Kidneys
Human kidneys derive their name from their unique shape. The bi-lateral organ structures are kidney bean-shaped, and are located at the back of the abdominal cavity (retroperitoneal). In most adults they are small--about the size of the average adult fist--but this size is not indicative of their importance. The kidney's role in the body's chemistry and operation is absolutely vital to life.
An estimated 1,000 milliliters (mL) of fluid flows through the kidneys each minute. The water that is reconditioned by the kidneys is responsible for maintaining the proper balance of acid and base and of ions (such as sodium and potassium) in cellular and tissue fluids and in blood plasma. Kidneys also keep the volume of water in the body constant, help regulate blood pressure, and stimulate the production of red blood cells via hormone signals. Abnormalities in the ionic balances can cause functional disruption to organs, specialized physiological functions, and illness.
Blood enters the kidney through the renal artery, is processed, and exits via the renal vein. The removed waste flow down channels called the urethers to the bladder for storage. When the bladder is full, the collected urine is excreted from the body through the urethra (which, in males, empties via the penis).
The kidney consists of four regions; the renal capsule (a thin, protective, outer membrane), cortex (a light colored outer region), medulla (a darker, inner region), and the renal pelvis (a flat, funnel-shaped cavity that collect the urine into the urethers). Within the cortex and medulla are millions of tiny structures called nephrons. These are the basic units of operation of the kidney. A nephron is constructed of a long thin tube (collecting duct), closed at one end (Bowman's capsule), that has two twisted regions (proximal and distal tubules) at either end of a hairpin loop (Loop of Henle). The nephron is surrounded by capillaries.
The filtration of fluids takes place in the nephrons. The fluids are passed under pressure though he walls of the capillaries and Bowman's capsule. Once insider the nephron, small molecules such as ions, glucose and amino acids are selectively reabsorbed by specialized proteins, called transporters, in an active energy-demanding process. Water reabsorption occurs passively. The molecules and water are routed back into circulation.
The kidney can adjust the pace of the filtering activity in relation to the volume of ingested water based on the level of a hormone called anti-diuretic hormone. The greater this hormone's concentration, the more water is able to pass through the kidney.
Malfunction of the kidney is serious. Each year in the United States more than three million people have some type of kidney condition or renal insufficiency caused by an infection, kidney stones, or cancer. Over 300,000 people suffer from kidney failure each year, necessitating kidney dialysis (where fluid is mechanically removed from the body, filtered to remove waste, and put back into the body) or removal and replacement of the defective kidney.
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