492 



VETERINARY PHYSIOLOGY 



V. Gases of the Blood. 

 A. The Oxygen of the Blood. 



The study of the pigments of the blood has shown that 

 the function of haemoglobin is to carry oxygen from the 

 lunsfs and to give it off to the tissues. It has been shown 

 that it is the coloured iron-containing hsematin, constituting 

 only about 4 per cent, of the molecule which acts as the 

 carrier. 



Haemoglobin carries oxygen in virtue of the fact that, 

 when it is exposed to a high partial pressure of the gas in 

 the lungs it takes it up, while it gives it off when exposed to 

 a low pressure in the tissues. 



The partial pressure of a gas in an atmosphere is got 

 by multiplying its percentage amount by the atmospheric 

 pressure and dividing by 100. Thus, taking the ox3^gen at 

 20 per cent, of atmospheric air, at normal pressure at sea- 

 level of 7 GO mm. Hg the partial pressure of the oxygen is — 



20 X 760 



100 



= 152 mm. Hg. 



The tension of a gas in a fluid, i.e. its tendency to escape, 

 may be measured by finding the partial pressure of the gas 

 in the atmosphere to which the fluid is exposed at which the 

 gas is neither given off nor taken up. Thus, if three vessels 

 containing oxygen in blood had over the fluid 2, 5, 10 

 per cent, of O2, i.e. 0, at partial pressures of 15'2, 38"0, 

 and 76"0 mm. Hg, and it were found that O2 came off in the 

 first and was taken up in the third, but remained constant 

 m the second, we should say that the tension of the gas was 



38 mm. Hg. 



Beginning 



0, 



in Air 



Blood 



End 

 in Air 



5% 



38-0 mm. Hg. 



5% 



% 



5% 



38*0 mm. Hg. 



38-0 mm. Hg. 



3S-0 mm. Hg. 



