TtiE CASES OF THE BLOOD 255 



tration is increased, as by addition of carbon dioxide (Fig. 121) or 

 lactic acid, the effect is to increase the dissociation tension of 

 o. \\-lKemoglobin, or what is a different way of expressing the same 

 thing, to diminish the amount of ox3<gen taken up under a given 

 oxygen partial pressure. So that to obtain a given percentage 

 saturation of the haemoglobin, a greater partial pressure of oxygen 

 must be employed than in the absence of the acid. 



So sensitive is the dissociation curve to changes in the hydrogen-ion 

 concentration, that a method of detecting and estimating alterations in 

 the acidity of the blood has been based upon the determination of the 

 percentage saturation of the blood 

 with oxygen, at a definite partial 

 pressure. The influence of the 

 carbon dioxide in the patient's 

 blood is eliminated by adding to 

 the gaseous mixture with which 

 the blood is shaken up a per- 

 centage of carbon dioxide, corre- 

 sponding to the partial pressure 

 of that gas in the blood, that is 

 to say, a percentage equal to the 

 percentage of carbon dioxide in 

 his alveolar air (p. 241). 



In blood haemoglobin, of 

 course, exists in the presence 

 of salts and of carbon dioxide 

 and other acid metabolites, and 

 therefore such curves as are 

 given in Figs. 120 and 121, 

 more nearly represent the dis- 

 sociation curves of the blood 

 pigment under physiological conditions, than those reproduced in 

 Fig. 119. 



The Distribution and Condition of the Carbon Dioxide in the 

 Blood. The question is much more complicated than for the 

 oxygen, which is practically confined to one of the morphological 

 elements of the blood (the erythrocytes), and exists in the form of a 

 single compound. Carbon dioxide is distributed over the entire 

 blood in important amounts, and is present in several forms. The 

 serum yields a larger percentage of carbon dioxide than the clot, 

 but this percentage is not great enough to allow us to assume that 

 the whole of the carbon dioxide is contained in the plasma. Some 

 what more than a third of it belongs to the corpuscles. 



As regards the condition of the carbon dioxide, it is known that 

 some of it is simply dissolved in the plasma and corpuscles; but 

 although this fraction, on account of the relatively high coefficient cf 

 absorption of the gas (p. 245), is much greater than the corresponding 

 oxygen fraction, it is insignificant in comparison with the quantity 

 chemically combined. Carbon dioxide is united in dissociable 



ill 30 40 50 60 10 60 90 (00 



Fig. i2i. Dissociation Curves of Blood, 

 with Different Tensions of CO 2 (o, 3, 

 20, 40, and go mm.). Ordinates = 

 percentage saturation. Abscissae = 

 oxygen pressure. (After Barcroft.) 



