TEXT-BOOK OF PHYSIOLOGY. 



ATMOSPHERIC AIR. 



O-I5S MM MG T OR 20. 85 P.C 



CO O.3 MM HG OR O.O4- P.C 



0-TENSlON : 

 22.. 0+ MM HQOR 



CO TENSION 

 4-1.0* MM HO OR 

 8.4 P.C. 



AL.VEOLUS 



The carbon dioxid pressure in the tissues being in excess of that 

 in the blood, it passes through the capillary wall into the blood, where 

 it exists in the free and combined states. On passing into the pul- 

 monic capillaries the blood enters a region in which the carbon 

 dioxid in the alveoli is less than in the blood. At once a diffusion and 

 dissociation of the carbon dioxid takes place through the alveolo- 

 capillary wall until equilibrium is established. This, however, is of 

 very short duration, for the carbon dioxid so eliminated is rapidly 

 removed from the lungs by the respiratory movements. 



While diffusion, in response to physical and chemic conditions, 



thus plays a large 

 part in, and is suf- 

 ficient to account 

 for, the exchanges 

 of gases, it is pos- 

 sible that the alve- 

 olar or respiratory 

 epithelium may 

 also play an essen- 

 tial role. It is be- 

 lieved by some in- 

 vestigators that it 

 is active in both 

 the absorption of 

 oxygen and the 

 excretion of carbon 

 dioxid. This view 

 has been suggested 

 as a means of in- 

 terpreting the re- 

 sults of the experi- 

 ments of more re- 

 cent investigators, 

 made with a view 

 of determining the 

 tension of the 

 blood gases. It 



was found by Bohr that the tension of the oxygen in arterial blood 

 was often as high as 101 to 144 mm. Hg, and in many instances 

 higher than the tension of the oxygen in the trachea, while the 

 carbon dioxid tension in the trachea was higher than in the blood. 

 Haldane and Smith by a different method found an oxygen tension in 

 the arterial blood of 200 mm. Hg. If these results should prove to 

 be correct, though they are at present subject to considerable criticism 

 and not generally accepted, some other force than diffusion would 



VENOUS 



BLOOD 



ARTERIAL 

 BLOOD 



0-TENSIO 



0-TCNSION 



29. 6* MM HC OR 

 3.9 P.C. 



CO z TENSION 

 21.28 M M HO OR 

 2.8 P.C. 



FIG. 177. DIAGRAM SHOWING THE RELATIVE TENSION 

 OF OXYGEN AND CARBON DIOXID IN THE LUNGS, 

 IN THE BLOOD, AND IN THE TISSUES. 



