CHANGES IN AIR AND BLOOD IN RESPIRATION. 689 



as that of the carbon dioxid in the alveoli of the lungs namely, 

 5.5 per cent, of the alveolar atmosphere (40 mms.). In the sys- 

 temic capillaries the blood comes into diffusion relations with the 

 tissues, and direct examination of the latter shows that the oxygen 

 in them exists under a very small pressure, practically zero pres- 

 sure,* while the CO 2 is present under a tension (Strassburg) of 

 7 to 9 per cent. The high tension of the C0 2 is explained by 

 the fact that it is being formed in the tissues constantly as a 

 result of their metabolism, while the low tension of the oxygen 

 is due to the fact that on entering the tissue this substance is 

 combined in some way in a chemical compound too firm to 

 dissociate. The physical conditions are, therefore, such as 

 would cause a stream of CO 2 from tissue to blood and a stream 

 of oxygen in the reverse direction. 



1 OXYGEN. CARBON DIOXID. 

 Arterial blood 100 mms. 35 mms. 



I 4- 



Wall of capillary 



Tissues mm. 50 to 70 mms. 



It is to be remembered that in this exchange the blood and 

 the lymph act as intermediaries. The CO 2 diffuses from lymph 

 to plasma and from tissues to lymph. The oxygen diffuses from 

 lymph to tissues, from plasma to lymph, and from oxyhemo- 

 globin to plasma. Bohr* has found experimentally that in 

 blood, when the oxygen tension is low, an increase in the CO 2 

 pressure tends to dissociate the oxy hemoglobin (Fig. 275). 

 Since these conditions prevail in the capillaries of the body, it 

 is probable that the presence of the C02 in increased amounts 

 facilitates the liberation of the oxygen. 



Suggested Secretory Activity in the Respiratory Exchange. The view 

 that the exchange of gases in the lungs and tissues is entirely explained by 

 the diffusion of the gases from points of high tension to points of low ten- 

 sion, and that the membranes interposed are entirely passive in the process 

 has not passed unchallenged. Certain observers (Bohr, Haldane, and Smith) f 

 claim that the tension of the oxygen in the arterial blood may be higher than 

 the pressure of oxygen in the alveolar air. In a recent discussion of the sub- 

 ject Haldane admits that the exchange of CO 2 is controlled entirely by- 

 physical diffusion, but he brings forward some facts which demonstrate, in his 

 opinion, that the oxygen under certain unusual conditions may be secreted 

 from the alveolar air into the blood through the agency of the epithelial cells 

 of the lungs. The condition on which he lays emphasis is that of diminished 

 oxygen-pressure in the alveolar air, such as occurs at high altitudes. The dis- 

 turbances produced at these altitudes, so-called mountain sickness, are now 

 recognized to be due to lack of oxygen. Those who live for some time under 



* This conclusion is doubted by some observers, see Barcroft, "Respirat- 

 ory Function of the Blood," 1914, p. 165. 



t"Skandinavisches Archiv f. Physiologic," 16, 402, 1896. 



j See Haldane and Smith, "Journal of Physiology," 20, 497, 1896. 



\ Haldane, General discussion in "Organism and Environment, etc.," 

 New Haven, 1917. 



44 



