Overview Of The Nephron
Eachkidney possesses an
average of 600,000 to 1,400,000 tubular structures called nephrons, which
contain a series of histologically distinct segments that alter the
concentration and contents of urine. The major segments of each nephron are
known as the glomerulus, proximal tubule, thin limb, distal tubule, and
collecting duct. The proximal and distal tubules are both divided into
convoluted and straight parts, while the thin limb is divided into descending
and ascending parts.
The arrangement of these different nephron segments gives rise to the two
grossly visible zones in the kidney, known as the cortex and medulla. The
medulla is divided into an outer zone (which is further subdivided into outer
and inner stripes) and an inner zone. The boundaries of these various regions
are marked by the transition sites between different nephron segments, as
described later.
Glomerulus And Proximal Convoluted Tubule
The initial formation of urine occurs at the interface between the
glomerular capillaries, which are arranged in a spherical tuft, and the first
part of the nephron, an epithelial-lined sac known as Bowman’s capsule. The
glomerular capillaries and Bowman’s capsule are together knows as the
glomerulus (or renal corpuscle). As blood from an afferent arteriole passes
through the glomerular capillaries, plasma and non–protein bound solutes are
filtered into the area bounded by Bowman’s capsule, known as Bowman’s space, to
form primitive urine. All nonfiltered blood is carried away from the glomerular
capillaries in an efferent arteriole.
Bowman’s space conveys the primitive urine to the first part of the
proximal tubule, known as the proximal convoluted tubule, which takes a very
tortuous course through a small region of the cortex. The proximal convoluted
tubule then transitions to the proximal straight tubule, which is the first part
of the loop of Henle.
Loop Of Henle
After the proximal convoluted tubule, each nephron plunges into the
medulla, makes a hairpin turn, and then returns to the cortex near its parent glomerulus.
This region of each nephron is known as the loop of Henle, and it contains the
proximal straight tubule, thin limb, and distal straight tubule (more commonly
known as the thick ascending limb).
The proximal straight tubule, described above, originates in the cortex
and courses to the border between the outer and inner stripes of the outer zone
of the medulla. It then transitions to the first part of the thin limb, known as
the descending thin limb.
The remaining structure of the loop of Henle differs based on the
location of the nephron’s parent glomerulus. In nephrons associated with
glomeruli in more superficial regions of the renal cortex, the descending thin
limb continues until reaching the border between the inner zone of the medulla
and the inner stripe of the outer zone of the medulla. At this point, it transitions
to the thick ascending limb, which makes a hairpin turn and courses back toward
the cortex.
In nephrons associated with glomeruli near the corticomedullary border
(known as juxtamedullary glomeruli), the descending thin limb plunges deep into
the medulla, makes a hairpin turn near the papilla, and continues as the
ascending thin limb until the border between
the outer and inner zones of the medulla. At this point it transitions to the thick ascending limb, which
courses back toward the cortex.
Thus, based on the above descriptions, two different populations of
nephrons can be distinguished: short- looped nephrons, which are associated
with superficial and midcortical glomeruli, and long-looped nephrons, which are
associated with juxtamedullary glomeruli. Long-looped nephrons have higher
urine-concentrating capabilities than short-looped nephrons (see Plate 3-15);
however, short-looped nephrons are far more numerous, accounting for 85% of the
total nephron population in humans.
Distal Convoluted Tubule, Connecting Tubule, And Collecting Duct
The thick ascending limb, as described in the previous section, courses
from the medulla toward the cortex, where
it transitions to the distal convoluted tubule. Near this transition point is a
specialized group of cells known as the macula densa, which make direct contact
with the nephron’s parent glomerulus.
The distal convoluted tubule, like the proximal convoluted tubule, takes
a very tortuous course within a small area of the cortex. It transitions to a
short connecting segment (or tubule), which in turn leads to the collecting
duct.
The collecting duct courses from the cortex toward the medulla adjacent to
ducts from neighboring nephrons. In the inner zone of the medulla, these
individual ducts join to form larger ducts. By a succession of several such
junctions, the papillary ducts are formed, which arrive at the cribriform area
of the papillae to drain urine into
the minor calyces.
