4:06 THE RESPIRATION 



be evident by studying the curves in Fig. 142 more minutely. Let us 

 suppose that all of the 18.1 volumes per cent of 2 in arterial blood be- 

 comes used up in the tissues. With the respiratory quotient of 0.8 (page 

 582) this will mean that 15 c.c. of C0 2 are carried away from the tissues in 

 every 100 c.c. of blood. If the blood when it -gained the lungs remained 

 completely unoxygenated it will be seen by examination of curve B 

 that to discharge this 15 c.c. C0 2 , the C0 2 -tension would have to fall 

 through 40 mm. e.g., from 80 mm. to 40 mm., or in other words, the al- 

 veolar C0 2 -tension would rise to this extent. But we know by actual 

 measurement of alveolar tension that no such change occurs. The ex- 

 planation is that the absorbed 2 forms oxyhemoglobin so that to find 

 the pressure difference necessary to drive out the 00 2 we must shift to 

 curve A, when we find that 22 mm. Hg. difference in tension is sufficient 

 to expel the 15 c.c. of C0 2 , as is shown in the curve by the straight line 

 joining A and B. Now we know that blood only yields up about one- 

 third of its oxygen during a circuit of the circulation; therefore, since 

 only 5 c.c. of C0 2 will be added to every 100 c.c. of blood, the necessary 

 difference in tension in the alveoli will require to be only a little over 

 7 mm. Hg., a difference which is not far removed from that actually 

 observed. Even when the pressure of C0 2 in the venous blood entering 

 the lungs is the same, or even somewhat less than that in the alveolar 

 air, some of the C0 2 will be discharged because of the arterialization of 

 the blood. 



How the C0 2 is Carried in the Blood. As already remarked this is 

 not fully understood. There are several conflicting observations for 

 which no satisfactory explanation can be given. If blood be separated 

 into corpuscles and serum and each exposed to atmospheres containing 

 equal percentages of C0 2 , the serum will absorb somewhat more C0 2 

 than the corpuscles. In both fractions it is carried, partly in simple so- 

 lution, its coefficient of solubility being relatively high (see page 354), 

 and partly combined with alkali to form bicarbonate. The bicarbonate 

 in the plasma of living blood is, as we have seen, the most important 

 regulator of the H-concentration, and indeed some recent work goes to 

 show that the bicarbonate is present for this specific purpose only and 

 not because it is an important carrier of 00 2 (Bayliss). In other words, 

 the CO, produced by metabolism in the tissues is carried by the blood 

 to the lungs mainly in combination with other things than alkali ; only 

 a small proportion of it becoming united with alkali to form bicarbonate 

 for the purpose of serving as a buffer substance. As to how the main 

 bulk of C0 2 is carried in the plasma cannot be said. It has been supposed 

 that it might be united with protein but there is incontrovertible evi- 

 dence to show that this is not the case; for example, Bayliss has found 



