330 



Comparative Anhnal Physiology 



buffers. For each fluid containing buffers a COo dissociation curve can be 

 constructed by equilibrating with a known CO2 tension and measuring the 

 volumes per cent taken up. Thus in the two examples above, in human 

 blood 40-50 volumes per cent corresponds to 40 mm. CO:-, whereas in sea 

 water 4.8 volumes per cent corresponds to 0.23 mm. COo. Typical COo 

 dissociation curves are given in Figure 84. These indicate differences in the 

 COo-combining ability of different species. Aquatic animals with a very 



TABLE 58. CO. CONTENT OF DRAWN BLOOD IN VOLUMES PER CENT 



AND CORRESPONDING CO, TENSIONS. VALUES FOR SEA WATER 



ARE GIVEN FOR PURPOSES OF COMPARISON. 



weak buttering supply of cations, such as ascidians, may actually contain less 

 total COo than the surrounding water.-'^-* 



1 he COo content of circulating blood depends on the total buffering capa- 

 city of the blood, the COo in body tissues, and the COo outside the respiratory 

 surface. In I able 58 are given the COo content in volumes per cent and 

 corresiwnding COo tensions for freshly drawn blood from several animals. In 

 terrestrial vertebrates the COo tension to which blood is exposed in the lungs 



