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.