RESPIRATION BEYOND THE LUNGS 



405 



height of the curve at any given tension of C0 2 varies somewhat for 

 different individuals, but not in proportion to the tension of C0 2 in the 

 alveolar air ; that is, a person with a low C0 2 -dissociation curve may have 

 a high alveolar C0 2 -tension and vice versa. On the other hand in a given 

 individual if the one of these be lowered by some physiological condition, 

 the other will become altered in the same direction. Thus in muscular 

 exercise the C0 2 -dissociation curve and the alveolar tension of C0 2 both 

 decline. 



It can be shown that it is the degree of reduction of hemoglobin which 

 is responsible for the alteration in C0 2 -carrying power. Thus, the dis- 



50 



yfr 



30 40 



50 



60 70 

 vrv rtvm. 



80 



90 



Fig. 142. Curve of CO 2 tension in blood. For description, see text. (From Christiansen, Doug- 

 las and Haldane.) 



sociation curve is the same whether pure oxygen or air is used as the 

 diluting gas. Did 2 per se have an influence these results should be 

 different. Another indication that it is the amount of reduced hemoglobin 

 that is the determining factor is that the curve is the same in the presence 

 of coal gas as in that of oxygen. 



When we were considering the unloading of 2 from the blood in the 

 tissues we saw that the local increase in C0 2 -tension must encourage it 

 because of depression of the dissociation curve for 2 (page 399). Of much 

 greater physiological importance, however, is the opposite relationship be- 

 tween the unloading of C0 2 from the blood in the lungs and the increase 

 in oxyhemoglobin due to the oxygen taken up from the alveoli. This will 



