ANATOMY OF THE KIDNEYS AND URINARY SYSTEM

The 2 kidneys lie outside the peritoneal cavity close to the posterior abdominal wall, 1 on each side of the vertebral column. Each of the 2 kidneys is a beanshaped structure. The rounded, outer convex surface of each kidney faces the side of the body, and the indented surface, called the hilum, is medial. Each hilum is penetrated by a renal artery, renal vein, nerves, and a ureter, which carries urine out of the kidney to the bladder. Each ureter within a kidney is formed from funnel-like structures called major calyces, which, in turn, are formed from minor calyces. The minor calyces fit over underlying cone-shaped renal tissue called pyramids. The tip of each pyramid is called a papilla and projects into a minor calyx. The calyces act as collecting cups for the urine formed by the renal tissue in the pyramids. The pyramids are arranged radially around the hilum, with the papillae pointing toward the hilum and the broad bases of the pyramids facing the outside, top, and bottom of the kidney (from the 12-o’clock to the 6-o’clock position). The pyramids constitute the medulla of the kidney. Overlying the medullary tissue is a cortex, and covering the cortical tissue on the very external surface of the kidney is a thin connective tissue capsule.
The working tissue mass of both the cortex and medulla is constructed almost entirely of tubules (nephrons and collecting tubules) and blood vessels (capillaries and capillary-like vessels). Tubules and blood vessels are intertwined (something like a plateful of spaghetti) or arranged in parallel arrays (like bundles of soda straws) and, in either case, are always close to each other. Between the tubules and blood vessels lies an interstitium, which comprises less than 10% of the renal volume. The interstitium contains fluid and scattered interstitial cells (fibroblasts and others) that synthesize an extracellular matrix of collagen, proteoglycans, and glycoproteins.
The cortex and medulla have very different properties both structurally and functionally. On closer examination, we see that (1) the cortex has a highly granular appearance, absent in the medulla, and (2) each medullary pyramid is divisible into an outer zone (adjacent to the cortex) and an inner zone, which includes the papilla. All these distinctions reflect the arrangement of the various tubules and blood vessels.

A, The urinary system. The urine formed by a kidney collects in the renal pelvis and then flows through the ureter into the bladder, from which it is eliminated via the urethra. B, Section of a human kidney. Half the kidney has been sliced away. Note that the structure shows regional differences. The outer portion (cortex) contains all the glomeruli. The collecting ducts form a large portion of the inner kidney (medulla), giving it a striped, pyramid-like appearance, and these drain into the renal pelvis. The papilla is in the inner portion of the medulla.