Some of the amœboid corpuscles from the blood make their way between the cells forming the walls of the capillaries. Lymph, then, is practically blood plasma plus some colorless corpuscles. It acts as the medium of exchange between the blood proper and the cells in the tissues of the body. By means of the food supply thus brought, the cells of the body are able to grow, the fluid food being changed to the protoplasm of the cells. By means of the oxygen passed over by the lymph, oxidation may take place within the cells. Lymph not only gives food to the cells of the body, but also takes away carbon dioxide and other waste materials, which are ultimately passed out of the body by means of the lungs, skin, and kidneys.
Internal Secretions.—In addition to all the functions given above, the blood has recently been shown to carry the secretions of a number of glands through which it passes, although these glands have no ducts to carry off their secretions. These internal secretions seem absolutely necessary for the health of the body. Several glands, the thyroid, adrenal bodies, the testes, and ovaries, as well as the pancreas, give off these remarkable substances.
The Amount of Blood and its Distribution.—Blood forms, by weight, about one sixteenth of the body. This would be about four quarts to a body weight of 130 pounds. Normally, about one half of the blood of the body is found in or near the organs lying in the body cavity below the diaphragm, about one fourth in the muscles, and the rest in the head, heart, lungs, large arteries, and veins.
Blood Temperature.—The temperature of blood in the human body is normally about 98.6° Fahrenheit when tested under the tongue by a thermometer, although the temperature drops almost two degrees after we have gone to sleep at night. It is highest about 5 P.M. and lowest about 4 A.M. In fevers, the temperature of the body sometimes rises to 107°; but unless this temperature is soon reduced, death follows. Any considerable drop in temperature below the normal also means death. Body heat results from the oxidation of food, and the circulation of blood keeps the temperature nearly uniform in all parts of the body.
Cold-blooded Animals.—In animals which are called cold-blooded, the blood has no fixed temperature, but varies with the temperature of the medium in which the animal lives. Frogs, in the summer, may sit for hours in water with a temperature of almost 100°. In winter, they often endure freezing so that the blood and lymph within the spaces under the loose skin are frozen into ice crystals. This change in body temperature is evidently an adaptation to the mode of life.
Circulation of the Blood in Man.—The blood is the carrying agent of the body. Like a railroad or express company, it takes materials from one part of the human organism to another. This it does by means of the organs of circulation,—the heart and blood vessels. These blood vessels are called arteries where they carry blood away from the heart, veins where they bring blood back to the heart, and capillaries where they connect the larger blood vessels. The organs of circulation thus form a system of connected tubes through which the blood flows.
The Heart; Position, Size, Protection.—The heart is a cone-shaped muscular organ about the size of a man's fist. It is located immediately above the diaphragm, and lies so that the muscular apex, which points downward, moves while beating against the fifth and sixth ribs, just a little to the left of the midline of the body. This fact gives rise to the notion that the heart is on the left side of the body. The heart is surrounded by a loose membranous bag called the pericardium, the inner lining of which secretes a fluid in which the heart lies. When, for any reason, the pericardial fluid is not secreted, inflammation arises in that region.
Diagram showing the front half of the heart cut away: a, aorta; l, arteries to the lungs; la, left auricle; lv, left ventricle; m, tricuspid valve open; n, bicuspid or mitral valve closed; p and r, veins from the lungs; ra, right auricle; rv, right ventricle; v, vena cava. Arrows show direction of circulation.
Internal Structure of Heart.—If we should cut open the heart of a mammal down the midline, we could divide it into a right and a left side, each of which would have no internal connection with the other. Each side is made up of an upper thin-walled portion with a rather large internal cavity, the auricle, which opens into a lower smaller portion with heavy muscular walls, the ventricle. Communication between auricles and ventricles is guarded by little flaps or valves. The auricles receive blood from the veins. The ventricles pump the blood into the arteries.