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)/100 = 124·33 millimetres; 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. The carbonic acid exhaled contains all the carbon of the digestible food, with the exception of a comparatively small quantity given off in urea. It amounts to about 900 grammes per diem.