Section 11. Kidney and Urinary Tract Disorders

Chapter 141. Biology of the Kidneys and Urinary Tract

Topics: Introduction | Kidneys | Ureters | Bladder | Urethra | Effects of Aging

Kidneys

The kidneys are bean-shaped organs, each about 4 to 5 inches long. One lies on each side of the spinal column, just behind the space that contains the digestive organs (abdominal cavity). 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 vessels that are called capillaries. Blood exits each glomerulus through another arteriole, which connects to a small vein. The small veins join to form a single large renal vein, which carries blood away from each kidney.

See the figure Viewing the Urinary Tract.

Nephrons are microscopic units that filter the blood and produce urine. Each kidney contains about one million nephrons. Each nephron consists of a glomerulus surrounded by a thin-walled, bowl-shaped structure (Bowman's capsule), a tiny tube (tubule) that drains fluid from a space in Bowman's capsule, and a collecting duct that drains urine (the fluid becomes urine by the time it reaches the collecting ducts) 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 (distal convoluted tubule) and an inner part (medulla). All of the glomeruli are located in the cortex, while the tubules reside 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.

The Kidney's Functions

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.

Filtration and Excretion of Waste Products: The primary functions of the kidneys are to filter the blood and eliminate (excrete) metabolic waste products and excess water and electrolytes (for example, sodium, potassium, chloride, glucose, and bicarbonate). In addition, many drugs are excreted by the kidneys.

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 the first part of the 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, sodium, potassium, and chloride are pumped out, and the remaining fluid becomes increasingly dilute. The dilute fluid passes through the next part of the tubule, where more 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, fluid (now urine) may continue as dilute urine, or water can be absorbed from the urine and returned to the blood, making the urine more concentrated. Antidiuretic hormone, which is produced by the pituitary gland, and other hormones help to regulate kidney function and control urine composition to maintain water and electrolyte balance.

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 Section 3, Chapter 22). 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 the red blood cells into the bloodstream.

Growth and maintenance of healthy bones is a complex process that depends on several organ systems of the body, including the kidneys. The kidneys help regulate levels of calcium and phosphorus, the minerals that are critical to bone health. They do so by converting an inactive form of vitamin D (a type of hormone), which is produced in the skin and is also present in many foods, to an active form of vitamin D (calcitriol). Calcitriol then stimulates absorption of calcium and phosphorus from the small intestine.