C h a p t e r 2 4
Structure and Function of the Kidney
603
The glomerular basement membrane consists of a
homogeneous acellular meshwork of collagen fibers,
glycoproteins, and mucopolysaccharides (Fig. 24-5C).
Because the endothelial and epithelial layers of the glo-
merular capillary have porous structures, the basement
membrane determines the permeability of the capillary
membrane. The spaces between the fibers that make up
the basement membrane represent the pores of a filter
and determine the size-dependent permeability barrier
of the glomerulus. The size of the pores in the basement
membrane normally prevents red blood cells and plasma
proteins from passing into the filtrate. Alterations in the
structure and function of the glomerular basement mem-
brane are responsible for the leakage of proteins and
blood cells into the filtrate that occurs in many forms of
glomerular disease.
The visceral layer of the Bowman capsule is com-
posed of epithelial cells that are highly modified to
perform a filtering function. These large cells, called
podocytes,
have numerous finger-like processes that
completely encircle the outer surface of the capillaries
(see Fig. 24-5C). The elongated spaces between the inter-
digitating foot processes, called
filtration slits,
function
as a size-selective filter that prevents proteins and mac-
romolecules that have crossed the basement membrane
from entering Bowman space.
Another important component of the glomerulus is
the
mesangium.
In some areas, the capillary endothe-
lium and the basement membrane do not completely
surround each capillary tuft. Instead, the mesangial
cells, which lie between the tufts, provide support for
the glomerulus in these areas (Fig. 24-5B). The mesan-
gial cells produce an intercellular substance similar to
that of the basement membrane. This substance cov-
ers the endothelial cells where they are not covered by
basement membrane. The mesangial cells also exhibit
contractile properties and are thought to contribute to
the regulation of blood flow through the glomerulus;
possess phagocytic properties and remove macromo-
lecular materials that enter the intercapillary spaces;
and are capable of proliferation. Although the mesan-
gial area is normally narrow and contains only a small
number of cells, mesangial hyperplasia and increased
mesangial matrix develop in many forms of glomerular
disease.
Tubular Components of the Nephron
The nephron tubule is divided into several segments: the
proximal tubule,
which drains Bowman capsule; a thin
looped structure, the
loop of Henle;
a distal coiled por-
tion, the
distal convoluted tubule;
and the final segment,
the
collecting tubule
(see Fig. 24-4A). The filtrate passes
through each of these segments before reaching the pel-
vis of the kidney.
The proximal tubule is a highly coiled structure that
lies in the cortex of the kidney and dips toward the renal
pelvis to become the loop of Henle. The loop consists of
a descending and ascending limb. The ascending loop
of Henle returns to the region of the renal corpuscle,
where it becomes much thicker, and is referred to as the
thick ascending limb
. Beyond the thick ascending limb
of Henle is the distal convoluted tubule, which like the
proximal tubule lies in the renal cortex. The distal con-
voluted tubule is divided into two segments: the
diluting
segment
and the
late distal tubule.
The late distal tubule
fuses with the collecting tubule The collecting tubule is
divided into two segments: the
cortical tubule
and the
medullary collecting tubule.
The initial parts of 8 or
10 cortical collecting tubules join to form a single large
tubule that moves down into the medulla to become the
medullary collecting tubule.
Throughout its course, the tubule is composed of
a single layer of epithelial cells resting on a basement
membrane. The structure of the epithelial cells varies
with tubular function. The cells of the proximal tubule
have a fine, villous structure that increases the surface
area for reabsorption; they also are rich in mitochon-
dria, which support active transport processes. The epi-
thelial layer thins in segments of the loop of Henle and
has few mitochondria, indicating minimal metabolic
activity and reabsorptive function.
Urine Formation
Urine formation involves the filtration of blood by the
glomerulus to form an
ultrafiltrate of urine,
the selective
reabsorption by the renal tubules of substances needed
SUMMARY CONCEPTS
■■
The kidneys are paired, bean-shaped organs that
lie outside the peritoneal cavity in the posterior
abdomen, one on either side of the vertebral
column. On longitudinal section, a kidney can
be divided into an outer cortex and an inner
medulla.The cortex and medulla are composed of
nephrons, blood vessels, and nerves.
■■
The nephrons, which are the functional units of
the kidney, consist of a renal capsule (glomerulus
and Bowman capsule) where blood is filtered,
and tubular structures (proximal tubule, loop of
Henle, distal tubule, and collecting tubule) where
water, electrolytes, and soluble nutrients are
reabsorbed into the blood and waste products are
secreted from the blood into the tubular fluid.
■■
The nephrons are supplied by two capillary
systems, a glomerulus and peritubular capillary
network.The glomerulus is a unique, high-
pressure capillary filtration system located
between two arterioles—the afferent and the
efferent arterioles.The peritubular capillaries,
which originate from the efferent arteriole and
surround the tubules, are low-pressure vessels
adapted for reabsorption rather than filtration.
