496 VETERINARY PHYSIOLOGY 



with the partial pressure in the air to which the blood is 

 exposed. 



As already indicated, the proteins of the blood plasma 

 and of the red cells, notably the globin, which is the chief 

 constituent of hgemoglobin, may combine with CO.2. 



Attempts have been made to determine the amounts of 

 CO., in the various combinations, but at present our knowledge 

 is too defective to allow of definite figures being given. 



It has been maintained that, since NaHCOg is not dis- 

 sociated at the temperature of the body with a partial 

 pressure of COj such as occurs in the lungs, therefore the 

 bicarbonate does not play the part of carrying the CO2 from 

 the tissues to the lungs, and that the main carrier is the haemo- 

 globin, the HbCOo being more unstable in the presence of 0,. 



This theory seems to ignore the significance of the 



H.,C0, 1 , . .,. 



proportion between ^ ^j. " = — and its adjustment under 



various conditions. 



It is certain that the amount of CO, which leaves the 

 blood in the lungs is a very small part of the amount held 

 in the blood (p. 498). 



The actual quantities of oxygen and of carbon dioxide in 

 the blood are of much less importance than their tension. 



Method of Determining. 

 (i.) They may be extracted by subjecting the blood to 

 the Torricellian vacuum over the barometric column of 

 mercury. Many forms of mercury gas pumps have been 

 devised. One is shown in fig. 203. By raising the mercury 

 ball 3LB., air may be driven out of the blood bulbs a-h by 

 filling them with mercury. On clamping at a and lowering 

 M.B., a Torricelhan vacuum is produced. The bulbs are 

 then detached and weighed, and blood is collected in them 

 from a vessel. The blood bulbs are then connected with the 

 apparatus and a vacuum produced in G.B., where the gases 

 are collected. By turning the two-way tap T., they can be 

 passed into the eudiometer tube E., and then analysed, the 

 carbon dioxide being absorbed by caustic soda and the 

 oxygen by alkaline sodium pyrogallate. 



