Structure of the Nephron
The functional unit of the kidney is the nephron, which consists of a renal tubule and its associated bloodvessels (Figure 40.10b and c). Each kidney contains a large number of nephrons-about a million in a human kidney, representing a total of about 80 km of tubules. Water, urea, salts, and other small molecules present in blood flow from capillaries into the renal tubule, where the fluid is now called filtrate. The transport epithelium lining the renal tubule processes the filtrate to form the urine, which is eventually excreted from the kidney.
From the 1100-2000 L of blood that flows through the human kidneys each day, the nephrons process about 180 L of filtrate, but excrete only about 1.5 L of urine. The rest of the filtrate, including about 99% of the water, is reabsorbed into the blood, The blind end of the renal tubule, which receives filtrate from the blood, is expanded to form a cup-shaped receptacle called Bowman's capsule, which embraces a ball of capillaries, the glomerulus.
From Bowman's capsule, the filtrate passes successively through three main regions of the renal tubule: (1) the proximal convoluted tubule; (2) the loop of Henle, a long hairpin turn with a descending limb ana an ascending limb; and (3) the distal convoluted tubule. This last portion of the renal tubule empties its filtrate into a collecting duct, which receives filtrate from many other renal tubules.
The many collecting ducts of the kidney then pass the filtrate, now called urine, into the renal pelvis, a chamber that in turn drains into the ureter.
The nephrons have a radial orientation in the kidney, with the loops of Henle and collecting ducts perpendicular to the kidney surface. Bowman's capsules, proximal convoluted tubules, and distal convoluted tubules are located in the outer zone of the kidney, the cortex. In the human kidney, about 80% of the nephrons, termed cortical nephrons, have reduced loops of Henle, and are almost entirely confined to the cortex. The other 20% of the nephrons, the jux-tamedullary nephrons, have well-developed loops that extend to the inner zone of the kidney, the medulla. Only mammals and birds have juxtamedullary nephrons; loops of Henle are absent in the nephrons of the other vertebrate classes. As we shall soon see, the juxtameduhary nephrons play an important role in the ability of mammals to excrete a urine that is hyper-osmotic to body fluids, an adaptation that conserves water.
Each nephron is supplied with blood by an afferent arteriole, a branch of the renal artery that subdivides to form the capillaries of the glomerulus. The capillaries converge as they leave the capsule to form an efferent arteriole, but then this vessel subdivides again into a second network of capillaries, the peritubular capillaries. These capillaries intermingle with the proximal and distal convoluted tubules of the nephron. Additional capillaries extend downward to form the vasa recta, the capillary system that serves the loop of Henle. The vasa recta is also a loop, with a descending vessel and an ascending vessel conveying blood in opposite directions.
Although the renal tubule and its surrounding capillaries are closely associated, they do not exchange materials directly across their walls; the tubules and capillaries are immersed in interstitial fluid, through which various substances pass back and forth between the plasma within capillaries and the filtrate within the nephron tubules.
Keeping in mind the correlation of structure and function, let us next investigate how the complex organization of the renal tubule and its blood vessels explains how the nephrons work.