Physiology and Hygiene for Secondary Schools. Francis M. Walters

Table of Contents

      What is the nature of oxygen? What is its purpose in the body and how does it serve this purpose? How is the blood able to take it up at the lungs and give it off at the cells? What becomes of it after being used? These are questions touching the maintenance of life and they deserve careful consideration.

      Nature of Oxygen.—To understand the relation which oxygen sustains to the body we must acquaint ourselves with certain of its chemical properties. It is an element44 of intense affinity, or combining power, and is one of the most active of all chemical agents. It is able to combine with most of the other elements to form chemical compounds. A familiar example of its combining action is found in ordinary combustion, or burning. On account of the part it plays in this process, oxygen is called the supporter of combustion; but it supports combustion by the simple method of uniting. The ashes that are left and the invisible gases that escape into the atmosphere are the compounds formed by the uniting process. It thus appears that oxygen, in common with the other elements, may exist in either of two forms:

      [pg 105]1. That in which it is in a free, or uncombined, condition—the form in which it exists in the atmosphere.

      2. That in which it is a part of compounds, such as the compounds formed in combustion.

      Oxygen manifests its activity to the best advantage when it is in a free state, or, more accurately speaking, when it is passing from the free state into one of combination. It is separated from its compounds and brought again into a free state by overcoming with heat, or some other force, the affinity which causes it to unite.

      How Oxygen unites.—The chemist believes oxygen, as well as all other substances, to be made up of exceedingly small particles, called atoms. The atoms do not exist singly in either elements or compounds, but are united with each other to form groups of atoms that are called molecules. In an element the molecules are made up of one kind of atoms, but in a compound the molecules are made up of as many kinds of atoms as there are elements in the compound. Changes in the composition of substances (called chemical changes) are due to rearrangements of the atoms and the formation of new molecules. The atoms, therefore, are the units of chemical combination. In the formation of new compounds they unite, and in the breaking up of existing compounds they separate.

      The uniting of oxygen is no exception to this general law. All of its combinations are brought about by the uniting of its atoms. In the burning of carbon, for example, the atoms of oxygen and the atoms of carbon unite, forming molecules of the compound known as carbon dioxide. The chemical formula of this compound, which is CO_2, shows the proportion in which the atoms unite—one atom of carbon uniting with two atoms of oxygen in each of the molecules. The affinity of oxygen for other[pg 106] elements, and the affinity of other elements for oxygen, and for each other, resides in their atoms.

      Oxidation.—The uniting of oxygen with other elements is termed oxidation. This may take place slowly or rapidly, the two rates being designated as slow oxidation and rapid oxidation. Examples of slow oxidation are found in certain kinds of decay and in the rusting of iron. Combustion is an example of rapid oxidation. Slow and rapid oxidation, while differing widely in their effects upon surrounding objects, are alike in that both produce heat and form compounds of oxygen. In slow oxidation, however, the heat may come off so gradually that it is not observed.

      Movement of Oxygen through the Body.—Oxygen has been shown in the preceding chapters to pass from the lungs into the blood and later to leave the blood and, passing through the lymph, to enter the cells. That oxygen does not become a permanent constituent of the cells is shown by the constancy of the body weight. Nearly two pounds of oxygen per day are known to enter the cells of the average-sized person. If this became a permanent part of the cells, the body would increase in weight from day to day. Since the body weight remains constant, or nearly so, we must conclude that oxygen leaves the body about as fast as it enters. Oxygen enters the body as a free element. The form in which it leaves the body will be understood when we realize the purpose which it serves and the method by which it serves this purpose.

      Purpose of Oxygen in the Body.—The question may be raised: Is it possible for oxygen to serve a purpose in the body without remaining in it? This, of course, depends upon what the purpose is. That it is possible for oxygen to serve a purpose and at the same time pass on through[pg 107] the place where it serves that purpose, is seen by studying the combustion in an ordinary stove (Fig. 54). Oxygen enters at the draft and for the most part passes out at the flue, but in passing through the stove it unites with, or oxidizes, the fuel, causing the combustion which produces the heat.

      Fig. 54—Coal stove illustrating rapid oxidation.

      Now it is found that certain chemical processes, mainly oxidations, are taking place in the body. These produce the heat for keeping it warm and also supply other forms of energy,45 including motion. It is the purpose of oxygen to keep up these oxidations and, by so doing, to aid in supplying the body with energy. It serves this purpose in much the same way that it supports combustion, i.e., by uniting with, or oxidizing, materials derived from foods that are present in the cells.

      Does Oxygen serve Other Purposes?—It has been suggested that oxygen may serve the purpose of oxidizing, or destroying, substances that are injurious and of acting, in this way, as a purifying agent in the body. In support of this view is the natural tendency of oxygen to unite with substances and the well-known fact that oxygen is an important natural agent in purifying water. It seems probable, therefore, that it may to a slight extent serve this purpose in the body. It is probable also that oxygen aids through its chemical activity in the formation of compounds[pg 108] which are to become a part of the cells. Both of these uses, however, are of minor importance when compared with the main use of oxygen, which is that of an aid in supplying energy to the body.

      Oxygen and the Maintenance of Life.—In the supplying of energy to the body, one of the conditions necessary to the maintenance of life is provided. Because oxygen is necessary to this process, and because death quickly results when the supply of it is cut off, oxygen is frequently called the supporter of life. This idea is misleading, for oxygen has no more to do with the maintenance of life than have the food materials with which it unites. Life appears to be more dependent upon oxygen than upon food, simply because the supply of it in the body at any time is exceedingly small. Being continually surrounded by an atmosphere containing free oxygen, the body depends upon this as a constant source of supply, and does not store it up. Food, on the other hand, is taken in excess of the body's needs and stored in the various tissues, the supply being sufficient to last for several days. When the supply of either oxygen or food is exhausted in the body, life must cease.

      The Oxygen Movement a Necessity.—Since free oxygen is required for keeping up the chemical changes in the cells, and since it ceases to be free as soon as it goes into combination, its continuous movement through the body is a necessity. The oxygen compounds must be removed as fast as formed in order to make room for more free oxygen. This movement has already been studied in connection with the blood and the organs of respiration, but the consideration of certain details has been deferred till now. By what means and in what form is the oxygen passed to and from the cells?

      [pg 109]Passage of Oxygen through the Blood.—In serving its purpose at the cells, the oxygen passes twice through the blood—once as it goes toward the cells and again as it passes from the cells to the exterior of the body:

      Passage toward the Cells.—This is effected mainly through the hemoglobin of the red corpuscles. At the lungs the oxygen and the hemoglobin form a weak chemical compound that breaks up and liberates the oxygen when it reaches the capillaries in the tissues. The separation of the oxygen from the hemoglobin at the tissues appears to be due to two causes: first, to the weakness of the chemical attraction between the atoms of oxygen and the atoms that make up the hemoglobin molecule; and second, to a difference in the so-called oxygen pressure at the lungs and at the tissues.46

      The attraction of the oxygen and the hemoglobin is sufficient to cause