KIDNEYS

A. General Organization: The kidneys are bean-shaped retroperitoneal organs encapsulated by dense connective tissue and surrounded by adipose tissue. Several components can be distinguished without the aid of a microscope.

1. Renal sinus. This medial concavity of each kidney contains the renal pelvis, the entering and exiting blood vessels and nerves, and adipose tissue.
2. Hilum, This consists of the renal sinus and its contents.
3. Cortex, This is the kidney's dark-staining outer region; it underlies the capsule. It contains the renal corpuscles, proximal and distal convoluted tubules, peritubular capillaries, and medullary rays.
4. Medulla, This is the kidney's light-staining inner region, which partly surrounds the renal sinus. It consists of 8-18 conical medullary pyramids whose bases abut the cortex and whose apices (renal papillae) point inward, toward the renal sinus. It also contains the collecting ducts, loops of Henle, and vasa recta. Each renal papilla, perforated by openings of the collecting ducts, is cradled by a minor calyx into which the ducts empty. Several minor calyces empty into a major calyx. The major calyces empty into the renal pelvis, which in turn drains into the ureter. 5. Medullary rays. These fingerlike extensions of medullary tissue that enter the cortex comprise clusters of collecting tubules and ducts. One medullary ray occupies the center of each renal lobule.
6. Renal lobes, Each human kidney has 8-18 lobes, the kidney's largest subdivisions. Each lobe, which consists of a medullary pyramid and its associated cortex, contains numerous renal lobules.
7. Reual lobules, Each of these subdivisions of the lobes consists of a central medullary ray and all the nephrons that empty into its collecting tubules. The borders between adjacent renal lobules are marked by interlobular arteries and veins