680 PHYSIOLOGY OF RESPIRATION. 



larger than the preceding, is chemically combined with the 

 alkali of the blood as a carbonate, most probably as a bicarbonate 

 (HNaCOa). Other facts tend to show that the available alkali 

 of the blood in combination as carbonate or phosphate is not 

 sufficient to combine with the quantity of carbon dioxid normally 

 present. In recent years an additional possibility has been sug- 

 gested by the discovery (Bohr) that carbon dioxid forms a disso- 

 ciable compound with hemoglobin (p. 429), and the probability 

 that a similar compound may be formed with the proteins of the 

 plasma. Accepting this suggestion it would seem that the carbon 

 dioxid exists in the blood in three forms. The amounts present 

 in each form is estimated by Loewy* as follows: In each 100 c.c. 

 of arterial blood, containing normally 40 volume per cent, of 

 carbon dioxid, there is 



Physically absorbed in plasma and corpuscles 1.9 per cent. 



He!d as sodium bicarbonate { JP*; ; - * } . . 18.8 " 



Held in organic combination { gj* ; ; Jj } . . !9.3 - 



When serum or plasma is exposed to a vacuum at body tem- 

 perature only a portion of the carbon dioxid is given off; to 

 obtain the balance it is necessary to add acid to the liquid. This 

 latter portion, liberated only by a stronger acid, is spoken of as 

 the "fixed carbon dioxid." If instead of exposing serum or 

 plasma to a vacuum one uses full blood, that is, plasma or serum 

 plus corpuscles, all the carbon dioxid may be obtained without 

 the necessity of adding acid. This fact has been explained on 

 the supposition that the hemoglobin under these conditions 

 plays the part of an acid in breaking up the compound in which 

 the carbon dioxid is firmly held. Presumably this fixed carbon 

 dioxid is the portion which in the above classification is repre- 

 sented as bicarbonate. Since the portion that is held in organic 

 combination is apparently more easily dissociated, it seems 

 likely that it furnishes the main compound which is physio- 

 logically useful in providing a means for the transportation of 

 carbon dioxid from the tissues, where it is formed, to the lungs, 

 where it is eliminated. 



The Physical Theory of Respiration. The physical theory 

 of respiration assumes that the gaseous exchange in the lungs and 

 in the tissues takes place in accordance with the physical laws of 

 diffusion of gases. If a permeable membrane separates two vol- 

 umes of any gas, or two solutions of any gas at different pressures, 

 the molecules of the gas will pass through the membrane in both 



* Loewy, "Handbuch d. Biochemie," 1908, IV. 



