FUNCTIONS OF BLOOD 89 



animal that the CO2 tension reached the figure of 40 mm in 

 the respiring cells while it was only 2 mm in the surrounding 

 medium, a transport of 5 volumes per cent, corresponding to 

 the tension difference, would be possible. 



For reasons which have nothing to do with the transport of 

 gases, pure water is never utilized by animals as an internal 

 medium, and in all the cases with which we have to deal 

 transport takes place by means of substances which combine 

 chemically with the gas in question. To be of use as a trans- 

 port mechanism such combination must be "dissociable'' in 

 the sense that, at least within a certain range, the combined 

 quantity of gas must increase with increasing gas-tension, 

 making the properties of the combination capable of expres- 

 sion in a "dissociation" curve relating tension to combined 

 quantity. This will be amply illustrated in the following. 

 It will be convenient to discuss separately first the transport of 

 CO2 and next the transport of 0>, although the two are to 

 some extent mutually interdependent. 



The transport capacity of blood for C0 2 . The fluids actually 

 transporting C0 2 in the animal body have a complicated 

 and varied composition, normally containing strong kations 

 (Na + ) and anions (CI - ) and also weak anions like phosphate, 

 lactate, and protein ions. It is not proposed to go into the 

 separate effects of these different ions. It is sufficient to 

 state that there is generally a surplus of strong kations over 

 strong anions which combines with C0 2 to form bicarbonates 

 and make up the "alkali reserve." The dissociation range of 

 bicarbonates alone is at very low COo tensions and would in 

 most cases be unsuitable for respiration purposes, but the range 

 is extended by the presence of other weak acids. When bio- 

 logical fluids, acting as carriers of COo. are exposed in "satura- 

 tors" to atmospheres containing known concentrations of the 

 gas, and the corresponding quantities of C0 2 in 100 volumes 

 of fluid are determined, curves can be constructed relating 

 CO2 tensions, expressed generally in mm Hg, to quantities in 

 volumes per cent. Examples of such curves are given in 



