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U N I T 7
Kidney and Urinary Tract Function
The osmolarity of body fluids relies heavily on the ability of the kidney to produce dilute
or concentrated urine. Urine concentration depends on three factors: (1) the osmolarity of
interstitial fluids in the urine-concentrating part of the kidney, (2) the antidiuretic hormone
(ADH), and (3) the action of ADH on the cells in the collecting tubules of the kidney.
Osmolality
(mOsm)
300
600
800
1200
NaCl
NaCl
NaCl
NaCl
NaCl
NaCl
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
K + Na +
Urea
Urea
NaCl
Urea
Vasa recta Loop of
Henle
Collecting
tubule
H
2
O
NaCl
Urea
NaCl
Urea
Cortex
Medulla
Osmolarity.
In approximately one
fifth of the juxtamedullary neph-
rons, the loops of Henle and special
hairpin-shaped capillaries called the
vasa recta
descend into the medullary
portion of the kidney to form a coun-
tercurrent system—a set of parallel
passages in which the contents flow
in opposite directions. The counter-
current design serves to increase the
osmolarity in this part of the kidney
by promoting the exchange of solutes
between the adjacent descending and
ascending loops of Henle and between
the descending and ascending sec-
tions of the vasa recta. Because of
these exchange processes, a high con-
centration of osmotically active par-
ticles (approximately 1200 mOsm/
kg of H
2
O) collects in the interstitium
surrounding the collecting tubules
where the ADH-mediated reabsorp-
tion of water takes place.
1
ADH
Osmoreceptors
Posterior
pituitary gland
Hypothalamus
Urine: decreased
flow and concentrated
2
Antidiuretic Hormone.
Antid
iuretic hormone, which regulates the
ability of the kidneys to concentrate
urine, is synthesized by neurons in
the hypothalamus and transported
down their axons to the posterior
pituitary gland and then released
into the circulation. One of the main
stimuli for synthesis and release of
ADH is an increase in serum osmo-
larity. Antidiuretic hormone release
is also controlled by cardiovascular
reflexes that respond to changes in
blood pressure or blood volume.
U N D E R S T A N D I N G
How the Kidney
