THE PURPOSE OF BREATHING.
We must now try to understand what is the object of the remarkable and complicated struc ture of the lungs and of the movements which have been described.
The Gases of the p. 295 it has been shown that the blood contains three gases, oxygen, carbonic acid gas, and nitrogen, partly dissolved in it, partly in chemical union with certain of its constituents. The nitrogen need not be taken into account. The oxygen is to be considered as part of its nourishing material, which the tissues, to which the blood is distri buted, require to carry on their processes. The carbonic acid gas is a waste substance which the tissues produce by their activity, and which the blood carries away from them. As the blood flows through the body its oxygen is removed from it, carbonic acid gas being substituted ; and if the efficiency of the blood to nourish the tissues, in this respect, is to be maintained, it must always be receiving new supplies of oxy gen, and a means must be at hand of ridding it of its excess of carbonic acid gas. This double function it is the business of the respiratory process to perform. The blood that is sent out from the left side of the heart, on its mission to supply the body, is of a bright scarlet hue (the colour of arterial blood), while the blood that returns to the right side of the heart, after its mission is accomplished, is of a dark purple colon r (the colour of venous. blood), This change in colour takes place in the capillaries, the ves sels whose walls are so delicate as to permit of free interchange between the blood in the ves sels and the tissues outside of them. In short, it is due to the fact that in the capillaries the blood gives up its oxygen to the tissues, and receives from them carbonic acid gas. This is proved by chemical analysis, which shows that arterial blood contains more oxygen and less carbonic acid than venous blood,' and by the experimental fact that if the dark - coloured venous blood be shaken up with oxygen it be comes of a scarlet colour, while arterial blood shaken up with carbonic acid gas becomes purple. Now, as mentioned on p. 302, the venous blood, returned from the body, is conveyed to the right side of the heart, and thence by the pulmonary artery to the lungs, through which it is distributed in capillary blood-vessels to be gathered up into the pulmonary veins and carried to the left side of the heart. But when it leaves the right side of the heart the blood is purple-coloured, and when it enters the left side it is scarlet. That is to say, while passing through the capillaries of the lungs it has been converted from venous into arterial blood. In other words, in its progress through the lungs it has given off its excess of carbonic acid gas and obtained a new supply of oxygen. So that while in the general capillaries of the body the blood is rendered impure by being deprived of much of its oxygen and being laden with car bonic acid, in the capillaries of the lungs the process is reversed, and the blood is purified by being rid of its excess of carbonic acid and by having its proper quantity of oxygen restored. Now it has been already stated (p. 343) that the capillaries of the pulmonary artery, through which the blood flows on its way from the right to the left side of the heart, are distributed over the walls of the air-cells of the lung, and that the air-cells are so numerous and closely packed, and their walls, as well as those of the capil laries, so thin that there is no obstacle to an interchange taking place between the blood in the vessels and the air in the air-cells. It is manifestly here, then, that the change occurs which transforms the dark-coloured, carbonic acid-laden venous blood into the bright-hued blood refreshed with oxygen. How does that conversion occur is the next question.
Exchanges between the Blood and the Air in the lungs. It is a well-known physical law that if two different liquids be placed in a vessel in contact with one another, and be left alone, without any disturbance whatever, they do not remain separate, but proceed straight way to mix, and in time there will be a perfect mingling of the two liquids. Suppose, for ex ample, that water and spirit are taken, and, for the sake of seeing the experiment going forward, that the spirit is coloured red, that the water is placed in a glass jar, and when it is perfectly still that the spirit is carefully poured on the top of it, so carefully that the two layers, one below of colourless water and the other above of coloured spirit, are quite distinct from one another. If the jar be set aside, it will be ob served that the coloured spirit does not long remain all on the top, though it is lighter than water, but that the colour gradually passes downwards into the water, until in time the whole is coloured—an equal mixture of water and spirit is found in the jar. This is called
diffusion of liquids. Now the same thing occurs with gases though the process is not visible. If one glass jar be filled with carbonic acid gas and a second with oxygen gas,and if the jar containing the oxygen be placed upside down on the top of the jar containing the carbonic acid, the jars being fitted mouth to mouth, then we have practically one glass vessel, composed of two jars united at their mouths, the lower part containing carbonic acid gas and the upper oxygen, the two being in contact with one another. In spite of the fact that the lower gas is much heavier than the upper one, the two proceed to diffuse, the heavy one passing up into the light, and the light one passing down into the heavy. In course of time—and the time is not long—the vessel contains a per fect mixture of carbonic acid gas and oxygen. This is diffusion of gases. On pp. 193 and 194 it is stated that two liquids will mingle even when separated from one another by a membrane, and similarly two gases will mingle even when separated from one another by a membrane. Thus if a bladder be filled with oxygen, and if, after being firmly closed, it be placed in a jar containing carbonic acid gas, the oxygen will pass through the walls of the membrane to mingle with the carbonic acid, and the carbonic acid gas will pass inwards to mix with oxygen, until the bladder and the jar contain equal mixtures of the two gases. Moreover, the process will be aided by the walls of the bladder being moistened with water. This is owing to the fact that liquids dissolve gases. Water exposed to an atmos phere of oxygen will lick up, or, to speak more correctly, will absorb some of the oxygen. It will do the same with other gases; and other liquids also absorb gases, though one liquid will absorb more of one gas than of another. Nay, one liquid, already containing a gas dis solved in it in quantity, is not thereby pre vented from absorbing a quantity of another gas. So that, if the walls of the bladder be wet, one side is moistened with water contain ing oxygen in solution, and the other side is moistened with water containing carbonic acid gas in solution, and an interchange takes place between the two solutions, as described on pp. 193 and 194, so that oxygen passes outwards and carbonic acid gas passes inwards. Not only do liquids dissolve gases naturally, but they may be made by pressure to lick up much more than the usual quantity. As everyone knows, a bottle of aerated water contains water charged with carbonic acid gas. The gas is forced into the bottle by great pressure and the water compelled to dissolve it. The pres sure is so great that, in order to compel the water to retain the gas, the bottle must be tightly corked and the cork bound down by wire. As soon as the cork is removed, the gas comes off with great force and produces the sparkling or effervescence of the water, while sometimes the pressure of gas in the water is so great as to blow out the cork or burst the bottle. Gas dissolved in a liquid exerts pres sure on that liquid in its efforts to escape. If the pressure outside the liquid be less than that of the gas in the liquid, the gas will escape and come off; if the outside pressure be equal to that of the gas in the liquid, or greater, it will remain dissolved. Suppose, then, water containing oxygen in solution be placed in a jar filled with oxygen. If the pressure of gas in the jar be greater than that in the liquid, the water will take up more oxygen, but if it be less it will give off some oxygen, the result being that, in the end, the pressure of gas in the liquid and that outside of it become the same. In the same way if a liquid containing both oxygen and carbonic acid gas be exposed to an atmosphere of mixed oxygen and car bonic acid gas, unless the pressures of the two gases are the same in the liquid and in the atmosphere, an exchange will take place. If the pressure of oxygen in the liquid be less than the pressure of that gas in the atmos phere, oxygen will pass into the liquid till both are equal; and if the pressure of carbonic acid gas in the liquid be greater than that out side, carbonic acid gas will pass off from the liquid till both are equal, so that the liquid will gain oxygen and lose carbonic acid gas, while the atmosphere will lose oxygen and gain carbonic acid. The same process will take place even though the liquid be separated from the atmosphere by a membrane, and will occur more readily, as we have seen, if the membrane be moistened on both sides.