Biology of the Kidneys and Urinary Tract | Merck Manual Consumer Version

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The kidneys are bean-shaped organs that figure prominently in the urinary tract. Each is about 4 to 5 inches (12 centimeters) long and weighs about one third of a pound (150 grams). One lies on each side of the spinal column, just behind the abdominal cavity, which contains the digestive organs.
Each kidney receives blood through a branch of the aorta, called the renal artery. Blood flows from the renal artery into progressively smaller arteries, the smallest being the arterioles. From the arterioles, blood flows into glomeruli, which are tufts of microscopic blood vessels called capillaries. Blood exits each glomerulus through an arteriole that connects to a small vein. The small veins join to form a single large renal vein, which carries blood away from each kidney.
Nephrons are microscopic units that filter the blood and produce urine. Each kidney contains about one million nephrons. Each nephron contains a glomerulus surrounded by a thin-walled, bowl-shaped structure (Bowman capsule). Also in the nephron is a tiny tube (tubule) that drains fluid (that soon becomes urine) from the space in Bowman capsule (Bowman space). Each tubule has three interconnected parts: the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. A third part of the nephron is a collecting duct that drains the fluid from the tubule. After the fluid leaves the collecting duct it is considered urine.
The kidneys consist of an outer part (cortex) and an inner part (medulla). All glomeruli are located in the cortex, while tubules are located in both the cortex and the medulla. The urine drains from the collecting ducts of many thousands of nephrons into a cuplike structure (calix). Each kidney has several calices, all of which drain into a single central chamber (renal pelvis). Urine drains from the renal pelvis of each kidney into a ureter.
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