626 PHYSIOLOGY OF RESPIRATION. 



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. 621, 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 

 which is chemically combined in the plasma is held partly in an 

 easily dissociated 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 CO 2 is given off; to obtain the remainder, the so-called 

 fixed CO 2 , it is necessary to add some acid. On the contrary, when 

 the whole blood (plasma and corpuscles) is exposed to a vacuum, at 

 the temperature of the body, all of the CO 2 may be given off. In 

 this latter case the hemoglobin evidently acts like a weak acid in 

 breaking up the firm combination of the so-called fixed carbon 

 dioxid. The portion of the carbon dioxid which exists in the form 

 of an easily dissociated compound, breaks up and gives off CO., when 

 the pressure of this gas in the surrounding medium is low. The 

 nature of this compound is not entirely clear. Some of the CO 2 in 

 the blood is undoubtedly combined with sodium to form sodium 

 carbonate or bicarbonate. An aqueous solution of sodium bicar- 

 bonate when exposed to a vacuum gives off some of its CO 2 and is 

 converted to the carbonate, while, on the other hand, a solution of 

 the carbonate exposed to an atmosphere containing CO., takes up 

 some of this gas, forming sodium bicarbonate. The reaction in 

 the two cases may be expressed by a reversible equation, as follows: 



Na 2 CO 3 + CO 2 -f H 2 O ^. 2HNaCO 3 



Sodium carbonate. Sodium bicarbonate. 



It has been suggested that the respiratory exchange of carbon 

 dioxid in the body takes place essentially according to this equation. 

 Experiments upon aqueous solutions of sodium bicarbonate show, 

 however, that dissociation with liberation of CO 2 does not take 

 place until the tension of the CO 2 in the surrounding medium falls 

 very low, lower than actually exists in the body. Some other 

 hypothesis must be sought, therefore, to explain the giving off 

 of the CO 2 in the lungs. A supplementary hypothesis of this char- 

 acter has been suggested. The proteins of the blood have the 

 property of combining with the bases (Na, K, etc.), and normally, 

 therefore, the bases present must be divided between the protein 

 and the acids of the blood. The sodium, for instance, will exist 

 partly in combination with the protein and partly as sodium car- 

 bonate or phosphate, taking account only of the combination 

 with the weak acids. When the tension of the carbon dioxid is 

 increased, as happens, for instance, in the active tissues, the 



