THE GASES OF THE BLOOD. 239 



THE GASES OF THE BLOOD. 



These are present in two conditions : i.e., (1) dissolved in it 

 in accordance with well-established physical laws,* and (2) chemi- 

 cally combined. But, since the latter 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 C. and 760 mm. pressure) that every 100 volumes 

 of arterial blood yield 20 volumes of oxygen, whilst 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 atmos- 

 pheric pressure ; (2) chemically combined, in which state nearly 

 all the oxygen exists, and forms with the haemoglobin a loose 

 combination called oxyhsemoglobin. This oxygen, therefore, does 

 not follow the laws of absorption by leaving the blood in propor- 

 tion as the pressure is reduced, but at a certain point of reduction 



* I. A given liquid absorbs the same volume of a given gas independent of 

 the pressure exercised by that gas. 



II. At the same temperature the volume of a gas varies inversely with the 

 pressure, so that with twice the pressure a given volume of gas is twice the 

 weight. 



III. Therefore, the amount by weight of gas absorbed by a liquid depends 

 directly on the pressure, being nil in vacuo. 



The weight of a given volume of gas decreases as the temperature in- 

 creases ; therefore, the amount of gas absorbed is in inverse proportion to the 

 temperature, being practically nil at boiling point. 



