The excretory system is a system of organs that removes waste products from the body. When cells in the body break down proteins (large molecules that are essential to the structure and functioning of all living cells), they produce wastes such as urea (a chemical compound of carbon, hydrogen, nitrogen, and oxygen). When cells break down carbohydrates (compounds consisting of carbon, hydrogen, and oxygen and used as a food), they produce water and carbon dioxide as waste products. If these useless waste products are allowed to accumulate in the body, they would become dangerous to the body's health. The kidneys, considered the main excretory organs in humans, eliminate water, urea, and other waste products from the body in the form of urine.
Other systems and organs in the body also play a part in excretion. The respiratory system eliminates water vapor and carbon dioxide through exhalation (the process of breathing out). The digestive system removes feces, the solid undigested wastes of digestion, by a process called defecation or elimination. The skin also acts as an organ of excretion by removing water and small amounts of urea and salts (as sweat).
The kidneys are bean-shaped organs located at the small of the back near the spinal column. The left kidney sits slightly higher than the right one. The size of an adult kidney is approximately 4 inches (10 centimeters) long and 2 inches (5 centimeters) wide. To maintain human life, it is necessary for at least one of the kidneys to function properly.
Blood carries waste products to the kidneys via the renal artery. Inside each kidney, blood is transported to 1.2 million filtering units called nephrons (pronounced NEFF-rons). The cells in nephrons take in the liquid portion of the blood and filter out impurities (urea, mineral salts, and other toxins). Necessary substances such as certain salts, water, glucose (sugar), and other nutrients are returned to the blood stream via the renal vein.
Antidiuretic hormone: Chemical secreted by the pituitary gland that regulates the amount of water excreted by the kidneys.
Hemodialysis: Process of separating wastes from the blood by passage through a semipermeable membrane.
Nephron: Filtering unit of the kidney.
Urea: Chemical compound of carbon, hydrogen, nitrogen, and oxygen produced as waste by cells that break down protein.
Ureter: Tube that carries urine from a kidney to the urinary bladder.
Urethra: Duct leading from the urinary bladder to outside the body through which urine is eliminated.
The waste-containing fluid that remains in the nephrons is called urine. Urine is 95 percent water, in which the waste products are dissolved. A pair of tubes called ureters carry urine from the kidneys to the urinary bladder. Each ureter is about 16 to 18 inches (40 to 45 centimeters) long. The bladder is a hollow muscular sac located in the pelvis that is collapsed when empty, but pear-shaped and distended when full. The bladder in an adult can hold more than 2 cups (0.6 liters) of urine. The bladder empties urine into the urethra, a duct leading to outside the body. In males, the urethra is about 8 inches (20 centimeters) long. In females, it is less than 2 inches (5 centimeters) long. A sphincter muscle around the urethra at the base of the bladder controls the flow of urine between the two.
The volume of urine excreted is controlled by the antidiuretic hormone (ADH), which is released by the pituitary gland (a small gland lying at the base of the skull). If an individual perspires a lot or fails to drink enough water, special nerve cells in the hypothalamus (a region of the brain controlling body temperature, hunger, and thirst) detect the low water concentration in the blood. They then signal the pituitary gland to release ADH into the blood, where it travels to the kidneys. With ADH present, the kidneys reabsorb more water from the urine and return it to the blood. The volume of urine is thus reduced. On the other hand, if an individual takes in too much water, production of ADH decreases. The kidneys do not reabsorb as much water, and the volume of urine is increased. Alcohol inhibits ADH production and therefore increases the output of urine.
Disorders of the urinary tract (kidneys, ureters, bladder, and urethra) include urinary tract infections (UTI). An example is cystitis, a disease in which bacteria infect the urinary bladder, causing inflammation. Most UTIs are treated with antibiotics. Sometimes kidney stones, solid salt crystals, form in the urinary tract. Kidney stones can obstruct the urinary passages and cause severe pain and bleeding. If they do not pass out of the body naturally, a physician may remove them surgically or disintegrate them using shock waves.
Chronic renal failure is the permanent loss of kidney function. Hemodialysis and kidney transplant are two medical treatments for this condition. In hemodialysis, an artificial kidney device cleans the blood of wastes. During the procedure, blood is taken out of an artery in the patient's arm and passed through a tubing that is semipermeable (allows certain materials to pass through its sides). The tubing is immersed in a solution. As the blood passes through the tubing, wastes pass out of the tubing and into the surrounding solution. The cleansed blood then returns to the body. In a kidney transplant, a surgeon replaces a diseased kidney with a closely matched donor kidney. Although about 23,000 people in the United States wait for donor kidneys each year, fewer than 8,000 receive kidney transplants.