
• 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
Increased metabolism
of amino acids
Increased concentration
of H+ in urine
Concentration of H+
in body fluids returns
toward normal
Increased secretion
of H+ into fluid of
renal tubules
Increased formation
of sulfuric acid and
phosphoric acid
Increased concentration
of H+ in body fluids
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Time Course of Hydrogen Ion Regulation
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• 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
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)
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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
7.35
7.45
7.8
8.0
Normal pH range
Survival range
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Acidosis & Alkalosis
• 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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Loss of
bases
Accumulation
of acids
Increased concentration of H+
Acidosis
pH drops
pH scale
7.4
pH rises
Alkalosis
Decreased concentration of H+
Loss of
acids
Accumulation
of bases
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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 CO2
Accumulation of nonrespiratory acids
Metabolic acidosis
Excessive loss of bases
Respiratory
acidosis
Prolonged diarrhea
with loss of alkaline
intestinal secretions
Prolonged vomiting
with loss of intestinal
secretions
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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
• Fever
• Poisoning
• High altitude
Gastric
drainage
Vomiting with loss
of gastric secretions
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
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Important Points in Chapter 21:
Outcomes to be Assessed
21.1: Introduction
 Explain the balance concept.
 Explain the importance of water and electrolyte balance.
21.2: Distribution of Body Fluids
 Describe how body fluids are distributed in compartments.
 Explain how fluid composition varies among compartments and how
fluids move from one compartment to another.
21.3: Water Balance
 List the routes by which water enters and leaves the body.
 Explain the regulation of water input and water output.
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Important Points in Chapter 21:
Outcomes to be Assessed
21.4: Electrolyte Balance
 List the routes by which electrolytes enter and leave the body.
 Explain the regulation of the input and the output of electrolytes.
21.5: Acid-Base Balance
 Explain acid-base balance.
 Identify how pH number describes the acidity and alkalinity of a body
fluid.
 List the major sources of hydrogen ions in the body.
 Distinguish between strong acids and weak acids.
 Explain how chemical buffer systems, the respiratory system, and the
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kidneys keep the pH of body fluids relatively constant.
Important Points in Chapter 21:
Outcomes to be Assessed
21.6: Acid-Base Imbalances
 Describe the causes and consequences of increase or decrease in body
fluid pH.
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