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 pressure[47] 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 air[48] (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).
