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8-3. THE KIDNEY

a. General.

(1) The kidneys have the same shape and color as kidney beans, but are about 8-10 centimeters (3-3 1/2 inches) in length.

(2) Each kidney has a fibrous capsule. On the concave, medial side of each kidney, there is a notch called the hilus. Through this hilus pass the ureter and the NAVL (nerve, artery, vein, and lymphatic) which service the kidney.

(3) Each kidney is attached to the posterior wall of the abdominal cavity, just above the waistline level. Each is held in place by special fascia and fat.

b. Gross Internal Structure. If we compare the structure of the kidney with that of a cantaloupe (muskmelon), the renal cortex would correspond to the hard rind, the renal medulla would correspond with the edible flesh of the melon, while the renal sinus would correspond to the hollow center (after the seeds have been removed). The medulla consists of pyramids with their bases at the cortex and forming peaks, papillae, which empty into the sinus.

PAPILLA = pimple, nipple

See figure 8-2 for a section of the kidney showing the inner structure.

c. The Nephron. See figure 8-3 for an illustration of a nephron. Nephrons are the functional units of the human kidney. Their primary function is to remove the wastes of protein usage from the blood. In addition, they serve to conserve water and other materials for continued use by the body. The end result of nephron function is a more or less concentrated fluid called urine. The kidneys contain great numbers of nephrons, about a million for each kidney. The main subdivisions of a nephron are the renal corpuscle and a tubular system.

(1) Renal corpuscle. The renal corpuscle has a hollow double- walled sac called the renal capsule ("Bowman's capsule"). Leading into the capsule is a very small artery called the afferent arteriole. Within the capsule, this artery becomes a mass of capillaries known as the glomerulus. An efferent arteriole drains the blood away from the capsule. The capsule and the glomerulus together are known as the renal corpuscle.

(2) Tubules. Each renal capsule is drained by a renal tubule. The first part of this tubule runs quite a distance in a coiled formation and is called the proximal convoluted tubule. A long loop, the renal loop (of Henle), extends down into the medulla with two straight parts and a sharp bend at the bottom. As the tube returns to the cortex layer, it once again becomes coiled and here is known as the distal convoluted tubule.

(3) Filtration/reabsorption. Except for the blood cells and the larger proteins, the fluid portion of the blood passes through the walls of the glomerulus into the cavity between the two layers of the renal capsule. This fluid is called the glomerular filtrate. By a process of taking back (resorption), the majority of the fluid is removed from the tubules and the concentrated fluid is called the urine.

d. The Collecting Tubule. The distal convoluted tubules of several nephrons empty into a collecting tubule. The urine is then passed from the collecting tubule at the papilla of the medullary pyramid. Several collecting tubules are present in each pyramid.

e. Renal Pelvis. The renal pelvis is a hollow sac within the sinus of the kidney. Urine from the pyramids collects into the funnel-shaped renal pelvis. The ureter then drains the urine from the renal pelvis.

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