The Kidneys Regulate the Composition of
Body Fluids 453
• Water is distributed among intracellular and extracellu-
lar fluid compartments. The extracellular compartment
includes interstitial fluid and blood plasma, as shown.
Within each fluid compartment, electrolytes are dissolved.
Sodium, chloride, and bicarbonate are the main electrolytes
in extracellular compartments. Potassium, magnesium,
bicarbonate, and phosphate are the primary electrolytes in
D is tr ib u tio n o f w a t e r in v a r io u s c o m p a r t m e n t s o f t h e
b o d y • F ig u r e 1 5 .9
• Water intake and water loss are balanced on a daily basis.
The kidneys regulate water balance by altering reabsorption
in the distal tubule and collecting duct under stimulation by
antidiuretic hormone (ADH). •
• The balance of water, sodium, and chloride are linked with
blood volume and blood pressure. If blood volume decreas-
es, the kidneys maintain sodium and chloride balance, blood
volume, and blood pressure by reabsorbing these ions in
the distal tubules under stimulation by the renin-angiotensin-
aldosterone system. When blood volume is increased,
the heart releases atrial natriuretic peptide (ANP), which
stimulates loss of sodium, chloride, and water in the distal
• The kidneys regulate blood plasma calcium levels by reab-
sorbing calcium in the distal convoluted tubule (DCT) when
stimulated by parathyroid hormone (PTH). The kidneys
also secrete calcitriol (vitamin D), which increases calcium
absorption in the intestine.
The Kidneys Help Maintain the Acid-Base
Balance of Body Fluids 458
• Your body produces acids (H+) through metabolism. More
acid is produced when you ingest a meat-rich diet than
when you ingest a diet rich in fruits and vegetables. To deal
with the acid produced, your fluids have three major buffer
systems: protein, carbonic acid-bicarbonate (shown below),
and phosphate buffers.
C a r b o n ic a c id - b ic a r b o n a t e b u ffe r s y s t e m
• F ig u r e 1 5 .1 4 b
When pH rises, the reaction will shift to the
right to produce additional H+ into the blood.
If the pH drops, the reaction will shift to the left,
using up some additional H+ that is in solution.
• The carbonic acid-bicarbonate system is a dynamic buffer
that keeps the blood pH in the range 7.35-7.45. The respira-
tory system can rapidly alter blood pH through changes in
the rate and depth of breathing. This changes the carbon
dioxide concentration of the blood and alters the equilib-
rium of the bicarbonate system. The kidneys can alter blood
pH by changing the reabsorption of H+ or HCO3-. The kidneys
change blood pH more slowly than the respiratory system,
but the changes are permanent.
• Several conditions cause alkalosis (increases in blood pH) or
acidosis (decreases in blood pH). The changes in blood pH
are compensated by actions of the respiratory system and
464 CHAPTER 15
The Urinary System and Fluid, Electrolyte, and Acid-Base Balance