This, then, we must remember,—that in every tissue, so long as the blood flows, and life lasts, this exchange takes place between the blood within the capillaries and the tissues without.
The stream of blood to the tissues carries to them the material, including the all-important oxygen, with which they build themselves up and do their work. The stream from the tissues carries into the blood the products of certain chemical changes which have taken place in these tissues. These products may represent simple waste matter to be cast out or material which may be of use to some other tissue.
In brief, the tissues by the help of the lymph live on the blood. Just as our bodies, as a whole, live on the things around us, the food and the air, so do the bodily tissues live on the blood which bathes them in an unceasing current, and which is their immediate air and food.
178. Physical Properties of Blood. The blood has been called the life of the body from the fact that upon it depends our bodily existence. The blood is so essentially the nutrient element that it is called sometimes very aptly “liquid flesh.” It is a red, warm, heavy, alkaline fluid, slightly salt in taste, and has a somewhat fetid odor. Its color varies from bright red in the arteries and when exposed to the air, to various tints from dark purple to red in the veins. The color of the blood is due to the coloring constituent of the red corpuscles, hæmoglobin, which is brighter or darker as it contains more or less oxygen.
Fig. 65.—Blood Corpuscles of Various Animals. (Magnified to the same scale.)
- A, from proteus, a kind of newt;
- B, salamander;
- C, frog;
- D, frog after addition of acetic acid, showing the central nucleus;
- E, bird;
- F, camel;
- G, fish;
- H, crab or other invertebrate animal
The temperature of the blood varies slightly in different parts of the circulation. Its average heat near the surface is in health about the same, viz. 98½° F. Blood is alkaline, but outside of the body it soon becomes neutral, then acid. The chloride of sodium, or common salt, which the blood contains, gives it a salty taste. In a hemorrhage from the lungs, the sufferer is quick to notice in the mouth the warm and saltish taste. The total amount of the blood in the body was formerly greatly overestimated. It is about 1/13 of the total weight of the body, and in a person weighing 156 pounds would amount to about 12 pounds.
179. Blood Corpuscles. If we put a drop of blood upon a glass slide, and place upon it a cover of thin glass, we can flatten it out until the color almost disappears. If we examine this thin film with a microscope, we see that the blood is not altogether fluid. We find that the liquid part, or plasma, is of a light straw color, and has floating in it a multitude of very minute bodies, called corpuscles. These are of two kinds, the red and the colorless. The former are much more numerous, and have been compared somewhat fancifully to countless myriads of tiny fishes in a swiftly flowing stream.