Chapter19 - Fluid, Electrolyte, Acid-Base Balance

Understanding Anatomy & Physiology
A Visual, Interactive Approach
Copyright © 2012 F.A. Davis
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Chapter 19
Water, Electrolyte, and
Acid-Base Balance

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Fluid compartments

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Fluid movement

What determines the amount and direction of
fluid that flows between body compartments?
A. The volume of fluid in one of the
compartments
B. The concentration of solutes
C. The pH of the fluid
D. The concentration of hydrogen ions in the
fluid
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Correct answer: B
Rationale:
None of the other factors influences fluid
shifts between body compartments.
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Regulation of intake & output
 Water loss varies in amount.
 To maintain balance, the body uses
mechanisms to adjust intake and output.
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Mechanisms to ↑ intake
1. Physical changes stimulate thirst center
in hypothalamus.
2. Salivation decreases, causing dry mouth
and thirst.
3. Water consumption occurs.

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Mechanisms to ↓ output
1. Physical changes stimulate the
hypothalamus.
2. This stimulates the posterior pituitary to
secrete antidiuretic hormone (ADH).
3. ADH prompts the kidneys to reabsorb
water and produce less urine.
4. Fluid loss slows until water is ingested.

Antidiuretic hormone (ADH) is released in
response to:
A. a rise in blood pressure.
B. an increase in fluid volume.
C. a decrease in both serum sodium and
osmolarity.
D. an increase in both serum sodium and
osmolarity.
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Correct answer: D
Rationale:
ADH is secreted in the volume of total body
water declines, which leads to a drop in
blood pressure and an increase in serum
sodim and osmolarity.
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Disorders of water balance
Can result from abnormality in:
 Fluid volume
 Fluid concentration
 Distribution of fluid between
compartments

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Fluid deficiency
 Volume depletion: results from blood loss
or loss of both water and sodium
 Dehydration: results when the body
eliminates more water than sodium

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Dehydration

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Fluid excess
 Kidneys usually compensate by producing
more urine.
 Specific type includes water intoxication.

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Fluid accumulation
 Involves the accumulation of fluid
between compartments
 Edema: Fluid accumulation in interstitial
spaces, causing tissue swelling

Which condition results when the body
eliminates more water than sodium?
A. Edema
B. Dehydration
C. Water intoxication
D. Volume depletion
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Correct answer: B
Rationale:
Edema is the accumulation of fluid in
interstitial spaces. Water intoxication is the
consumption of more water than sodium.
Volume depletion is the loss of both sodium
and water.
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Electrolyte balance
 Crucial for proper body functioning
 Major cations: Na+, K+ Ca+, H+
 Major anions: Cl−, HCO−, P3
i

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Sodium
 Main electrolyte in extracellular fluid
 Determines volume of total body water
 Influences how body water is distributed
 Plays a key role in depolarization

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Sodium regulation
↓Serum Na+
 Serum osmolarity ↓
 Aldosterone prompts renal tubules to
reabsorb Na+
 Antidiuretic hormone (ADH) suppressed →
kidneys secrete water
 Serum Na+ levels increase

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Sodium regulation
↑ Serum Na+
 Serum osmolarity ↑
 Aldosterone prompts renal tubules to
reabsorb Na+.
 Antidiuretic hormone (ADH) causes kidneys
to reabsorb water.
 ADH stimulates thirst.
 Serum Na+ levels decline.

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Sodium imbalances
Hypernatremia
 Plasma concentration
greater than 146 mEq/L
 Indicates fluid deficit
 Usually self-corrects by
triggering thirst
Hyponatremia
less than 139 mEq/L
 Results from excess body
water
 Usually corrected by
excretion of excess
water

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Potassium imbalances
Hyperkalemia
above 5.0 mEq/L
 May occur suddenly or
gradually
 Makes nerve and muscle
cells irritable
Hypokalemia
less than 3.5 mEq/L
 May result from
diuretics, vomiting, or
chronic diarrhea
 Makes cells less excitable

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Calcium imbalances
Hypercalcemia
greater than 5.8 mEq/L
 May result from
hyperparathyroidism,
hypothyroidism, alkalosis
 Inhibits depolarization
Hypocalcemia
less than 4.5 mEq/L
 May result from
hypoparathyroidism,
hyperthyroidism,
acidosis, diarrhea
 Increases excitation of
nerves and muscles

What is the main cation of extracellular
fluid?
A. Potassium
B. Sodium
C. Chloride
D. Calcium
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Correct answer: B
Rationale:
Potassium is the main cation in intracellular
fluid. Chloride is the most abundant
extracellular anion and is linked to sodium.
Calcium exists mostly outside the cell but is
not the chief extracellular cation.
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Acid-base balance
 Influences homeostasis
 Slight deviations can be fatal
 pH of blood ranges from 7.35 to 7.45

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Chemical buffers
 Includes bicarbonate, phosphate, and
protein buffer systems
 Use weak base to bind H+ ions and weak
acid to release them

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Physiological buffers
 Includes respiratory and urinary systems.
 Lungs expel CO2 to lower pH.
 Kidneys expel H+ ions to lower pH.

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Respiratory control of pH
 Central chemoreceptors in the brainstem detect
a decline in pH from an accumulation of CO2.
 They signal the respiratory centers to increase
the rate and depth of breathing.
 The lungs blow off CO2.
 Less CO2 is available to combine with water to
form carbonic acid; the concentration of H+ ions
decreases and pH rises.

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Renal control of pH
 Expels H+ ions and reabsorbs bicarbonate
 Is the most powerful buffer system
 Is also the slowest to respond
<View animation “Renal control of pH”

When the body’s pH rises above normal,
which reaction would occur first?
A. Respiratory rate would increase
B. Kidneys would excrete hydrogen ions
C. Hydrogen would bind with bicarbonate
D. Hydrogen would bind with carbonic acid
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Correct answer: C
Rationale:
Although both an increase in respiratory
rate and the excretion of hydrogen ions
from the kidneys would help lower pH,
neither would be the first response.
Hydrogen does not bind with carbonic acid.
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Acid-base imbalances
 Respiratory imbalances result from an
excess or deficiency of CO2
 Metabolic imbalances result from an
excess or deficiency of bicarbonate

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CAUSES OF ACID GAIN
(Acidosis)
CAUSES OF ACID LOSS
(Alkalosis)
RESPIRATORY  Retention of CO2
(hypoventilation—such as
from emphysema or
pneumonia—as well as
apnea)
 Loss of CO2
(hyperventilation)
METABOLIC  Increased production of
acids (such as ketone
bodies in diabetes mellitus
or lactic acid in anaerobic
metabolism)
 Consumption of acidic
drugs (such as aspirin)
 Inability of the kidneys to
excrete H+ ions
 Loss of bicarbonate (such
as chronic diarrhea)
 Loss of gastric juices (such
as through vomiting or
suctioning)
 Excessive ingestion of
bicarbonates (such as
antacids)

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Compensation for acid-base
imbalances
 Respiratory system responds to metabolic
disturbances by adjusting ventilation.
 Renal system responds by adjusting the
rate of H+ ion excretion.

Which electrolyte disturbance would result
from acidosis?
A. Hypernatremia
B. Hyponatremia
C. Hyperkalemia
D. Hypokalemia
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Correct answer: C
Rationale:
Acidosis does not affect the concentration
of sodium in the blood. Hypokalemia would
result from alkalosis, caused as potassium
moves into the cell to balance the
movement of hydrogen ions out of the cell.
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Chapter19 - Fluid, Electrolyte, Acid-Base Balance

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