Kidney Anatomy

Human kidneys are paired, reddish brown, bean shapped structures about 11 cm (4.4 in) long and function as excretory system. They are located in back of the body cavity, one on each side of the spine just above the waist. The kidneys are loosely held in place by a mass of fat and by fibrous tissue. The outer margin is convex, the inner border concave. On the inner surface is a slit, the hilus, through which pass the arteries, veins, nerves and the renal pelvis, a funnellike structure. Urine from each kidney is collected in the renal pelvis and passes into the hollow tube, the ureter, which extends downward, emptying into the urinary bladder. A shorter, single tube, the urethra, eliminates urine the bladder.

The cut surface of the kidney reveals two distinct areas; the cortex - a dark band along the outer border, about 1 cm (0.4 in) thickness, and the inner medulla. The medulla is divided into 8 to 18 conical tissues termed renal pyramids (cortical arches) and extends down between each pyramid as the renal columns.

Nephrons:
Each kidney contains at least 1 million microscopic structures called nephrons, or renal tubules. These are the urine forming units. Each nephron has thin walls and is divided into a Browman's capsule and a tubule. The nephron begins with a renal corpuscle, which is a granulus. A winding, con voluted section of the nephron tubule. The region is followed by Henle's loop, which extends into the medulla and loops back to the cortex. The final section, the distal convulted tubule, is located near the proximal convoluted portion. It continues on to connect other tubules in a branching structure, the collecting tubules, which descend into the medulla and termnate at the papillae, thus conducting the urine to the renal pelvis.

Blood Circulation:
Arterial blood enters the kidneys, at the hilus, by renal arteries, which subdivide into smaller and yet smaller arteries, and finally into afferent arteries. They lead into the 30 to 40 capillary loops of each glomerulus. Blood is recollected from each glomerulus by the efferent arterioles which are smaller in diameter than the afferent arterioles and therefore create a relatively higher backup blood pressure in the capillary of the glomerulus than in other capillaries of the body. This is significant in the efficient filtering function of the nephron. The efferent vessel divide into capillaries surround the tubules, thus supplying blood to the medulla. Eventually they rejoin into vein and exit through the renal hilus.

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