kidney and urinary tract disorders
Kidneys
Functions of the Kidneys
All of the functions normally done 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 for transplantation into another person with Overview of Kidney Failure . In other cases, one kidney may be severely damaged by disease or injury. Theprimary function of the kidneysis to Maintain the proper balance of water and Overview of Minerals (including electrolytes) in the body Additional kidney functionsinclude Filtration and excretion of waste products from the processing of food, drugs, and harmful substances (toxins) Regulation of blood pressure Secretion of certain hormones 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 aglomerulusat 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 space and passes into the tubule leading from Bowman 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% of the fluid 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 thetubule(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 (reabsorbed). 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. . 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 Overview of Electrolytes . 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 isurea, which comes from protein metabolism.Ureapasses 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). 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 Regulating Blood Pressure: The Renin-Angiotensin-Aldosterone System , which in turn raises blood pressure. The kidneys also produce urotensin, which causes blood vessels to constrict and helps raise blood pressure. A person with kidney failure is less able to regulate blood pressure and tends to have high blood pressure. 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 callederythropoietin, 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 ofvitamin D, which is produced in the skin and is also present in many foods, to an active form ofvitamin D(calcitriol) that acts like a hormone to stimulate absorption of calcium and phosphorus from the small intestine. 