406 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 indeed even somewhat less than that in the 

 alveolar air, some of the C0 2 will be discharged because of the arterial- 

 ization of the blood. In normal human blood equilibrated with C0 2 at 

 a partial pressure of 40 mm. at 38 C. the total C0 2 varies between 43 

 and 55 vols. per cent for whole blood (Peters and Barr) 85 and it is about 



8 vols. per cent higher for plasma (Joffe and Poulton). 86 



How the C0 2 is Carried in the Blood. Most of the carbon dioxide 

 is carried in combination with alkali which is set free for this purpose 

 from hemoglobin. The latter is therefore a carrier of C0 2 in the sense 

 that it furnishes the necessary alkali. It does this in virtue of being 

 a weak acid. This acidity is however decidedly greater for HbO than 

 for Hb so that as 2 leaves the blood in the tissue capillaries and HbO 

 is changed into Hb, alkali that was previously in combination is set free 

 and can combine with C0 2 . This is the explanation of the curve in Fig. 

 142. Even although no reduction of HbO to Hb should occur, an in- 

 creased tension of C0 2 in the blood would also cause some alkali to be 

 split off from HbO as well as some from phosphates. The H 2 C0 3 fixed 

 in these ways must pass through the envelopes of the corpuscles. But, 

 it may be asked, how does this explain fixation of C0 2 by the plasma 

 alone? This occurs because the hemoglobin increases the alkali of the 

 plasma by withdrawing Cl into the corpuscles from the Nad of the 

 plasma, thereby leaving the Na to form NaHC0 3 . The Cl like the 

 H 2 C0 3 unites in the corpuscles with the alkali set free from Hb and HbO 

 and by interchange with phosphates. In other words, when the tension 



