Passage of Oxygen through the Body : Part 2

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 9 of 25)

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?

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.

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 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.

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.

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 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. 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.

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. 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.

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. 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.

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.

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 percent 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?

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