676 PHYSIOLOGY OF RESPIRATION. 



venous. Not more than 2 to 2.5 c.c. of this amount can be held 

 in physical solution, the remainder must be present in chemical 

 combination. As stated above, a considerable portion, as much 

 perhaps as 20 c.c, is contained in the plasma in combination with 

 the sodium as sodium bicarbonate.* As for the remainder it has 

 been suggested that it is held in some form of combination with 

 the proteins of the plasma and the corpuscles. We know that the 

 blood takes up some CO2 in the tissues and gives off this new quota 

 to the alveolar air while circulating through the lungs. Conse- 

 quently, a portion of the CO2 in the blood must be held in some 

 form of chemical combination which undergoes dissociation or a 

 reaction of some kind in the lungs that results in the giving off of 

 its CO2. It has proved to be a difficult problem to explain the 

 nature of this carrier of the CO2. The sodium bicarbonate in solu- 

 tion in the plasma cannot well discharge the function. Aqueous 

 solutions of sodium bicarbonate do undergo dissociation with the 

 liberation of CO2 when exposed to low pressures of CO2, as low as 

 0.3 per cent, of an atmosphere, but in the lungs the alveolar air 

 contains CO2 at a pressure of 5 to 5.5 per cent, of an atmosphere, 

 so that under these conditions a liberation of CO2 from the sodium 

 bicarbonate is not possible. It has been suggested, therefore, that 

 the CO2 combined with the proteins of the plasma and corpuscles 

 may be so held as to be capable of functioning in the way demanded 

 by the theory of respiration. Recent work has tended to support 

 this view, but with the modification that the CO2 is not combined 

 directly with the protein, but with the alkali contained in the 

 protein (Van Slykef). In this connection the hemoglobin is of 

 especial importance. Hemoglobin has acid properties and can 

 combine with a certain amount of alkali, sodium or potassium. 

 When CO2 is added to the blood, as happens in the tissues, this 

 CO2 or the carbonic acid which it forms with the water of the blood 

 and tissues, can enter the corpuscles and react with the alkali 

 combined with the hemoglobin, forming a sodium or potassium 

 bicarbonate. In the lungs this compound breaks down, liberating 

 the CO2, while the alkali goes back to its combination with the 

 hemoglobin. To explain this reaction in the lungs it is pointed 

 out that oxyhemoglobin has stronger acid properties than the 

 reduced hemoglobin. Consequently, as the oxyhemoglobin is 

 formed by the absorption of the oxygen from the alveolar air it 

 will tend to break up the bicarbonate, binding the alkali and liber- 

 ating the CO2. On the other hand, in the tissues a certain propor- 

 tion of the hemoglobin passes from the condition of oxyhemo- 



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



t Van Slyke, "Physiological Reviews," 1, 141, 1921. 



