These are present in two conditions: (i) dissolved in accordance with well-established physical laws,* and (2) chemically combined. But since those present in the latter state are but loosely combined they may be separated by the same means as the former, and thus the oxygen, carbon dioxide, and nitrogen can all be removed by reducing the pressure with the air pump. For this purpose a mercurial pump must be used, by means of which a practically perfect vacuum can be formed and all the gases obtained in a manner which facilitates further analysis. Together they are found to measure about 60 volumes for every 100 volumes of blood.

## Oxygen

The amount of oxygen in the blood is found to vary much with circumstances. In arterial blood the quantity is much more constant, and always exceeds that in venous blood. It is estimated (at o° C. and 760 mm. pressure) that every 100 volumes of arterial blood yield 20 volumes of oxygen, while the volume of oxygen in venous blood varies from 8 to 12 per cent.

The oxygen which comes off in the Torricellian vacuum exists in the blood in two distinct states: (1) a very small quantity simply absorbed, - about as much as water absorbs under atmospheric pressure; (2) chemically combined, in which state nearly all the oxygen exists, and forms with the haemoglobin the loose combination called oxyhemoglobin. This oxygen therefore does not follow the laws of absorption by leaving the blood in proportion as the pressure is reduced, but when a certain point of reduction of pressure (20 - 30 mm. mercury, according to the temperature) is reached, the oxygen comes off almost completely.

* 1. At the same temperature the volume of a gas varies inversely with the pressure, so that with twice the pressure a given volume of a gas is twice the weight.

2. A given liquid absorbs the same volume of a given gas, to which it is exposed, independent of the pressure exercised by that gas.

3. Therefore the amount by weight of gas absorbed by a liquid, at a given temperature, depends directly on the pressure, being nil in vacuo.

4. The weight of a given volume of a gas decreases and the coefficient of absorption of a liquid diminishes, as the temperature increases.

5. Therefore the amount of gas absorbed is in inverse proportion to the temperature, being practically nil at boiling point.

## Carbon Dioxide (C02)

The amount of carbon dioxide also varies more in venous than in arterial blood, for under certain circumstances (suffocation) it may rise to over 60 volumes per cent., although ordinary venous blood on an average contains only 46 volumes in every 100 of blood. On the other hand, the amount of this gas in arterial blood varies little from 39 volumes per cent.

Nearly all the carbon dioxide exists in the plasma, where some of it appears to be chemically combined with soda salts.

## Nitrogen

The amount of nitrogen does not vary much, being in both venous and arterial blood about 1.5 volume per cent., and it would appear to be simply absorbed.

For further details about the gases of arterial and venous blood, see Respiration.