8 The Excretory System
What is Excretory System?
8.1 Your excretory system removes the waste products of metabolism from your body. It also removes excess water and minerals. Your kidneys are the main organ of excretion, but your respiratory and digestive systems also play a role. In addition, a small amount of waste products are excreted by the sweat glands.
The Kidneys and Urinary Tract
8.2 In addition to its role in digestion, the liver converts the toxic waste products of metabolism into less toxic ones: urea and uric acid. These and other toxic substances in the bloodstream must be removed. The renal arteries carry blood containing toxic substances to the kidneys. The kidneys remove urea and uric acid from the bloodstream and turn them into the liquid called urine. Peristaltic contractions move urine from the kidneys through the ureters to the bladder. Urine collects in the bladder until it leaves the body through the urethra.
8.3 In addition to forming urine, the kidneys maintain the body’s internal chemical balance by first removing water and minerals from the bloodstream and then reabsorbing the amount of water and minerals needed by the body. The clean blood with its adjusted chemical composition then leaves the kidneys through the renal veins. The kidneys are amazingly efficient; one healthy kidney can easily meet the body’s needs.
8.4 Each kidney contains a little over 1 million blood-filtering units called nephrons. Nephrons consist of a tiny tube, or tubule, with a cup-shaped structure at one end, and they produce urine in a two-phase process. First, filtration occurs. Blood coming from the renal artery flows into the capillary network in the cup. Pressure forces water and small molecules, such as urea, out of the capillaries and into the long tubule connected to the cup. Second, reabsorption takes place. About 99 percent of the water and some of the other substances that were filtered out of the blood move into capillaries that surround the tubule. The fluid that remains in the tubule is urine. The tubules join to form larger tubes, which connect with the ureter where it joins the kidney. Urine is a concentrated mixture of water, urea, and various mineral salts. Complex mechanisms regulate urine formation, and the contents of the urine can vary depending on the state of the body. This is why an analysis of the urine, called a urinalysis, is so useful in diagnosing some diseases.