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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 them to unite where the oxygen pressure is more than one half pound to the square inch, but it is not sufficiently strong to cause them to unite or to prevent their separation, if already united, where the oxygen pressure is less than one half pound to the square inch. The oxygen pressure at the lungs, which amounts to nearly three pounds to the square inch, easily causes the oxygen and the hemoglobin to unite, while the almost complete absence of any oxygen pressure at the tissues, permits their separation. The blood in its circulation constantly flows from the place of high oxygen pressure at the lungs[pg 110] to the place of low oxygen pressure at the tissues and, in so doing, loads up with oxygen at one place and unloads it at the other (Fig. 55).

Passage from the Cells.—Since oxygen leaves the free state at the cells and becomes a part of compounds, we are able to trace it from the body only by following the course of these compounds. Three waste compounds of importance are formed at the cells—carbon dioxide (CO2), water (H2O), and urea (N2H4CO). The first is formed by the union of oxygen with carbon, the second by its union with hydrogen, and the third by its union with nitrogen, hydrogen, and carbon. These compounds are carried by the blood to the organs of excretion, where they are removed from the body. The water leaves the body chiefly as a liquid, the urea as a solid dissolved in water, and the carbon dioxide as a gas. The passage of carbon dioxide through the blood requires special consideration.

Fig. 55—Diagram illustrating movement, of oxygen and carbon dioxide through the body (S.D. Magers). Each moves from a place of relatively high to a place of relatively low pressure. (See text.)

Passage of Carbon Dioxide through the Blood.—Part of the carbon dioxide is dissolved in the plasma of the blood, and part of it is in weak chemical combination with substances found in the plasma and in the corpuscles. Its passage through the blood is accounted for in the same[pg 111] way as the passage of the oxygen. Its ability to dissolve in liquids and to enter into chemical combination varies as the carbon dioxide pressure47 This in turn varies with the amount of the carbon dioxide, which is greatest at the cells (where it is formed), less in the blood, and still less in the lungs. Because of these differences, the blood is able to take it up at the cells and release it at the lungs (Fig. 55).

Fig. 56—Soap bubble floating in a vessel of carbon dioxide, illustrating the difference in weight between air and carbon dioxide gas.

Properties of Carbon Dioxide.—Carbon dioxide is a colorless gas with little or no odor. It is classed as a heavy gas, being about one third heavier than air48 (Fig. 56). It does not support combustion, but on the contrary is used to some extent to extinguish fires. It is formed by the oxidation of carbon in the body, and by the combustion of carbon outside of the body. It is also formed by the decay of animal and vegetable matter. From these sources it is continually finding its way into the atmosphere. Although not a poisonous gas, carbon dioxide may, if it surround the body, shut out the supply of oxygen and cause death.49

[pg 112]Final Disposition of Carbon Dioxide.—It is readily seen that the union of carbon and oxygen, which is continually removing oxygen from the air and replacing it with carbon dioxide, tends to make the whole atmosphere deficient in the one and to have an excess of the other. This tendency is counteracted through the agency of vegetation. Green plants absorb the carbon dioxide from the air, decompose it, build the carbon into compounds (starch, etc.) that become a part of the plant, and return the free oxygen to the air (Fig. 57). In doing this, they not only preserve the necessary proportion of oxygen and carbon dioxide in the atmosphere, but also put the carbon and oxygen in such a condition that they can again unite. The force which enables the plant cells to decompose the carbon dioxide is supplied by the sunlight (Chapter XII).

Fig. 57—Under surface of a geranium leaf showing breathing pores, highly magnified (O.H.).

Summary.—Oxygen, by uniting with materials at the cells, keeps up a condition of chemical activity (oxidation) in the body. This supplies heat and the other forms of bodily energy. Entering as a free element, oxygen leaves the body as a part of the waste compounds which it helps to form. The free oxygen is transported from the lungs to the cells by means of the hemoglobin of the red corpuscles, while the combined oxygen in carbon dioxide and other compounds from the cells is carried mainly by the plasma. The limited supply of free oxygen in the body at any time makes necessary its continuous introduction into the body.

[pg 113]Exercises.—1. Describe the properties of oxygen. How does it unite with other elements? How does it support combustion?

2. State the purpose of oxygen in the body. What properties enable it to fulfill this purpose?

3. What is the proof that oxygen does not remain permanently in the body? How does the oxygen entering the body differ from the same oxygen as it leaves the body?

4. What is the necessity for the continuous introduction of oxygen into the body, while food is introduced only at intervals?

5. How are the red corpuscles able to take up and give off oxygen? How is the plasma able to take up and give off carbon dioxide?

6. If thirty cubic inches of air pass from the lungs at each expiration and 4.5 per cent of this is carbon dioxide, calculate the number of cubic feet of the gas expelled in twenty-four hours, estimating the number of respirations at eighteen per minute.

7. What is the weight of this volume of carbon dioxide, if one cubic foot weigh 1.79 ounces?

8. What portion of this weight is oxygen and what carbon, the ratio by weight of carbon to oxygen in carbon dioxide being twelve to thirty-two?

9. What is the final disposition of carbon dioxide in the atmosphere?