252 



ESSENTIALS OF PHYSIOLOGY. 



dead space, the composition of which differs but little from that of the 

 atmosphere. 



The air is analysed by shaking up a known volume with caustic potash , 

 which absorbs the carbonic acid ; the diminution in volume represents the 

 amount of carbonic acid present in the air. The air is then shaken up with 

 pyrogallic acid, which absorbs oxygen, and the diminution in volume is again 

 measured. The residual gas is regarded as being nitrogen. 



THE GASES IN THE BLOOD. 



Before discussing the means by which the interchange of oxygen and 

 carbonic acid between the blood and air in the. lungs is effected, it is 

 necessary to determine, first, the amount of these gases in the blood, and, 

 secondly, the conditions in which they are held in it. 



If blood is exposed to a vacuum, there is a considerable evolution 

 of gas, which may be collected and analysed. For this purpose, some 

 form of gas pump is usually employed, the composition of the gas 

 being subsequently ascertained by exposing it first to potash, which 

 absorbs carbonic acid, and secondly to pyrogallic acid, which absorbs 

 oxygen. By this means it is found that 100 volumes of blood give off 

 about 60 volumes of gas, the composition of which varies according 

 to whether the blood is arterial or venous. The total quantity of 

 oxygen which 100 volumes of blood can take up or give off is called its 

 oxygen capacity. 



GASES IN THE BLOOD (Doo). 



This method necessitates the use of comparatively large quantities 

 of blood (10 to 20 c.c.) in order to give accurate results ; and Barcroft 

 has devised an apparatus, by means of which the estimation of the 

 blood gases can be carried out with very small quantities (1 c.c. or even 

 O'l c.c.) of blood. The method has the advantage of being readily 

 applicable to man, and depends upon the fact that, when potassium 

 ferricyanide and a trace of alkali (usually ammonia) are added to blood, 

 all the oxygen previously in combination with haemoglobin is evolved, 

 and the amount of oxygen given off. from a known volume of blood can 



