CHANGES IN AIR AND BLOOD IN RESPIRATION. 605 



chemical combination in the plasma; (3) the part in chemical com- 

 bination in the corpuscles. Regarding the part in solution, we may 

 estimate approximately its value from our knowledge of the absorp- 

 tion coefficient of this gas at the temperature of the body. As stated 

 on p. 600, the calculation would account for 2.6 c.c. of the gas in each 

 100 c.c. of blood, that is, about 5 or 6 per cent, of the total amount 

 present in venous blood. The part of the carbon dioxid present 

 in the plasma is held partly in loose chemical combination, partly in 

 a more fixed form. That is to say, if serum or plasma is exposed 

 to a vacuum only a portion of the C0 2 is given off; to obtain the 

 remainder, the so-called fixed C0 2 , it is necessary to add some 

 weak acid. If the full blood is used in such an experiment all of the 

 C0 2 may be obtained, a fact which indicates that the red corpuscles 

 (hemoglobin) play the part, in this respect, of a weak acid. The 

 form in which the C0 2 is held in the plasma is not entirely 

 understood. It is supposed that it is combined with the alkali 

 of the blood as sodium carbonate (Na 2 C0 3 ) or sodium bicar- 

 bonate (HNaC0 3 ). When the carbon dioxid pressure in the sur- 

 rounding medium is lowered, some of the bicarbonate dissociates, 

 giving off CO 2 ; and the reverse takes place when the pressure of the 

 carbon dioxid is raised. Essentially, therefore, the process of taking 

 up and giving off C0 2 from the plasma may be represented by the 

 reaction: Na 2 C0 3 + C0 2 +H 2 0^ 2(HNaC0 3 ). In the blood, how- 

 ever, the conditions are more complex than in a simple aqueous solu- 

 tion of carbonate of soda. The proteid present has also an affinity for 

 the alkali and thus acts like a weak acid in aiding the dissociation 

 of the bicarbonate. One may say that the alkali of the blood is 

 distributed between the C0 2 and the proteid in accordance with 

 the law of mass action, and that the proteid thus plays an important 

 part as well as the alkali in controlling the conditions under which 

 the CO 2 is held. The phosphoric acid in the blood, so far as it is 

 present, plays naturally a similar role in combining with the alkali 

 and thus influencing the dissociation of the compound of the C0 2 

 with the alkali. If the proteids (and phosphates) were not present 

 the combination between the alkali and C0 2 would be so strong that 

 the compound would fail to fulfill its respiratory functions, that is, 

 ijt would not dissociate readily when the pressure of the C0 2 in the 

 surrounding medium is lowered. 



The portion of the C0 2 that is held in the corpuscles is contained 

 in part in combination with the alkali present, under the same con- 

 ditions as those described for the plasma. But, according to 

 Bohr,* the hemoglobin is capable of combining directly with the 

 C0 2 , and, indeed, independently of the amount of oxygen that it 

 may hold in combination. He suggests that the C0 2 combines with 

 * Bohr, " Skandinavisches Arehiv f. Physiologie, " 16, 402, 1904. 



