RESPIRATION 85 



per cent, while actually about 4 volumes per cent are given off. It 

 is evident, therefore, that the giving off of CO 2 in the lungs is 

 almost entirely dependent on the dissociation of its chemical 

 combinations in the blood. 



In what form is CO 2 chemically combined in the blood? We 

 cannot answer this question in the same comparatively simple 

 and definite manner as in the case of the combination of oxygen 

 with blood. CO 2 dissolved in water has acid properties, and by the 

 addition of other stronger acids to blood the dissociable chemical 

 combinations with CO 2 are entirely broken up and CO 2 liberated. 

 It is thus quite evidently as an acid (i.e., as H 2 CO 3 ) that CO 2 

 enters into combination with blood. On analysis blood is found to 

 contain an excess of alkali (for the greater part soda) not com- 

 bined with mineral acids. In other words hydrochloric, phosphoric, 

 and small amounts of sulphuric, acids are present in blood, but 

 not in sufficient amounts to saturate the alkali. Hence CO 2 is ap- 

 parently free to combine with the excess of alkali, forming, since 

 an excess of free CO 2 is present, bicarbonates. As Zuntz 3 pointed 

 out, if blood were nothing but a solution of the well-recognized 

 acids and bases present in it, we could account for the quantity of 

 CO 2 which it is capable of combining with chemically. Zuntz cal- 

 culated that the excess of alkali present in the blood is equivalent 

 to at least a 0.2 per cent solution of soda. This could take up as 

 bicarbonate as much CO 2 as blood can take up in combination. 

 Nevertheless the properties of such a solution in respect to the 

 carriage of CO 2 would not approach to those of blood: for the 

 soda would remain completely saturated as bicarbonate when ex- 

 posed to the CO 2 in the alveolar air, and there would not be any 

 appreciable dissociation, so that the solution would be no better 

 than distilled water as a physiological carrier of CO 2 . This point 

 has been rendered specially clear by Bohr, who investigated the 

 dissociation curve for CO 2 of a dilute sodium bicarbonate solution. 



To reach an insight into the actual behavior of blood as a car- 

 rier of CO 2 we have to take into consideration another factor. 

 Proteins have the very peculiar property of being able to act either 

 as weak alkalies towards acids or as weak acids towards alkalies. 

 This is shown, for instance, by the familiar fact that an ordinary 

 indicator such as litmus ceases to give a sharp end-point when a 

 protein is present, and that not only neutral but even slightly acid 



'Zuntz, Hermann's Handbuch d,er Physiol., IV, 2, p. 65, 1882. To Zuntz's 

 admirably clear and thorough discussion of the subject I am greatly indebted. This 

 discussion is far ahead of most of what has appeared in later textbooks and papers. 



