*Pa.s.sage of Oxygen through the Blood.*-In serving its purpose at the cells, the oxygen pa.s.ses twice through the blood-once as it goes toward the cells and again as it pa.s.ses from the cells to the exterior of the body:

_Pa.s.sage 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 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).

_Pa.s.sage 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 pa.s.sage of carbon dioxide through the blood requires special consideration.

[Fig. 55]

Fig. 55-*Diagram ill.u.s.trating 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.)

*Pa.s.sage 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 pa.s.sage through the blood is accounted for in the same way as the pa.s.sage 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]

Fig. 56-*Soap bubble* floating in a vessel of carbon dioxide, ill.u.s.trating 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 cla.s.sed 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)

*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]

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.

*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 pa.s.s 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?

PRACTICAL WORK

*To show the Difference between Free Oxygen and Oxygen in Combination.*-Examine some crystals of pota.s.sium chlorate (KClO3). They contain oxygen _in combination_ with pota.s.sium and chlorine. Place a few of these in a small test tube and heat strongly in a gas or alcohol flame.

The crystals first melt, and the liquid which they form soon appears to boil. If a splinter, having a spark on the end, is now inserted in the tube, it is kindled into a flame. This shows the presence of _free_ oxygen, the heat having caused the pota.s.sium chlorate to decompose. The difference between free and combined oxygen may also be shown by decomposing other compounds of oxygen, such as water and mercuric oxide.

*Preparation and Properties of Oxygen.*-Intimately mix 3 grams (1/2 teaspoonful) of pota.s.sium chlorate with half its bulk of manganese dioxide, and place the mixture in a large test tube. Close the test tube with a tight-fitting stopper which bears a gla.s.s tube of sufficient length and of the right shape to convey the escaping gas to a small trough or pan partly filled with water, on the table. Fill four large-mouthed bottles with water and, by covering with cardboard, invert each in the trough of water. Arrange the test tube conveniently for heating, letting the end of the gla.s.s tube terminate under the mouth of one of the bottles (Fig. 58).

Using an alcohol lamp or a Bunsen burner, heat over the greater portion of the tube at first, but gradually concentrate the flame upon the mixture.

Do not heat too strongly, and when the gas is coming off rapidly, remove the flame entirely, putting it back as the action slows down. After all the bottles have been filled, remove the end of the gla.s.s tube from the water, but leave the bottles of oxygen inverted in the trough until they are to be used. On removing the bottles from the trough, keep the tops covered with wet cardboard.

[Fig. 58]

Fig. 58-*Apparatus* for generating oxygen.

1. Examine a bottle of oxygen, noting its lack of color. Insert a small burning splinter in the upper part of the bottle and observe the change in the rate of burning. The air contains free oxygen, but it is diluted with nitrogen. Compare this with the undiluted oxygen in the bottle as to effect in causing the splinter to burn.

2. In a second bottle of oxygen insert a splinter without the flame, but having a small spark on the end. As soon as the oxygen kindles the spark into a flame, withdraw from the bottle and blow out the flame, but again insert the spark. Repeat the experiment as long as the spark is kindled by the oxygen into a flame. This experiment is usually performed as a test for undiluted oxygen.

3. Make a hollow cavity in the end of a short piece of crayon. Fasten a wire to the crayon, and fill the cavity with powdered sulphur. Ignite the sulphur in the flame of an alcohol lamp or Bunsen burner, and lower it into a bottle of oxygen. Observe the change in the rate of burning, the color of the flame, and the material formed in the bottle by the burning.

The gas remaining in the bottle is sulphur dioxide (SO2), formed by the _uniting_ of the sulphur and the oxygen.

4. Bend a small loop on the end of a piece of picture wire. Heat the loop in a flame and insert it in some powdered sulphur. Ignite the melted sulphur which adheres, and insert it quickly in a bottle of oxygen.

Observe the dark, brittle material which is formed by the burning of the iron. It is a compound of the iron with oxygen, similar to iron rust, and formed by their uniting.

*Preparation and Properties of Carbon Dioxide.*-1. (_a_) Attach a piece of carbon (charcoal) no larger than the end of the thumb to a piece of wire.

Ignite the charcoal in a hot flame and lower it into a vessel of oxygen.

Observe its combustion, letting it remain in the bottle until it ceases to burn. Note that the burning has consumed a part of the carbon and has used up the free oxygen. Has anything been formed in their stead?

(_b_) Remove the charcoal and add a little limewater. Cover the bottle with a piece of cardboard, and bring the gas and the limewater in contact by shaking. Note any change in the color of the limewater. If it turns white, the presence of carbon dioxide is proved.

2. Burn a splinter in a large vessel of air, keeping the top covered. Add limewater and shake. Note and account for the result.

3. Place several pieces of marble (limestone) in a jar holding at least half a gallon. Barely cover the marble with water, and then add hydrochloric acid until a gas is rapidly evolved. This gas is carbon dioxide.

(_a_) Does it possess color?

(_b_) Insert a burning splinter to see if it supports combustion.

(_c_) Place a bottle of oxygen by the side of the vessel of carbon dioxide. Light a splinter and extinguish the flame by lowering it into the vessel of carbon dioxide. Withdraw immediately, and if a spark remains on the splinter, thrust it into the bottle of oxygen. Then insert the relighted splinter into the carbon dioxide. Repeat several times, kindling the flame in one gas and extinguishing it in the other. Finally show that the spark also may be extinguished by holding the splinter a little longer in the carbon dioxide.