THE CHEMISTRY OF RESPIRATION 505 



presence of this carbon dioxid which may seriously interfere with the 

 determination of the dissociation curve of hemoglobin and oxygen in 

 whole blood. Its action is similar to that of weak acids, such as lactic 

 acid, because the greater its tension, or the greater the acidity of the 

 blood, the greater is the dissociation of the oxygen. It possesses, there- 

 fore, a solvent action which, however, it does not unfold unless the 

 oxygen tension is markedly diminished. To illustrate, under a partial 

 pressure of the oxygen of 150 mm. Hg, the blood remains practically 

 saturated even if its carbon dioxid tension is varied within physio- 

 logical limits. If the oxygen pressure is now reduced to 20 mm. Hg and 

 the carbon dioxid pressure to 5 mm. Hg, the oxyhemoglobin content 

 of the blood is changed to 67.5 per cent. This value may be further 

 decreased by raising the carbon dioxid tension. This is a matter of 

 great importance to the body, because it facilitates the liberation of 

 oxygen in those parts of the body in which the tension of this gas is 

 low, i.e., in the tissues. By means of this peculiar action of the carbon 

 dioxid, the hemoglobin is relieved of all available oxygen, in fact, of 

 more than it would allow to be transferred to the cells under ordinary 

 conditions of oxygen diffusion. 



The Condition of Carbon Dioxid in the Blood. — While the amount 

 of carbon dioxid absorbed by blood, is dependent upon its partial 

 pressure in the surrounding medium, a direct relationship between 

 these factors does not exist. In fact, the volume of this gas actually 

 acquired by a certain quantity of blood, is much greater than the 

 volume which could theoretically be allotted to it upon the basis of 

 its absorption coefficient. It is evident, therefore, that only a part of 

 the carbon dioxid is retained in a physical state,while another part 

 forms a dissociable chemical compound with some constituent of the 

 blood. Conditions, however, are not so simple as they are in the case of 

 oxygen, which gas unites with only one element of the blood, whereas 

 the carbon dioxid is bound to several, i.e., to the plasma as well as to 

 the corpuscles. 



If the venous blood of the dog is exposed to .the vacuum of an air pump, from 

 45 to 50 c.c. of carbon dioxid may be extracted from each 100 c.c. of blood. It 

 has also been ascertained with the help of the aerotonometer that this gas is held 

 in the venous current under a pressure of about 40 mm. Hg, or 5-6 per cent, of an 

 atmosphere. This coexistence of a relatively high carbon dioxid content and 

 low degrees of pressure, immediately assumes a greater significance if these values 

 are compared with those obtained with pure solutions of this gas. Thus, if water 

 and carbon dioxid are shaken under a pressure of 760 mm. Hg and at the tem- 

 perature of the body, about 50 per cent, of the gas will be absorbed. Quite 

 similarly, if blood plasma is treated in this way, it will take up an almost equally 

 large amount of this gas, while whole blood assimilates almost 150 c.c. But 

 naturally, under normal conditions the blood is not exposed to a carbon dioxid 

 pressure of one atmosphere (760 mm. Hg), but only to a pressure of about 40 

 mm. Hg = 3^9 of an atmosphere. Hence, all the carbon dioxid, excepting 2.01 

 c.c. for every 100 c.c. of blood, must be held in chemical combination, and further- 

 more, if the volume of the corpuscles is reckoned at ^i of the total volume of the 

 blood, these bodies must contain 0.59 c.c. and the plasma 1.42 c.c. of this gas in a 



