In the urinary system, the kidneys form urine, pass it to the bladder for storage, and excrete it. Kidneys filter blood that passes through them, then reabsorb what the body neeeds. Reabsorption recovers materials that are filtered but needed by the body.
Kidneys form urine in nephrons, which empty into collecting ducts that merge into papillary ducts. The kidneys are retroperitoneal (posterior to the peritoneal membrane of the abdominal cavity). They are covered with a renal capsule that helps maintain shape and adipose tissue that cushions and supports them. Renal fascia anchors the kidneys to the posterior abdominal wall.
The renal cortex is the outer layer of the kidneys. It extends deeply as renal columns. The renal medulla is the inner layer. It is organized into renal pyramids that end in renal papillae. The renal hilum is the entry site for the ureters and blood vessels.
Distal tubules of multiple nephrons pass the filtrate to collecting ducts for excretion. Collecting ducts empty into the renal pelvis of the kidney and enter the ureters. The flow of urine is from the papillary duct, to the renal papilla, minor calyx, major calyx, renal pelvis, and ureter.
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