THE CHEMISTRY OF RESPIRATION 



1179 



becomes completely saturated with oxygen. No carbon dioxide is given off, 

 since this combines with the weak ammonia. If the two bloods contain the same 

 amount of oxyhsemoglobin no difference will be produced in the level of the oil 

 in the two tubes. If, however, one be arterial and the other venous, the venous 

 blood will absorb more oxygen from its bottle than the arterial blood from its 

 side of the apparatus, so that the oil will rise in the tube on the side of the 

 venous blood. From the amount of rise the difference in the amount of 

 oxygen taken up by the blood on the two sides can be reckoned, and this figure 

 will express the relative saturation of the haemoglobin in the two samples of 

 blood. 



For clinical purposes it is possible to work with 0-1 c.c. of blood. Fig. 492 B 

 represents the form of apparatus devised by Barcroft for dealing with these 

 minute quantities. The principle of the apparatus is the same as that of the 

 larger type. 



The condition of the gases in the blood can be judged by the 

 amount of gas which the blood will take up when exposed to different 

 pressures of the gas. If a gas is in simple solution the amount of it 

 dissolved varies directly with the pressure. Thus, if water takes up 

 a certain bulk of a gas at a given temperature and pressure, it will 

 take up twice as much if the pressure of the gas be doubled. 

 Since the volume of a gas varies inversely as the pressure, we may 

 say that a fluid will dissolve the same volume of gas whatever the 

 pressure. The absorption coefficient of a liquid for a gas is expressed 

 by the number of cubic centimetres of gas which will be taken up at 

 C. by 1 c.c. of the liquid when the gas is at a pressure of 760 mm. 

 Hg. The absorption coefficient diminishes with rise of temperature. 

 The following Table represents the absorption coefficient for oxygen, 

 carbon dioxide, carbon monoxide, and nitrogen, in water at various 

 temperatures between and 40 C. : 



From this Table we see that 100 c.c. of water at C. will absorb 

 4-89 C:C. oxygen at 760 mm. Hg., i.e. at one atmosphere. If the 

 pressure be raised to two atmospheres the volume of gas absorbed 

 will be the same, but if these gases be measured at the original pressure, 

 i.e. at one atmosphere, the amount dissolved will be 9'78 volumes. 

 If therefore we plot out the absorption of the gas on a curve of which 

 the ordinates represent the amount of gas dissolved and the abscissa 

 the different pressures of the gas, we shall find that the curve is a 



