Visibly Human Health and Disease in the Human Body

The Urinary System

In the human adult water makes up approximately 60% of body weight. The kidney maintains the volume and composition of fluid within the body.

The kidneys are paired, bean-shaped organs that lie at the back of the abdominal cavity on either side of the spinal column. Each adult kidney weighs about 150 grams (6 ounces).

The kidneys have several important functions, making them more than simple "garbage disposal" organs that rid the body of wastes. Functions include fluid regulation, hormone synthesis and nitrogen excretion. The kidneys also filter some drugs and their end products, food additives and other foreign substances from the body.

Our kidneys regulate the amount of water, salt, sugars, proteins and other constituents in our bodies by filtering blood plasma and reabsorbing into the blood those substances the body needs.

The kidneys release three important hormones: erythropoietin affects red blood cell production and renin and kallikrein influence blood pressure.

The ureters are drainage tubes that open into the wall of the bladder. Their openings are normally closed by the pressure of the urine in the bladder. They open only when urine is forced into them from the renal pelvis.

The bladder is the holding tank for the urinary system. It resembles a balloon. When it is empty, it is wrinkled and collapsed. As the bladder begins to fill, the first urge to urinate is felt when the bladder contains about 150 ml (5 oz). The pressure is greatest when the bladder is "full" at about 400 ml (13 oz).

Urine passes out of the body via the urethra in both males and females. The release of the contents of a full bladder is mainly controlled by a decrease in levels of antidiuretic hormone or ADH which occurs when increased amounts of water are absorbed in the blood.

Normal kidney, plastinated. NMHM 1998.0033.43, NMHM 1990.0099.10	Atherosclerosis The renal arteries can be affected by the buildup of thickened fatty plaque in the walls of arteries which constrict vessels and disrupts the blood supply and nutrient flow to the kidneys. Excessive deposits of plaque can result in tissue death and kidney failure. NMHM 1987.0017.63
Renal cell carcinoma Renal cell carcinoma is the most common kidney cancer in adults. The large white-colored tumor is clearly delineated. Tumors such as this have a tendency to spread to the lungs, liver, central nervous system, thyroid and bone. They are more common in men than women, usually occurring after age 50. AFIP 641030	Kidney stones Urinary "stones" may occur anywhere in the urinary tract and are common causes of obstruction and infection. They are composed primarily of inorganic crystals of calcium, phosphates and other minerals that normally pass harmlessly through the system. Dehydration can contribute to the formation of stones when reduced urine volume leads to an increased rate of excretion of stone-forming crystals.
Horseshoe kidney This anatomical variant results when the two kidneys fuse during early embryonic development as the kidneys migrate to their anatomical position. The horseshoe shape gives the condition its name. One third of individuals with this developmental anomaly have complications such as hydronephrosis, kidney stones, and cancer. AFIP 24162	Duplicated ureters Duplication of the ureters and renal pelvis is a congenital abnormality. The condition arises during embryonic development. In most cases this is a harmless variant, though it may contribute to ureteropelvic obstruction, hydronephrosis and pyleonephritis. AFIP 13498, NMHM 1998.0033.17