The unloading of oxygen from the blood 163 



Our procedure is to expose each sample of blood to suitable known 

 pressures of C0 2 in a tonometer at 37 C. and to analyse the blood in 

 order to ascertain the amounts of C0 2 which it holds at these pressures. 

 The crosses in Fig. 87 represent such analyses. A graph is made for 

 each blood, relating the quantity of CO 2 to known pressures of the 

 gas ; from this graph we can read the CO 2 pressure corresponding to 

 the quantity held by each sample of blood. Were the samples alike 

 in the matter of other acids one graph would serve for the three, but 

 no assumption of that kind can be made, indeed it was known that 

 the reverse was the case. 



These three graphs are shown in Fig. 87. From them it appears 

 that the carbonic acid pressures in the three samples of blood were 

 approximately : 



(1) Pressure of C0 2 in arterial blood 36 mm. 



(2) Pressure of C0 2 in venous (resting) blood 46 mm. 



(3) Pressure of CO 2 in venous (active) blood 34 mm. 



We now proceed to determine the oxygen-dissociation curves for 

 the three samples of blood at or near these C0 2 pressures. For this 

 purpose the following data were obtained : 



(1) Oxygen pressure 36mm. percentage saturation 60, 62 %> 



mean 61 % K = '000200. 



(2) Oxygen pressure 43mm. percentage saturation 62, 65 %> 



mean 63'5 u / . K = '000147. 



(3) Oxygen pressure 40 mm. percentage saturation 54, 54 / 



mean 54 %. K = '000116. 



The curves corresponding to these values of K are given in Fig. 88. 

 The arterial blood has been assumed to be the same throughout the 

 experiment, the curves given for it in the two portions of Fig. 88 are 

 identical. On the curve may be read off the pressures corresponding 

 to the observed percentage saturations of the blood with oxygen, 

 we thus find that the oxygen pressure is : 



(1) In the arterial blood 93 mm. 



(2) In the venous (resting) blood 39 mm. 



(3) In the venous (chorda) blood 49 mm. 



The two latter form the "final capillary pressure-head" for the diffusion 

 of oxygen from the capillary into the tissue under the circumstances 

 of rest and activity respectively. 



This completes our survey of the general and the local mechanisms 

 for securing an efficient oxygen pressure in the capillary circulation. 

 In the examples which we have taken from the submaxillary gland, 



112 



