The Kidneys and Regulation of Water and Inorganic Ions
487
important in the control of blood pressure and sodium bal-
ance (described later in this chapter).
Structure of the Kidneys
and Urinary System
The two kidneys lie in the back of the abdominal wall but
not actually in the abdominal cavity. They are retroperitoneal,
meaning they are just behind the peritoneum, the lining of
this cavity. The urine fl ows from the kidneys through the
ure-
ters
into the
bladder,
and then is eliminated via the
urethra
(
Figure 14–1
).
Each kidney contains approximately 1 million similar
subunits called
nephrons.
Each nephron consists of (1) an ini-
tial fi ltering component called the
renal corpuscle,
and (2) a
tubule
that extends from the renal corpuscle (
Figure 14–2
).
The renal corpuscle forms a fi ltrate from blood that is free of
cells and proteins. This fi ltrate then leaves the renal corpuscle
and enters the tubule. As it fl ows through the tubule, sub-
stances are added to or removed from it. Ultimately, the fl
uid
remaining at the end of each nephron combines in the collect-
ing ducts and exits the kidneys as urine.
Let us look fi rst at the anatomy of the renal corpus-
cles—the fi lters. Each renal corpuscle contains a compact
tuft of interconnected capillary loops called the
glomerulus
(plural,
glomeruli
), or
glomerular capillaries
(Figure 14–2
and
Figure 14–3
). Each glomerulus is supplied with blood
by an arteriole called an
afferent arteriole.
The glomerulus
protrudes into a fl
uid-fi lled capsule called
Bowman’s cap-
sule.
The combination of a glomerulus and a Bowman’s cap-
sule constitutes a renal corpuscle. As blood fl ows through
the glomerulus, about 20 percent of the plasma fi lters into
Bowman’s capsule. The remaining blood then leaves the
glomerulus by the
efferent arteriole.
One way of visualizing the relationships within the renal
corpuscle is to imagine a loosely clenched fi st—the glomerulus—
punched into a balloon—the Bowman’s capsule. The part of
Bowman’s capsule in contact with the glomerulus becomes
pushed inward but does not make contact with the opposite
side of the capsule. Accordingly, a fl
uid-fi lled space called the
Bowman’s space
exists within the capsule. Protein-free fl
uid
fi lters from the glomerulus into this space.
Blood in the glomerulus is separated from the fl
uid in
Bowman’s space by a fi ltration barrier consisting of three
layers (
Figures 14–3b
and
14–3c
). These include (1) the
single-celled capillary endothelium, (2) a noncellular pro-
teinaceous layer of basement membrane (also termed basal
lamina) between the endothelium and the next layer, which
is (3) the single-celled epithelial lining of Bowman’s cap-
sule. The epithelial cells in this region, called
podocytes,
are quite different from the simple fl attened cells that line
the rest of Bowman’s capsule (the part of the “balloon” not
in contact with the “fi st”). They have an octopus-like struc-
ture in that they possess a large number of extensions, or foot
processes. Fluid fi lters fi rst across the endothelial cells, then
through the basement membrane, and fi nally between the
foot processes of the podocytes.
In addition to the capillary endothelial cells and the
podocytes, there is a third cell type,
mesangial cells,
which
are modifi ed smooth muscle cells that surround the glomeru-
lar capillary loops but are not part of the fi ltration pathway.
Their function will be described later.
The renal tubule is continuous with a Bowman’s capsule.
It is a very narrow, hollow cylinder made up of a single layer
of epithelial cells (resting on a basement membrane). The epi-
thelial cells differ in structure and function along the length
of the tubule, and at least eight distinct segments are now rec-
ognized (see Figure 14–2). It is customary, however, to group
two or more contiguous tubular segments when discussing
function, and we will follow this practice. Thus, the segment
of the tubule that drains Bowman’s capsule is the
proximal
tubule,
comprising the proximal convoluted tubule and the
proximal straight tubule shown in Figure 14–2. The next
portion of the tubule is the
loop of Henle,
which is a sharp,
hairpin-like loop consisting of a
descending limb
coming
from the proximal tubule and an
ascending limb
leading
to the next tubular segment, the
distal convoluted tubule.
Fluid fl ows from the distal convoluted tubule into the
collect-
ing duct system,
comprised of the
cortical collecting duct
and then the
medullary collecting duct.
The reasons for the
terms
cortical
and
medullary
will be apparent shortly.
From Bowman’s capsule to the collecting duct system,
each nephron is completely separate from the others. This
separation ends when multiple cortical collecting ducts merge.
Kidney
Ureter
Bladder
Urethra
Diaphragm
Figure 14–1
Urinary system in a woman. In the male, the urethra passes through
the penis (Chapter 17). The diaphragm is shown for orientation.
previous page 515 Vander's Human Physiology The Mechanisms of Body Function read online next page 517 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off