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The kidneys are bean-shaped organs, each about 4 to 5 inches (12 centimeters) long. 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.

Viewing the Urinary Tract

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's capsule). Also in the nephron is a tiny tube (tubule) that drains fluid (now considered urine) from the space in Bowman's capsule (Bowman's space). A third part of the nephron is a collecting duct that drains urine from the tubule. Each tubule has three interconnected parts: the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. 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.

FUNCTIONS OF THE KIDNEYS

All of the functions normally performed by two kidneys can be carried out adequately by one healthy kidney. Some people are born with only one kidney and others choose to donate one kidney. In other cases, one kidney may be severely damaged by disease or injury.

The primary function of the kidneys is to maintain the proper balance of water and minerals (including electrolytes) in the body. Additional functions include filtration and excretion of waste products from the processing of food, drugs, and harmful substances (toxins); regulation of blood pressure; and secretion of certain hormones.

Water and Electrolyte Balance: People consume water regularly in order to maintain life. More water is produced by the processing (metabolism) of food. If the amount of water added to the body is not matched by an equal amount going out, water accumulates rapidly and the person becomes ill and may even die. Excess water dilutes the body's electrolytes, whereas water restriction concentrates them. The body's electrolytes must be maintained at very precise concentrations. The kidneys regulate and help maintain the proper balance of water and electrolytes.

Blood enters a glomerulus at high pressure. Much of the fluid part of blood is filtered through small pores in the glomerulus, leaving behind blood cells and most large molecules, such as proteins. The clear, filtered fluid enters Bowman's space and passes into the tubule leading from Bowman's capsule. In healthy adults, about 47 gallons (180 liters) of fluid is filtered into the kidney tubules each day. Nearly all this fluid (and the electrolytes contained in it) is reabsorbed by the kidney; only about 1.5 to 2% is excreted as urine. For this reabsorption to occur, different parts of the nephron actively secrete and reabsorb different electrolytes, which pull the water along, and other parts of the nephron vary their permeability to water, allowing more or less water to return to the circulation. The details of these processes are a bit complicated.

In the first part of the tubule (the proximal convoluted tubule) most of the sodium, water, glucose, and other filtered substances are reabsorbed and ultimately returned to the blood. In the next part of the tubule (the loop of Henle), sodium, potassium, and chloride are pumped out; thus the remaining fluid becomes increasingly dilute. The dilute fluid passes through the next part of the tubule (the distal convoluted tubule), where most of the remaining sodium is pumped out in exchange for potassium and acid, which are pumped in.

Fluid from the tubules of several nephrons enters a collecting duct. In the collecting ducts, the fluid may remain dilute, or water can be absorbed from the fluid and returned to the blood, making the urine more concentrated. Water reabsorption is regulated by antidiuretic hormone (produced by the pituitary gland) and other hormones. These hormones help to regulate kidney function and control urine composition to maintain body water and electrolyte balance.

Filtration and Excretion: As the body metabolizes food, certain waste products are created, and these products need to be removed from the body. One of the main waste products is urea, which comes from protein metabolism. Urea passes freely through the glomerulus into the tubular fluid and, because it is not reabsorbed, is passed into the urine.

Other undesirable substances, including metabolic waste products such as acids, and many toxins and drugs, are actively secreted into the urine by cells in the renal tubule (and give urine its characteristic odor).

Regulation of Blood Pressure: Another function of the kidneys is to help regulate the body's blood pressure by excreting excess sodium. If too little sodium is excreted, blood pressure is likely to increase. The kidneys also help regulate blood pressure by producing an enzyme called renin. When blood pressure falls below normal levels, the kidneys secrete renin into the bloodstream, thereby activating the renin-angiotensin-aldosterone system, which in turn raises blood pressure (see High Blood Pressure: Regulating Blood Pressure: The Renin-Angiotensin-Aldosterone System). A person with kidney failure is less able to regulate blood pressure and tends to have high blood pressure.

Secretion of Hormones: Through the secretion of hormones, the kidneys help regulate other important functions, such as the production of red blood cells and the growth and maintenance of bones.

The kidneys produce a hormone called erythropoietin, which stimulates the production of red blood cells in the bone marrow. The bone marrow then releases red blood cells into the bloodstream.

Growth and maintenance of healthy bones is a complex process that depends on several organ systems, including the kidneys. The kidneys help regulate levels of calcium and phosphorus, minerals that are critical to bone health. They do so by converting an inactive form of vitamin D

which is produced in the skin and is also present in many foods, to an active form of vitamin D

( calcitriol

) that acts like a hormone to stimulate absorption of calcium and phosphorus from the small intestine

Ureters

The ureters are muscular tubes—about 16 inches (40 centimeters) long—that attach at their upper end to the kidneys and at their lower end to the bladder.

Urine formed in the kidneys flows down the ureters into the bladder, but it does not flow passively. The ureters push each small amount of urine along in waves of contraction, at low pressure. At the bladder, each ureter passes through an opening in the bladder wall, which closes when the bladder contracts to prevent urine from flowing back into the ureter (reflux).