The vertebrate kidney is part of many
interlocking mechanisms that maintain
homeostasis. The kidney plays a
prominent role in this regulatory council
because it is the principal organ that
regulates the volume and composition
of the internal fluid environment.
While we commonly describe the vertebrate
kidney as an organ of excretion,
the removal of metabolic wastes
is incidental to its regulatory function.
The organization of kidneys differs
somewhat in different groups of
vertebrates, but in all the basic functional
unit is the nephron, and urine
is formed by three well-defined physiological
processes: filtration, reabsorption,
. The following
discussion focuses mainly on
the mammalian kidney, which is the
most completely understood regulatory
The two human kidneys are small
organs comprising less than 1% of
the body weight. Yet they receive a
remarkable 20% to 25% of the total
cardiac output, some 2000 liters of
blood each day. This vast blood flow is
channeled to approximately 2 million
nephrons, which make up the bulk of
the two kidneys. Each nephron begins
with an expanded chamber, the renal
containing a tuft of capillaries
called the glomerulus (glomer
'yoo-lus). Blood pressure in the
capillaries forces a protein-free filtrate
into a renal tubule
, consisting of several
segments that perform different
functions in the process of urine formation.
The filtrate passes first into a
proximal convoluted tubule,
into a long, thin-walled loop of
which drops deep into the
inner portion of the kidney (the
medulla) before returning to the outer
portion (the cortex) where it joins a
distal convoluted tubule.
distal tubule the fluid empties into a collecting duct
which drains into
the renal pelvis.
Here the urine is
collected before being carried by the
ureter to the urinary bladder.
anatomical relationships are shown in
|Figure 32-10 Urinary system of humans, with enlargements showing detail of the kidney and a single nephron.
The urine that leaves the collecting
duct is very different from the filtrate
produced in the renal corpuscle. During
its travels through the renal tubule
and collecting duct, both the composition
and concentration of the original
filtrate change. Some solutes such as
glucose and sodium have been reabsorbed
while other materials, such as
hydrogen ions and urea, have been
concentrated in the urine.
|Figure 32-11 Scanning electron micrograph of
a cast of the
microcirculation of the mammalian
showing several glomeruli and associated
vessels. The capsular epithelium, which
surrounds each glomerulus, has
away in preparing the cast.
The nephron, with its pressure filter
and tubule, is intimately associated
with blood circulation (Figure 32-11).
Blood from the aorta enters each kidney
through a large renal artery,
which divides into a branching system
of smaller arteries. The arterial blood
reaches the renal corpuscle through an
and leaves by way
of an efferent arteriole
. From the
efferent arteriole the blood travels to
an extensive capillary network that
surrounds and supplies the proximal
and distal convoluted tubules and the
loop of Henle (Figure 32-10). This capillary network provides a means for
the pickup and delivery of materials
that are reabsorbed or secreted by the
kidney tubules. From these capillaries
blood is collected by veins that unite to
form the renal vein.
This vein returns
the blood to the vena cava.
Let us now return to the glomerulus,
where the process of urine formation
begins. The glomerulus acts as a specialized
mechanical filter in which a
protein-free filtrate of the plasma is
driven by the blood pressure across
the capillary walls and into the fluidfilled
space of the renal corpuscle.
Solute molecules small enough to pass
through the slit pores of the capillary
wall are carried through with the water
in which they are dissolved. Red blood
cells and plasma proteins, however,
are withheld because they are too
large to pass through these pores (Figure
The filtrate continues through the
renal tubular system where it will
undergo extensive modification before
becoming urine. Human kidneys form approximately 180 liters (nearly 50 gallons)
of filtrate each day, a volume
many times exceeding the total blood
volume. If this volume of water and
the valuable nutrients and salts it contains
were lost, the body would soon
be depleted of these compounds.
Depletion does not happen because
nearly all of the filtrate is reabsorbed.
The final urine volume in humans
averages 1.2 liters per day.
Conversion of filtrate into urine
involves two processes: (1) modification
of the composition of the filtrate through
tubular reabsorption and secretion, and
(2) changes in the total osmotic concentration
of the urine through the regulation
of water excretion.