Each kidney contains over a million nephrons,
which are the anatomical units responsible for forming urine.
Figure 4.7 depicts the detailed structure and the relative
positioning of the nephrons in the kidney.
Each nephron consists of two major structures: a
glomerulus (a capillary knot) and a renal tubule.
During embryologic development, each renal tubule begins as a
blind-ended tubule that gradually encloses an adjactent
capillary cluster, or glomerulus. The enlarged end of the tubule
encasing the glomerulus is the glomerular (Bowman’s)
capsule, and its inner, or visceral, wall consists of highly
specialized cells called podocytes. Podocytes have long,
branching processes (foot processes) that interdigitate
with those of other podocytes and cling to the endothelial wall
of the glomerular capillaries, thus forming a very porous
epithelial membrane surrounding the glomerulus. The
glomerulus-capsule complex is sometimes called the renal
The rest of the tubule is approximately 3 cm
(1.25 inches) long. As it emerges from the glomerular capsule,
it becomes highly coiled and convoluted, drops down into a long
hairpin loop, and then again coils and twists before entering a
collecting duct. In order from the glomerular capsule, the
anatomical areas of the renal tubule are: the proximal
convoluted tubule, loop of Henle (descending and ascending
limbs), and the distal convoluted tubule. The wall of the
renal tubule is composed almost entirely of cuboidal epithelial
cells, with the exception of part of the descending limb (and
sometimes part of the ascending limb) of the loop of Henle,
which is simple squamous epithelium. The lumen surfaces of the
cuboidal cells in the proximal convoluted tubule but in greatly
reduced numbers, revealing its less significant role in
reclaiming filtrate contents.
Most nephrons, called cortical nephrons, are
located entirely within the cortex. However, parts of the loops
of Henle of the juxtamedullary nephrons (located close to
the cortex-medulla junction) penetrate well into the medulla.
The collecting ducts, each of which receives urine from
many nephrons, run downward throught eh medullary pyramids,
giving them their striped appearance. As the collecting ducts
approach the renal pelvis, they fuse to form larger papillary
ducts, which empty the final urinary product into the
calyces and pelvis of the kidney.
The function of the nephron depends on several
unique features of the renal circulation. The capillary vascular
supply consists of two distinct capillary beds, the glomerulus
and the peritubular capillary bed. Vessels leading to
and from the glomerulus, the first capillary bed, are
both arterioles: the afferent arteriole feeds the bed
while the efferent arteriole drains it. The glomerular
capillary bed has no parallel elsewhere in the body. It is a
high-pressure bed along its entire length. Its high pressure is
a result of two major factors: (1) the bed is fed and drained
by arterioles (arterioles are high-resistance vessels as
opposed to venules, which are low-resistance vessels), and (2)
the afferent feeder arteriole is larger in diameter than the
efferent arteriole draining the bed. The high hydrostatic
pressure created by these two anatomical features forces out
fluid and blood components smaller than proteins from the
glomerulus into the glomerular capsule. That is, it forms the
filtrate which is processed by the nephron tubule.
The peritubular capillary bed arises from the efferent
arteriole draining the glomerulus. This set of capillaries cling
intimately to the renal tubule and empty into the interlobular
veins that leave the cortex. The peritubular capillaries are low-pressure
very porous capillaries adapted for absorption rather than
filtration and readily take up the solutes and water reabsorbed
from the filtrate by the tubule cells. The juxtamedullary
nephrons have additional looping vessels, called the nephrons
have additional looping vessels, called the vasa recta (“straight
vessels”, that parallel their long loops of Henle in the
medulla. Hence, the two capillary beds of the nephron have very
different, but complementary, roles: The glomerulus produces the
filtrate and the peritubular capillaries reclaim most of the
Urine formation is a result of three processes: filtration,
reabsorption, and secretion (Figure 4.8 and 4.9).
Filtration, the role of the glomerulus, is
largely a passive process in which a portion of the blood passes
from the glomerular bed into the glomerular capsule. This
filtrate then enters the proximal convoluted tublue where
tubular reabsorption and secretion begin.
During tubular reabsorption, many of the
filtrate components move through the tubule cells and return to
the blood in the peritubular capillaries. Some of this
reabsorption is passive, such as that of water, which passes by
osmosis, but the reabsorption of most substances depends on
active transport processes and is highly selective. Which
substances are reabsorbed at a particular time depends on the
composition of the blood and needs of the body at that time.
Substances that are almost entirely reabsorbed from the filtrate
include water, glucose, and amino acids. Various ions are
selectively reabsorbed or allowed to go out in the urine
according to what is required to maintain appropriate blood pH
and electrolyte composition. Waste products (urea, creatinine,
uric acid, and drug metabolites) are reabsorbed to a much lesser
degree or not at all. Most (75% to 80%) of tubular reabsorption
occurs in the proximal convoluted tubule; the balance occurs in
other areas, especially the distal convoluted tubules and
Tubular secretion is essentially the reverse
process of tubular reabsorption. Substances such as hydrogen and
potassium ions and creatinine move either from the blood of the
peritubular capillaries through the tubular cells or from the
tubular cells into the filtrate to be disposed of in the urine.
This process is particularly important for the disposal of
substances not already in the filtrate (such as drug
metabolites), and as a device for controlling blood pH.
Observe photos of slides of the nephron
1) Identify a glomerulus, which appears as a
ball of tightly packed material containing many small nuclei
(Figure 4.10). It is usually delineated by a vacant-appearing
region (corresponding to the space between the visceral and
parietal layers of the glomerular capsule) that surrounds it.
2) Notice that the renal tubules are cut at various angles.
Also try to differentiate between the thin-walled loop of Henle portion of the tubules and the cuboidal epithelium of
the proximal convoluted tubule, which has dense microvilli.
|Figure 4.8 and 4.9
the nephron and label:
Glomerulus, Renal tubule, Bowman’s capsule, Afferent
arteriole, Artery, Proximal convoluted tubule, Loop of Henle,
convoluted tubule, Peritubular capillaries, Vein, Descending
loop, Collecting duct, Renal cortex, Renal medulla
Sketch of the longitudinal
section of the kidney
(Figure 4.8 and 4.9):
Renal tubules, Glomeruli, Bowman’s capsule