of Regula'on body uids (also excre'on)
Sources of water
and routes of
water loss.
Why is the water
content of the
body so
important?
The kidneys are the
principal organs of salt
and water balance in
mammals and birds.
(Other animal groups
may use dierent
organs.)
They are also the
principal organs for
excre'ng hydrophilic
substances, such as
nitrogenous wastes,
which are produced
when amino acids are
used in energy
metabolism.
In the kidneys, epithelial tubes
called nephrons work with blood
vessels to regulate the content,
composi'on, and volume of all
three body uid compartments.
They accomplish this result via
ltra'on, reabsorp'on, and
secre'on (which has a dierent
meaning in kidneys than it does
in endocrine physiology).
Water and solutes can be exchanged between the body uid
compartments based on driving forces and permeabili'es. Here are three
common examples:
Structure and loca'on of kidneys in the mammalian body
Heres a closer look at the
structure of a nephron
and its associated blood
vessels.
The epithelium in
each segment of a
nephron has unique
proper'es. As a
result, what moves
out as tubular uid
progresses along a
nephron varies from
segment to segment.
Based on the
permeabili'es and
driving forces along a
nephron, the
concentra'on of tubular
uid changes as it moves
along a nephron. These
concentra'ons are
important in kidney
func'on.
No'ce that the
concentra'on is
constant along the
proximal tubule, but it
increases along the
descending limb of the
loop of Henle.
As tubular uid moves
along the ascending loop
of Henle and the along
the distal tubule, the
concentra'on steadily
drops.
As with all func'on in a
nephron, this change
depends on the
permeability proper'es of
the epithelium and on the
driving forces.
When the uid reaches
the collec'ng duct, the
concentra'on
con'nues to change.
The values shown on
this gure are typical in
a normally hydrated
human being. (Stay
tuned to know what
that means.)
Where do solutes the and
water go?
Into the peritubular
capillaries or into the
vasa recta.
The permeability of the collec'ng duct (but no other part of the kidney)
to water depends on the concentra'on of a pep'de hormone,
an'diure'c hormone (ADH), is in circula'ng blood. The higher the
concentra'on of ADH is in the plasma, the more permeable to water the
epithelium of the collec'ng duct is.
(Remember that vasopressin/an'diure'c hormone (two names for the
same molecule) is released in the posterior pituitary.)
Heres how ADH works:
The net eect is to excrete salt, while
saving water.
A second hormone, aldosterone, also regulates kidney func'on, but the
pathway is more complex.
Aldosterone is a steroid hormone secreted by cells in the adrenal cortex.
We will consider this complex path
in stages.
First, low blood volume or low blood
pressure reduces ltra'on into
nephrons, and as a result, a small
cluster of cells associated with each
nephron releases an enzyme called
renin.
(Thats NOT rennin, which is an
enzyme from the stomachs of young
calves.)
Renin catalyzes hydrolysis of
specic bonds in angiotensinogen,
a substrate synthesized
con'nuously in the liver.
This reac'on produces a pep'de
called angiotensin I.
Angiotensin I is further clipped to a
10amino acid pep'de called
angiotensin II. The reac'on is
catalyzed by angiotensin conver'ng
enzyme, which is plen'ful in the
endothelial walls of pulmonary
capillaries.
Angiotensin II is a vasoconstrictor.
Angiotensin II also s'mulates cells in
the adrenal cortex to secrete
aldosterone.
Aldosterone acts on the epithelial
cells along the collec'ng duct
(nowhere else in the kidney) to
increase reabsorp'on of salt and
water (as long as ADH is present).
The net eect is to keep the blood
volume from decreasing even more
due to the loss of water in the urine.
The vertebrate kidney (and in
some species other epithelial
organs) have evolved to
produce salt and water balance
in a wide variety of condi'ons.
Here are three examples: