CHANGES IN AIR AND BLOOD IN RESPIRATION. 681 



the lymph act as intermediaries. The CO2 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 COj 

 pressure tends to dissociate the oxyhemoglobin (Fig. 275). 

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

 is probable that the presence of the CO2 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 hings 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 

 claun 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 CO2 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 

 such conditions gradually become acclimated, and Haldane believes that the 

 essential factor in the acclimatization consists in the assumption of a secretory 

 activity for oxygen upon the part of the lungs. His reason for this view is the 

 discovery that the oxygen-pressure in the arterial blood under these conditions 

 is greater than the oxygen-pressure in the alveolar air. The oxygen-pressure 

 of the blood was estimated by means of his carbon monoxid method. When 

 an individual is allowed to breathe a mixture of oxygen and carbon monoxid in 

 which the latter is present in small but known percentage, the hemoglobin 

 combines partly with the oxygen and partly with the carbon monoxid in defi- 

 nite proportions. Aiter equilibrium has been established to this mixture the 

 percentage saturation of the hemoglobin with carbon monoxid may be deter- 

 mined upon a sample of blood, and from this figure the percentage saturation 

 with oxygen may be estimated. Experiments of this kind made at high alti- 

 tudes (Pike's Peak, 14,100 feet) indicated that the ox-ygen pressure in the blood 

 was 35 mms. greater than in the alveolar air. As collateral evidence he cites the 

 case of the swim-bladder in deep-sea fishes. The gas in this bladder is known 

 to be nearly pure oxygen, and it may exist under a pressure of 100 atmospheres, 

 although in the blood of the fishes the oxygen-pressure cannot exceed ^ to j^ 

 of an atmosphere. 



* "Skandinavisches Archiv f. Physiologie," 16, 402, 1896. 

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

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

 New Haven, 1917. 



