The capacity of the blood for rapidly absorbing oxygen in the lungs and readily parting with it to the tissues is easily and completely explained by the property which hæmoglobin possesses of forming an unstable compound with this gas.
It is quite otherwise with regard to the liberation of carbonic acid. The problems presented by the solution of this gas in blood and its elimination in the lungs are difficult to solve. Less than one-tenth of the volume of carbonic acid which can be extracted from blood by the air-pump is simply in solution. The remainder is in loose chemical combination, the chief agents in holding it being the alkaline carbonates which the plasma contains. With an excess of carbonic acid they form acid carbonates, which give up carbonic acid and again become normal carbonates in the lungs. About one-third of the carbonic acid is, however, held by the blood-corpuscles—partly in virtue of their alkaline carbonates and phosphates, partly in combination with their globulin. The affinity of these several vehicles for carbonic acid is sufficient to enable them to take it from the lymph, and to hold it while the blood is in the veins. When they reach the capillaries of the lungs, they part with their burden of carbonic acid to the air. It is in connection with this renunciation that certain difficulties remain to be explained. The carbonic acid is given up with greater readiness than our knowledge of the chemistry of the compounds into which it enters in the blood would lead us to expect.
Why does oxygen enter blood as it circulates through the lungs, and carbonic acid leave it? We have referred to the immense surface which the lungs expose to air. If a soap-bubble be filled with a mixture of oxygen, nitrogen, and carbonic acid, and if the oxygen be in smaller proportion, and the carbonic acid be in greater proportion, than in the air of the room, oxygen will enter the bubble, and carbonic acid will leave it, by diffusion. If, instead of filling a bubble with gas, we fill a bladder with water charged with carbonic acid, but destitute of dissolved oxygen, a similar exchange with the gases of the air will take place. It is merely a question of “gaseous tension.” The tension of the gases in the lungs is measured by passing a small tube down the trachea, and along one of the two chief bronchi until it becomes blocked in a bronchus just large enough to admit it. Respiration is carried on under normal conditions in the remainder of the lung; but in the lobe which the catheter blocks diffusion from stationary air to tidal is no longer allowed. At the same time, since the circulation is not interfered with, the gases in the blood of the occluded lobe of the lung are not in markedly different proportions from those in the air-chambers of other parts. If at the end of a sufficient interval the air of the occluded lobe is drawn off and its gases measured, their tensions can be compared with the tensions of gases in specimens of arterial and of venous blood. If from 10 c.c. of fluid 1 c.c. of gas can be removed by the air-pump, the volume of gas dissolved is 10 per cent. of the volume of the fluid which dissolved it. Commonly this is written “10 volumes per cent.” To ascertain experimentally the tension of a particular gas in a particular fluid when dissolved to the amount of 10 volumes per cent. at the ordinary pressure of the atmosphere and at the temperature of the body, it would be necessary to place it in an open vessel in air containing a sufficient admixture of the gas to prevent its escape from the fluid. Suppose that it were found that, when the fluid containing the dissolved gas was placed in air mixed with the same gas to the extent of one-tenth of its volume, the fluid neither gave up gas nor absorbed more gas, the tension of the gas would be equal to one-tenth of an atmosphere. Since the pressure of the atmosphere equals 760 millimetres of mercury, the tension of the dissolved gas would be 76 millimetres. If more gas were added to the air, more would dissolve in the fluid; if some of the gas were removed from the air, gas would escape from the fluid. Gas passes from the medium in which its tension is high to the medium in which its tension is low. The tension of carbonic acid in tissues, particularly in muscles and glands, is higher than in lymph; in lymph higher than in blood; in blood higher than in air. Hence it passes by these several stages from the tissues in which it is formed to the air in the lungs. Much ingenuity has been devoted to perfecting methods for the determination of the tension of carbonic acid in lymph and in venous blood. Frequently results have been obtained which seemed opposed to the doctrine that carbonic acid progresses from one medium to another in accordance with the law of pressures; but such perplexing results were probably due either to imperfections in method or to the establishment of abnormal physiological conditions during the course of the observations. When, for example, it was found that the tension in lymph was less than the tension in blood, the specimen of lymph examined was probably not in the same condition as the lymph in the tissue-spaces where the exchange occurs. The experimenter in such a case was in error in supposing that the specimen of lymph which he examined contained as much carbonic acid as did the lymph in the tissue-spaces from which the blood which he compared with it received its supply of this gas.
We have already given the figures for the composition of the air in the air-chambers of the lungs. The figures commonly accepted as correct for the percentages of the several gases in the blood are, at 0° C. and 760 millimetres of mercury pressure:
| Oxygen. | Carbonic anhydride. | Nitrogen. | |
|---|---|---|---|
| In 100 vol. of arterial blood | 20 | 39 | 1-2 |
| In 100 vol. of venous blood | 8-12 | 46 | 1-2 |
This table shows the gain in oxygen and the loss in carbonic acid which results from the passage of blood through the capillaries of the lungs. The aerated blood returned to the heart by the pulmonary veins contains 8 to 12 volumes per cent. more oxygen, and about 7 volumes per cent. less carbonic acid, than the blood which the pulmonary artery carries to the lungs.
As to the physics of this exchange, the air in the recesses of the lungs contains about 16·36 per cent. of oxygen, and an amount of carbonic acid variously estimated at from 2·57 per cent. to 3·84 per cent. Of the 760 millimetres of mercury which the atmosphere holds up in a barometric tube, the oxygen in the alveoli of the lungs supports
| 760 × 16·36 | |
| ———— | = 124·33 millimetres; |
| 100 |
the carbonic acid, at the lower figure quoted (2·57 per cent.), 19·5 millimetres.
The tension of gases in arterial blood is ascertained by opening an artery into a closed vessel which contains nitrogen mixed with oxygen and carbonic acid at about the tensions which it is computed that they have in the blood. If the amounts of these gases are exactly right, no exchange occurs between the blood and the mixture of gases. The mean of many observations made in this way by various physiologists is, for oxygen in the blood 72·2 millimetres mercury pressure, for carbonic acid 20·5 millimetres mercury pressure. At a glance it is seen that, since the tension of oxygen in the blood never exceeds 72 millimetres, whereas its tension in pulmonary air never falls beneath 124 millimetres, there is no difficulty in accounting for its passage from air to blood. The position is somewhat otherwise with regard to carbonic acid. Aeration continues in the lungs until the tension of this gas in the blood returning to the heart does not exceed 20·5 millimetres; whereas the tension in pulmonary air, even accepting the lowest figure obtained by experimental means, is as high as 19·5 millimetres. This leaves a very small margin of pressure to account for the escape—and it is undoubtedly a rapid escape—of carbonic acid from blood as it circulates through the lungs. As was said regarding the fixation of carbonic acid in the blood, it is somewhat doubtful whether the problem has been completely solved.