HUMAN BIOLOGY

the UrInary SyStem 237

summary

section 12.1 Extracellular fluid (ECF)
contains various types and amounts of
substances dissolved in water. The ECF
includes tissue fluid and blood plasma. The
following processes maintain a healthy balance in the volume
and chemical composition of ECF:
The body absorbs water from the GI tract and gains it from
condensation reactions of metabolism. Water is lost by urinary
excretion, evapo ration from the lungs and skin, sweating, and
elimination in feces.
Solutes are gained by absorption from the GI tract, secretion
by cells, respiration, and metabolism. They are lost by excretion,
respiration, and sweating. These solutes include electrolytes
(sodium, potassium, calcium ions).
The kidneys control losses of water and solutes by adjusting
the volume and chemical makeup of urine.

section 12.2 The urinary system con-
sists of two kidneys, two ureters, a urinary
bladder, and a urethra. In the kidneys, blood
is filtered and urine forms in nephrons.
A nephron starts as a cup-shaped capsule
that is followed by three tubelike regions:
the proximal tubule, loop of Henle, and distal
tubule, which empties into a collecting duct.
The Bowman’s (glomerular) capsule surrounds a set of
highly permeable capillaries. Together, they are a blood-
filtering unit, the glomerulus.

section 12.3 Urine forms through a
sequence of steps: filtration, reabsorption, and
secretion (Table 12.4).
Filtration of blood at the glomerulus of a
nephron transfers water and small solutes into
the nephron.
In reabsorption, needed water and solutes leave the nephron
tubule and enter the peritubular capillaries that thread around
the tubule. Many solutes are reabsorbed when they diffuse
down their concentration gradients back into the bloodstream.
Others, such as sodium, are reabsorbed by active transport.
Water is reabsorbed by osmosis. A small amount of water and
solutes remains in the nephron.
In secretion some ions and a few other substances leave
the peritubular capillaries and enter the nephron for disposal
in urine.

section 12.4 During reabsorption in
kidney nephrons, water and salt are reab-
sorbed or excreted as required to conserve
or eliminate water. The mechanisms that
concentrate urine also help regulate blood
volume and blood pressure.
Urine becomes more or less concentrated by the action
of two hormones, ADH and aldosterone, on cells of distal
tubules and collecting ducts.
ADH is secreted when the body must conserve water; it
increases reabsorption from the distal nephron tubule and
collecting ducts. Inhibition of ADH allows more water to
be excreted.

Process characteristics

Filtration Pressure generated by heartbeats drives

water and small solutes (not proteins) out of

glomerulus capillaries and into Bowman’s

capsule, the entrance to the nephron.

Reabsorption Most water and solutes in the filtrate move

from a nephron’s tubule into interstitial fluid

around the nephron, then into blood inside

the peritubular capillaries.

Secretion Urea, H^1 , and some other solutes move

out of peritubular capillaries, into interstitial

fluid, then into the filtrate inside the nephron

for excretion in urine.

Table 12.4 Processes of urine Formation

Aldosterone conserves sodium by increasing its reabsorp-

tion in the distal tubule. It is secreted when cells in the

juxtaglomerular apparatus (next to the distal tubule) secrete

renin, an enzyme that triggers reactions that lead to aldo-

sterone secretion. More sodium is excreted when aldosterone is

inhibited. Because “water follows salt,” aldosterone influences

how much water is reabsorbed into the bloodstream.

section 12.5 Together with the

respiratory system and other mechanisms,

the kidneys also help maintain the body’s

overall acid–base balance. They help regulate pH by

eliminating excess hydrogen ions and replenishing the supply

of bicarbonate, which acts as a buffer elsewhere in the body.

reVieW Questions

1. Label the parts of this kidney and nephron.

© Cengage Learning

2. How does the formation of urine help maintain the body’s
   internal environment?


3. Explain what is meant when we talk about filtration,
   reabsorption, and secretion in the kidneys.


4. Which hormone or hormones promote (a) water conserva-
   tion, (b) sodium conservation, and (c) thirst behavior?


5. Explain how the kidneys help to maintain the balance of
   acids and bases in extracellular fluid.

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