The unloading of oxygen from the blood 165 



from the active gland. So much then for the pressure of oxygen in 

 the capillaries. 



The amount of oxygen which will diffuse through the wall 

 depends, other things being equal, on the gradient of pressure 

 across the capillary wall ; let us therefore turn to the consideration 

 of the pressure of oxygen in the tissues. 



The quantitative measurement of the extra-vascular oxygen 

 pressure in certain organs expressed in mm. of mercury has been 

 attempted by Verzar (1) during the portion of his Wissenschaftliche 

 Reise which he spent at Cambridge. The principle on which these 

 determinations were based was the following. The quantity of 

 oxygen which diffuses through the wall will depend, other things 

 being equal, upon the difference of oxygen pressure within and 

 without the capillary, and inversely on the distance of diffusion, i.e. 

 altogether on the pressure gradient. 



If therefore Q be the quantity of oxygen which passes out per 

 minute, p the intra-capillary pressure and p' the extra-capillary or 

 intra -cellular pressure 



Q cc p-p'. 



Naturally Verzar's first endeavour was to test the current view 

 of intra-cellular oxygen pressure, which is that the pressure in the 

 tissues is nil. Though no quantitative experiments of this character 

 are simple or easy, this fortunately is one of the least difficult, for if 



p' = 0, Q oc p. 



On this hypothesis it is possible to institute a direct experimental 

 test as to whether the amount of oxygen which left the capillary 

 varied directly with the pressure of oxygen in the capillary. The 

 amount of oxygen can of course be directly measured from a know- 

 ledge of the oxygen in the blood going to and leaving the organ in 

 question, and of the quantity of blood which goes through it in a 

 given time. The measurement of p offers a much more difficult 

 problem. Theoretically Verzar should in each experiment have gone 

 through the whole gamut of determinations we have set forth in the 

 example given above. In practice we had to approximate. A first 

 approximation to the capillary oxygen pressure will be arrived at by 

 an application of the percentage saturation of the arterial and venous 

 blood to the dissociation curve of the arterial blood of an animal of 

 the same species. The measurement of Q involves the amount of 

 oxygen in the blood : if the oxygen capacity be also known the 



