Now these facts explain to some extent the changes that occur in the blood in the lungs, for the conditions we have been speaking of are exactly fulfilled. Blood containing oxygen and carbonic acid gas is flowing in a multitude of tiny streams through the walls of the air cells of the lungs. The air-cells contain a mixture of the same two gases. The blood is separated from the air in the air-cells by a thin membrane, namely, the delicate walls of the capillaries and of the air-cells, and the mem brane is kept moist by the blood on the one side and the secretion of the membrane on the other. It has been found that the pressure of oxygen in the blood is less than that in the air-cells, and that the pressure of carbonic acid gas in the blood is greater than the pressure of the same gas in the air-cells. Consequently oxygen passes through the membrane from the air-cells into the blood, and carbonic acid gas passes through the membrane from the blood into the air-cells. The blood thus gains oxygen and loses carbonic acid, while the air - cells lose oxygen and gain the latter gas. At the same time the blood, by having its proper quantity of oxygen restored to it, and its excess of carbonic acid gas removed, changes in colour from purple to scarlet, from venous to arterial blood. It is re-invigorated and purified.
This much must further be noted, that the gases are not simply dissolved in the blood, but are partly in chemical union with it, and this affects to some extent, probably aids, the process.
It will readily occur to anyone that the result of this process will speedily be that the air-cells will be largely deprived of their oxygen and will contain chiefly carbonic acid gas. The pressure of oxygen will soon be even less in the air-cells than it is in the blood, and that of carbonic acid gas greater, so that the process would be reversed, oxygen taken from instead of added to the blood, and carbonic acid gas added to it instead of taken from it. This would make the blood more unfit than ever to nourish the body and more poisonous to it than before. It is evident that if the exchange is to continue of the proper character the air in the air-cells must be constantly renewed, its oxygen restored and its carbonic acid gas removed. How this is jccomplished is the next question to consider.
Exchanges between the Outside Air and that of the Air-cells.--The air which we exhale during the act of expiration is very different in character from the external air which we inhale. Both contain principally the same three gases, though in different quantities, as the following table shows:— In 100 parts of Air inspired. Air expired.
Oxygen, 20.8 15.4 Nitrogen, 79.2 79'3 Carbonic Acid, 04 4.3 In other words, exhaled air contains roughly 5 per cent less oxygen and 5 per cent more carbonic acid than inspired air; the air taken into the lungs loses oxygen and gains carbonic acid. There are also other differences. Expired air is hotter because it has been in contact with the warm air-passages, and it contains more moisture than the external air also from contact with the moist lining membrane of the passages. This is readily observed by breathing on a cold surface, on which the moisture condenses. In expired air there is also a small quantity of animal matters, which gives to the air its stuffy smell.
The quantity of oxygen thus removed from the external air in 24 hours by the breathing of an adult person, as well as the quantities of carbonic acid gas and water given out in the same time, have been carefully estimated. About 18 cubic feet of oxygen are consumed daily by an adult man at rest. The same amount of carbonic acid gas is given out, and would be represented by a piece of pure char coal weighing 9 ounces avoirdupois. The quantity of carbonic acid, however, varies according to circumstances, increasing up to the age of thirty and then diminishing, being increased also by external cold and by exer cise, and being affected by the kind of food taken. The amount of water varies from 6 to 20 ounces daily, on an average it is about half a pint.
The explanation of the difference in the com position of the air inhaled and the air exhaled is simple. During the pause that follows an act of expiration the lungs, that is the bronchial tubes and air-cells of the lungs, are still filled with air. That air, particularly in the air cells, must be different from fresh air, because the blood is continually drawing oxygen from it and adding carbonic acid to it. When an inspiration occurs the lungs distend, and 30 cubic inches of fresh air enter to fill up the in creased space. The new supply occupies only the upper air-passages. An expiration imme diately follows the inspiration, but the :30 cubic inches that have previously entered are not expelled. A like quantity is exhaled, but it contains only about one-third of the 30 cubic inches just inhaled. For the supply of fresh air has no sooner entered the lungs than it proceeds to mingle with the air already there, to diffuse into it, and two-thirds of it have already passed down a considerable way to wards the air-cells before the expiration which follows its entrance occurs. By the process of diffusion the fresh supply passes downwards towards the air-cells, increasing the quantity of oxygen already in the air in the lungs and diluting the carbonic acid. Of the air given out of the lungs in a breath, while one-third is formed of air that has entered just previously to the expiration, the remainder is air from the lungs charged with carbonic acid and deficient in oxygen, which has been displaced by the fresh air. The purpose of breathing is thus apparent: it is to restore to the air in the air cells of the lungs the quantity of oxygen of which it is being regularly deprived by the blood, and to rid it of the excess of carbonic acid imparted to it by the blood. The two processes that have been now described must keep pace with one another: the process by which the blood takes oxygen from the air in the air-cells, and gives to it carbonic acid, must be counterbalanced by the process by which ;a • certain quantity of fresh air is drawn into the lungs to restore the lost oxygen, and a certain quantity of air is expelled from the lungs to remove the excess of the hurtful gas.
A survey of the ground that has been gone over reveals how the structure of the lungs, the arrangement of the blood-vessels in the walls of the air-cells, and the action of the distention and recoil of the chest and lungs are all adapted to work together for this one end, namely, to facilitate exchanges of gases between the blood and the air.