Chapter XXV

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The urinary system clears the body of liquid waste through filtration and maintains the homeostasis (Grk. lit. "same standing") of water, pH, blood pressure, and other such factors. The process begins when blood is pumped via the heart to the kidneys, which are located in the retroperitoneum, that is, the membrane that envelops (“stretches around”) the abdominal cavity; the etymology of “kidney” is debated, but the Latin word for this organ, ren (pl. renes), gives us the adjectival form “.” Each kidney is surrounded by the , the protective tissue “around the kidney.”

Under this is the renal cortex, where filtration occurs. Embedded in the renal cortex are very many tiny bodies known as “nephrons” (or “”; note the variation in plural forms, for which see pp. 256-57). At the end of each nephron is the renal corpuscule, which is made up of the (Lat. “little clump”) surrounded by a glomerular capsule. This structure is a tuft of capillaries where fluid and waste are filtered out and sent into a tubule. Here any nutrients can be reabsorbed by the blood, while waste continues to move toward a collecting duct in the renal medulla, the portion of the kidney beneath the cortex, where it travels through structures known as the “renal ,” which have a striated appearance due to the number of tubules passing through them (these are surrounded by the renal calices, from the Greek calyx, “pod, husk”). At this point the liquid waste, urine, is collected into a basin called the “renal .” During this filtration process, the kidneys also manipulate the amount of water absorbed in order to maintain proper pH levels and blood pressure, and they regulate the concentration of sodium, hydrogen, potassium, and other ions in the body.

Urine passes from the renal pelvis into the , a tube that transports the waste matter to the bladder, which in Latin is known as the urinaria (a valve at the end prevents backflow), from which it may then pass through the and out of the body. When the bladder collects a certain amount of urine, the brain receives a signal that urination is needed, and this signal increases in intensity with the gradual accumulation of urine in the bladder. The muscle (Lat. “muscle that pushes downward”) contracts in order to empty the bladder.