Waste Materials.The chemical changes in the body have been compared to what goes on in a furnace, where coal is burned and heat is given off to warm a house or to run an engine. As a result of the burning of fuel in the furnace, certain waste materials are formedunburned clinkers, ash, and gases. The clinkers and ash drop into the ash pan and are removed; the gases pass off by way of the chimney. So in our bodies, as a result of the oxidation or “burning” of the food, waste products are formed. The carbohydrates and fats become carbon dioxide and water. The proteins change into other waste products, such as urea. The in-digestible remainder which cannot be taken into the body is left in the intestines, somewhat as the clinkers are left in the ash pan.
Wastes from the Intestines.Most of the material which is discharged from the intestines has never really been made a part of the living body. You remember that the alimentary canal runs from the mouth to the lower opening of the large intestine. As the food passes through the alimentary canal, the usable parts which the body needs are absorbed through the walls (mainly of the small intestine) into the blood stream, and the indigestible material remains finally in the large intestine, from which it must be discharged.
The Process of Excretion.The real wastes of the body are the substances formed by the life processes in the cells.
These are carried away from the tissues in the blood stream, and must be removed from the blood or they would ac-cumulate and poison the body. The removal of these wastes, such as carbon dioxide and urea, is brought about chiefly by the organs of excretionthe lungs, kidneys, liver, and skin.
In the lungs, the blood in the capillaries and the air in the air sacs are separated, as we have seen, by a thin membrane or layer of living cells. The carbon dioxide and some of the water vapor from the blood pass through this membrane into the air of the lungs and are discharged with the air which we exhale. The process of excretion is somewhat the same in kidneys, liver, and skin, except that in each of these organs the wastes pass from the blood into a special liquid, which is later discharged from the body. The wastes of the kidneys are discharged in the urine; the wastes of the liver, in the bile; and those of the skin, in the perspiration.
The Kidneys.The kidneys are two bean-shaped, dark red bodies which lie in the lower part of the back. Each kidney, in a grown person, measures about four inches by one and one half inches and weighs from four to six ounces. There are many blood vessels in the kidneys, and from the blood, the cells of the kidneys take out water and waste products, especially the wastes from protein foods. The liquid thus filtered from the blood in the kidneys is called the urine.
The urine gathers in very small tubes in the kidneys and passes by two larger tubes into the urinary bladder, a muscular sac below the kidneys, where it is stored until discharged. The bladder holds about half a pint. Since some three pints of urine are formed every twenty-four hours, the bladder should be emptied about six times a day. An equal quantity of water is given off through the lungs and the skin, making a total daily excretion of about six pints.
In order to keep the kidneys working well, it is important to drink plenty of water and to eat freely of fruits, green vegetables, and other foods that contain water. The wastes of the body must be eliminated regularly, and the kidneys can do their work better if plenty of water passes through, along with the waste products.
The Liver as an Organ of Excretion.Besides the in-digestible waste material which it discharges, the digestive canal carries off a true bodily excretion, which it receives from the bile,the fluid discharged by the liver.
The liver is the largest gland in the body, weighing three or four pounds. It lies in the upper part of the abdominal cavity just below the diaphragm. The bile which it forms may be discharged by the bile duct directly into the small intestine, or it may be stored for a time in a small sac called the gall bladder, connected with the bile duct. The bile has double work to do. It contains digestive juices which help in the digestion of fats, after the bile is discharged into the intestine. It also serves to get rid of some of the wastes of the body, which are excreted into it in the liver.
The Skin and the Perspiration.The skin is not so necessary for getting rid of wastes as are the lungs, the kidneys, and the liver, but it plays its own part in this important work.
Throughout the skin there are two or three million little pockets called sweat glands, which pour out perspiration. With a magnifying glass, you can see the tiny mouths of these glands as dark spots on the fine ridges of the skin. Usually the perspiration “dries up,” or evaporates, as fast as it is poured out, but when one is hot or is doing hard muscular work, so much is formed that it gathers on the skin. The perspiration is, of course, mostly water, but it also contains waste products like those found in the urine.
The sweat glands, like the other organs of the body, are under the control of the nervous system, as is shown by the fact that a person who is frightened or embarrassed often “breaks out in a perspiration.”
Effect of Alcohol on the Kidneys and the Liver.Alcoholic drinks exert a very harmful effect upon the kidneys, often bringing on the serious trouble called Bright’s disease. Old people may have Bright’s disease because their kidneys have become worn out by long use, but alcohol often wears out the kidneys prematurely and brings on this trouble much earlier. The liver also may easily become diseased from the heavy burden of getting rid of too much alcohol or other poisonous material.
QUESTIONS FOR DISCUSSION AND REVIEW
1. What would happen to a furnace if ashes were not re-moved, if clinkers were allowed to stay in the grate, and if the chimney were stopped up? Compare this to similar conditions in the body.
2. What difference is there between the wastes that pass off through the large intestine and those that are carried away from the various organs by the blood stream?
3. What are the organs of excretion by which the wastes are removed from the blood?
4. In what special form does each organ of excretion throw off the wastes from the body?
5. Describe the process of excretion in the lungs.
6. Describe the kidneys. Where is the urine stored? How much is excreted daily?
7. How can we help the work of the kidneys by our diet?
8. Describe the liver. What double work does the bile perform?
9. What part does the skin play in the excretion of wastes?
10. In how many ways is water given off by the body? How is this loss made up?
11. Does the perspiration contain other substances besides water? If so, what becomes of them when the water evaporates?