827
CHAPTER TWENTY-ONE
Water, Electrolyte, and Acid-Base Balance
b. The regulation of sodium ions involves the
secretion of aldosterone from the adrenal glands.
c. The regulation of potassium ions also involves
aldosterone.
d. Parathyroid hormone from the parathyroid glands
regulate calcium ion concentration.
e. The mechanisms that control positively charged
ions secondarily regulate negatively charged ions.
(1) Chloride ions are passively reabsorbed in
renal tubules as sodium ions are actively
reabsorbed.
(2) Some negatively charged ions, such as
phosphate ions, are reabsorbed partially by
limited-capacity active transport mechanisms.
21.5
ACID-BASE BALANCE (PAGE 819)
Acids are electrolytes that release hydrogen ions. Bases
combine with hydrogen ions.
1. Sources of hydrogen ions.
a. Aerobic respiration of glucose produces carbon
dioxide, which reacts with water to form carbonic
acid. Carbonic acid dissociates to release hydrogen
and bicarbonate ions.
b. Anaerobic respiration of glucose produces lactic
acid.
c. Incomplete oxidation of fatty acids releases acidic
ketone bodies.
d. Oxidation of sulfur-containing amino acids
produces sulfuric acid.
e. Hydrolysis of phosphoproteins and nucleic acids
gives rise to phosphoric acid.
2. Strengths of acids and bases
a. Acids vary in the extent to which they ionize.
(1) Strong acids, such as hydrochloric acid, ionize
more completely.
(2) Weak acids, such as carbonic acid, ionize less
completely.
b. Bases vary in strength also.
(1) Strong bases, such as hydroxide ions, combine
readily with hydrogen ions.
(2) Weak bases, such as bicarbonate ions,
combine with hydrogen ions less readily.
3. Regulation of hydrogen ion concentration
a. Acid-base buffer systems minimize pH changes.
(1) Buffer systems are composed of sets of two or
more chemicals.
(2) They convert strong acids into weaker acids or
strong bases into weaker bases.
(3) They include the bicarbonate buffer system,
phosphate buffer system, and protein buffer
system.
b. The respiratory system excretes carbon dioxide.
(1) The respiratory center is located in the
brainstem.
(2) It helps regulate pH by controlling the rate and
depth of breathing.
(3) Increasing carbon dioxide and hydrogen ion
concentrations stimulates chemoreceptors
associated with the respiratory center;
breathing rate and depth increase, and carbon
dioxide concentration decreases.
(3) Osmotic pressure regulates fl
uid movement in
and out of cells.
b. Sodium ion concentrations are especially
important in fl
uid movement regulation.
21.3
WATER BALANCE (PAGE 813)
1. Water intake
a. The volume of water taken in varies from person
to person.
b. Most water comes from consuming liquid or moist
foods.
c. Oxidative metabolism produces some water.
2. Regulation of water intake
a. The thirst mechanism is the primary regulator of
water intake.
b. Drinking and the resulting stomach distension
inhibit the thirst mechanism.
3. Water output
a. Water is lost in a variety of ways.
(1) It is excreted in the urine, feces, and sweat.
(2) Insensible loss occurs through evaporation
from the skin and lungs.
b. Urine production regulates water output.
4. Regulation of water output
a. The distal convoluted tubules and collecting ducts
of the nephrons regulate water output.
(1) ADH from the hypothalamus and posterior
pituitary gland stimulates water reabsorption
in these segments.
(2) The mechanism involving ADH can reduce
normal output of 1,500 milliliters to 500
milliliters per day.
b. If excess water is taken in, the ADH mechanism is
inhibited.
21.4
ELECTROLYTE BALANCE (PAGE 815)
1. Electrolyte intake
a. The most important electrolytes in the body fl
uids
are those that release ions of sodium, potassium,
calcium, magnesium, chloride, sulfate, phosphate,
and bicarbonate.
b. These ions are obtained in foods and beverages or
as by-products of metabolic processes.
2. Regulation of electrolyte intake
a. Electrolytes are usually obtained in sufF
cient
quantities in response to hunger and thirst
mechanisms.
b. In a severe electrolyte deF
ciency, a person may
experience a salt craving.
3. Electrolyte output
a. Electrolytes are lost through perspiration, feces,
and urine.
b. Quantities lost vary with temperature and physical
exercise.
c. The greatest electrolyte loss occurs as a result of
kidney functions.
4. Regulation of electrolyte output
a. Concentrations of sodium, potassium, and calcium
ions in the body fl
uids are particularly important.
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