The Blood : Part 1

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 4 of 26)

Two liquids of similar nature are found in the body, known as the blood and the lymph. These are closely related in function and together they form the nutrient fluid referred to in the preceding chapter. The blood is the more familiar of the two liquids, and the one which can best be considered at this time.

The Blood: where Found. - The blood occupies and moves through a system of closed tubes, known as the blood vessels. By means of these vessels the blood is made to circulate through all parts of the body, but from them it does not escape under normal conditions. Though provisions exist whereby liquid materials may both enter and leave the blood stream, it is only when the blood vessels are cut or broken that the blood, as blood, is able to escape from its enclosures.

Physical Properties of the Blood. - Experiments such as those described at the close of this chapter reveal the more important physical properties of the blood. It may be shown to be heavier and denser than water; to have a faint odor and a slightly salty taste; to have a bright red color when it contains oxygen and a dark red color when oxygen is absent; and to undergo, when exposed to certain conditions, a change called coagulation. These properties are all accounted for through the different materials that enter into the formation of the blood.

Composition of the Blood. - To the naked eye the blood appears as a thick but simple liquid; but when examined with a compound microscope, it is seen to be complex in nature, consisting of at least two distinct portions. One of these is a clear, transparent liquid; while the other is made up of many small, round bodies that float in the liquid. The liquid portion of the blood is called the plasma; the small bodies are known as corpuscles. Two varieties of corpuscles are described - the red corpuscles and the white corpuscles. Other round particles, smaller than the corpuscles, may also be seen under favorable conditions. These latter are known as blood platelets.

Red Corpuscles. - The red corpuscles are classed as cells, although, as found in the blood of man and the other mammals, they have no nuclei. Each one consists of a little mass of protoplasm, called the stroma, which contains a substance having a red color, known as hemoglobin. The shape of the red corpuscle is that of a circular disk with concave sides. It has a width of about 1/3200 of an inch (7.9 microns) and a thickness of about 1/13000 of an inch (1.9 microns). The red corpuscles are exceedingly numerous, there being as many as five millions in a small drop (one cubic millimeter) of healthy blood. But the number varies somewhat and is greatly diminished during certain forms of disease.

It is the function of the red corpuscles to serve as oxygen carriers for the cells. They take up oxygen at the lungs and release it at the cells in the different tissues. The performance of this function depends upon the hemoglobin.

Hemoglobin. - This substance has the remarkable property of forming, under certain conditions, a weak chemical union with oxygen and, when the conditions are reversed, of separating from it. It forms about nine tenths of the solid matter of the red corpuscles and to it is due the colors of the blood. When united with the oxygen it forms a compound, called oxy hemoglobin, which has a bright red color; the hemoglobin alone has a dark red color. These colors are the same as those of the blood as it takes on and gives off oxygen. The stroma, which forms only about one tenth of the solid matter of the corpuscles, serves as a contrivance for holding the hemoglobin. The conditions which cause the hemoglobin to unite with oxygen in the lungs and to separate from it in the tissues, will be considered later.

Disappearance and Origin of Red Corpuscles. - The red corpuscles, being cells without nuclei, are necessarily short-lived. It has been estimated that during a period of one to two months, all the red corpuscles in the body at a given time will have disappeared and their places taken by new ones. The origin of new corpuscles, however, and the manner of ridding the blood of old ones are problems that are not as yet fully solved. The removal of the products of broken down corpuscles is supposed to take place both in the liver and in the spleen.

Regarding the origin of the red corpuscles, the evidence now seems conclusive that large numbers of them are formed in the red marrow of the bones. The red marrow is located in what is known as the spongy substance of the bones and consists, to a large extent, of cells somewhat like the red corpuscles, but differing from them in having nuclei. These appear to be constantly in a state of reproduction. The blood, flowing through the minute cavities containing these cells, carries those that have been loosened out into the blood stream. Nuclei appear in the red corpuscles at the time of their formation, but these quickly separate and, according to some authorities, form the blood platelets.

White Corpuscles. - The white corpuscles, or leucocytes, are cells of a general spherical shape, each containing one, two, or more nuclei. They are much less numerous than the red, there being on the average only one white corpuscle to about every five hundred of the red ones. On the other hand, the white corpuscles are larger than the red, one of the former being equal in volume to about three of the latter.

The white corpuscles are found, when studied under favorable conditions, to possess the power of changing their shape and, by this means, of moving from place to place. This property enables them to penetrate the walls of capillaries and to pass with the lymph in between the cells of the tissues. The white corpuscles are, therefore, not confined to the blood vessels, as are the red corpuscles, but migrate through the intercellular spaces. If any part of the body becomes inflamed, the white corpuscles collect there in large numbers; and, on breaking down, they form most of the white portion of the sore, called the pus.

New white corpuscles are formed from old ones, by cell-division. Their production may occur in almost any part of the body, but usually takes place in the lymphatic glands and in the spleen, where conditions for their development are especially favorable. In these places they are found in great abundance and in various stages of development.

D.C. Heath and Co. - Publishers
Original copyright 1909