Location and external anatomy
The bean-shaped kidneys lie in retroperitoneal position (between the dorsal body wall and the parietal peritoneum) in the superior lumbar region. Extending approximately from T12 to L3, the kidneys receive some protection from the lower part of the rib cage. The right kidney is crowded by the liver and lies slightly lower than the left. An adult kidney has a mass about 150g (5 ounces) and its average dimensions are 12 cm long, 6cm wide and 3 cm thick – about the size of a large bar of soap. The lateral surface is convex. The medial surface is concave and has a vertical cleft called the renal hilum that leads into an internal space within the kidney called the renal sinus. The ureter, renal blood vessels, lymphatics and nerves all join each kidney at the hilum and occupy the sinus. Atop each kidney is an adrenal (or suprarenal) gland, an endocrine gland that is functionally unrelated to the kidney.
Three layers of supportive tissue surround each kidney:
- The renal fascia, an outer layer of dense fibrous connective tissue that anchors the kidney and the adrenal gland to surrounding structures
- The perineal fat capsule, a fatty mass that surrounds the kidney and cushions it against blows
- The fibrous capsule, a transparent capsule that prevents infections in surrounding regions from spreading to the kidney
The fatty encasement of the kidneys is important in holding the kidneys in their normal body position. If the amount of fatty tissue dwindles (as with extreme emaciation or rapid weight loss), one or both kidneys may drop to a lower position, an event called renal ptosis. renal ptosis may cause a ureter to become kinked, which creates problems because the urine, unable to drain, backs up into the kidney and exerts pressure on its tissue. Back up of urine from ureteral obstruction or other causes is called hydronephrosis. Hydronephrosis can severely damage the kidney, leading to necrosis (tissue death) and renal failure.
A frontal section through a kidney reveals three distinct regions: cortex, medulla and pelvis. The most superficial region, the renal cortex, is light in color and has a granular appearance. Deep to the cortex is the darker, reddish-brown renal medulla, which exhibits cone-shaped tissue masses called medullary or renal pyramids. The broad base of each pyramids faces toward the cortex, and its apex, or papilla, points internally. The pyramids appear striped because they are formed almost entirely of parallel bundles of microscopic urine-collecting tubules and capillaries. The renal columns, inward extensions of cortical tissue, separate the pyramids. Each pyramid and its surrounding cortical tissue constitutes one of approximately eight lobes of a kidney.
The renal pelvis, a funnel-shaped tube, is continuos with the ureter leaving the hilum. Branching extensions of the pelvis form two or three major calyces. Each one subdivides to form several minor calyces, cup-shaped areas that enclose the papillae.
The calyces collect urine, which drains continuosly from the papillae, and empty it into the renal pelvis. The urine then flows through the renal pelvis and into ureter, which moves it to the bladder to be stored. The walls of the calyces, pelvis and ureter contain smooth muscles that contracts rhytmically to propel urine along its course by peristalsis.