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EXCRETION - Ch. 19DATE______
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EXCRETION
The metabolic activities of the cell of the body result in the production of various waste products, which are called metabolic wastes. In high concentrations such wastes are quite poisonous, thus they must be gotten rid of quickly and efficiently. Excretion is the process by which an organism eliminates its metabolic wastes.
Metabolic wastes include carbon dioxide, salts, water, and nitrogenous wastes. The nitrogenous wastes--urea, uric acid, and ammonia--are the nitrogen-containing end products of protein metabolism. Urea is the most important nitrogenous waste. Uric acid and ammonia are present in relatively small quantities. Notice that the undigested food and other materials that make up the feces are not metabolic wastes. They have merely passed through the alimentary canal without becoming involved in cellular metabolism.
We have already discussed the elimination of carbon dioxide from the body through the lungs. There is also a loss of water vapor from the body through the lungs. This loss is the result of evaporation.
The skin also functions to some extent in excretion. Water and some salts are lost through the skin in the process of sweating. However, sweating is primarily a mechanism for regulating body temperature. The evaporation of water from the skin cools the body.
Most of the nitrogenous wastes of the body are produced in the liver. These and other waste products are then removed from the blood by the kidneys. The wastes are concentrated in the urine and excreted from the body.
Both the liver and kidneys function in removing toxic substances from the blood, so we will discuss first the liver, and second the kidneys.
THE LIVER. The liver has many important functions of the body. We have already discussed the production and secretion of bile. We have also mentioned the fact that the liver removes excess glucose from the blood and stores it in the form of glycogen. This helps maintain the blood sugar level within normal limits (see
Figure 19-l).
Among its other functions, the cells of the liver metabolize and render harmless various toxic substances ingested into the body. Among these substances is alcohol. Chronic abuse of alcohol eventually does permanent damage to the liver. If the abuse continues, the entire liver may stop functioning, and death follow.
The liver, spleen, and bone marrow are the major sites for the breakdown of worn out red blood cells, In the course of this process, the iron from the hemoglobin is extracted and used in the formation of new hemoglobin. The rest of the hemoglobin molecule is further broken down to a substance called bilirubin
The bilirubin is secreted with the bile into the intestine, and it is eventually excreted with the feces.
Excess amino acids are broken down in the liver. The carbon, hydrogen, and oxygen of these compounds are converted into glycogen and other compounds. The nitrogen-containing amino groups, NH2, are converted into urea, which enters the bloodstream. The urea is carried to the kidneys, where it is filtered from the blood, and excreted in the urine (see Figure 19-2).
THE KIDNEYS AND URINARY SYSTEM. The kidneys are the main excretory organs of the body. However, their function is not simply to remove all metabolic wastes. Instead, by selectively excreting or retaining certain substances at certain times, the kidneys play a major role in regulating the body biochemistry. The kidneys and their associated excretory structures make up the urinary system
(see Figure 19-3).
The kidneys are two small organs located in the small of the back. A person can live perfectly well with only one kidney, but if both kidneys cease to function, poisons quickly accumulate in the blood. Unless these poisons are removed, death will follow.
In the past, failure of both kidneys was always fatal. However, this condition can now be dealt with in two ways. First, there are dialysis machines (artificial kidney machines) that can filter the poisons from the blood. The second method is kidney transplant, where a kidney from a healthy donor is transplanted into the body of the person with kidney failure. Such transplants are becoming increasingly successful.
The kidneys function by filtering the blood. Wastes and substances present in excess are excreted. All others are reabsorbed for use by the body. This filtering is carried out by highly specialized microscopic structures called nephrons. Each kidney contains about one million nephrons. By examining the way in which one nephron works, we can understand how the kidneys operate (see Figure 19-3).
The nephron is a long, coiled tubule, which is surrounded by capillaries. One end of the tubule is closed, and this end forms a capsule. The capsule partly surrounds a mass of capillaries called the glomerulus. The other end of the nephron opens into a collecting duct, into which the urine passes.
Because of the small size of the capillaries, the blood in the glomerulus is under relatively high pressure. This forces some of the blood plasma out of the glomerular capillaries and into the closed end of the nephron. Blood cells and proteins remain in the capillaries.
As the plasma passes through the nephron, water containing dissolved materials passes back into the surrounding capillaries by osmosis. Some substances, such as glucose, may be reabsorbed into the blood by active transport. Other substances, such as urea, are removed from the blood and concentrated in the nephron by active transport. Thus as the fluid passes down the tubule of the nepron, substanced needed by the body are reabsorbed into the blood, and the wastes become concentrated in the fluid. By the time the fluid reaches the collecting tubule, its composition has been substantially changed, and it is now called urine. The urine consists mainly of water, urea, and salts.
From the collecting tubules, urine flows into the ureters, which are the tubes connecting the kidneys to the bladder.The urine collects and is stored in the bladder until it is eliminated from the body.