268 RESPIRATION 



Recent work has indicated, however, that the oxygen pressure 

 in some tissues is far from negligible. In the resting submaxillary 

 gland, indeed, it seems to be practically equal to the oxygen pres- 

 sure in the venous blood leaving the organ (over 40 mm. of mercury 

 in certain experiments). In general, the same was found to be 

 the case for the kidney. This does not mean, of course, that in 

 such organs there is no slope of oxygen pressure from blood to 

 tissue, in virtue of which oxygen can be moved out of the blood 

 by diffusion. For at the beginning of the capillary tract the whole 

 difference in oxygen pressure between the arterial and the venous 

 blood would be available. At the end of the capillary tract, the 

 blood by the loss of oxygen to the tissues would have come approxi- 

 mately into equilibrium with them. These observations enable us 

 to understand better the manner in which the tissues themselves 

 can regulate their intake of oxygen. If the glands are using little 

 oxygen their oxygen tension will be relatively high, and the passage 

 of a comparatively small amount of oxygen from the blood in the 

 capillaries will suffice to bring it into equilibrium with the tissues, 

 after which the diffusion of oxygen will cease. If, on the other 

 hand, the consumption of oxygen in the glands is suddenly in- 

 creased, their oxygen pressure will fall, the pressure gradient be- 

 tween them and the blood will become steeper, and oxygen will 

 diffuse more rapidly into them from the capillaries. It must be 

 remarked, also, that up to a certain point the existence of a sub- 

 stantial oxygen pressure in the tissues permits them at once to 

 increase their intake of oxygen from the blood without the neces- 

 sity of an immediate increase in the blood flow. For, as the cells 

 consume the oxygen already present in and between them, the 

 tissue oxygen tension falls, and the gradient between the interior 

 and the exterior of the capillaries is thus rendered more abrupt. 



These results were not obtained by direct tonometric measurement 

 of the oxygen pressure in the glands investigated, but were deduced 

 from observations on the quantities of oxygen taken up by them when 

 different oxygen pressures were produced in the capillaries by altering 

 the oxygen tension in the inspired air. The quantity of oxygen which 

 diffuses out of the capillaries in a given time will be proportional to 

 the difference of oxygen pressure on the two sides of the capillary wall. 

 If the pressure on the side of the tissues is always zero, then the quantity 

 of oxygen passing out will vary directly as the intracapillary oxygen 

 pressure. Far from doing this, the amount of oxygen taken up by the 

 glands was found to be about the same with intra-capillary oxygen 

 pressures as different as 61 mm. and 16 mm. of mercury. It should be 

 distinctly understood that no great importance must be attached to 

 the actual numbers in experiments of this kind. But the general con- 

 clusion that in some tissues, at any rate in the resting condition, a 

 sensible partial pressure of oxygen exists, seems to be established 

 (Yerzar). The application of the same method to muscle has led to 

 a different result. Here, the possible partial pressure of oxygen was 

 found decidedly less than that of the venous blood, although at times 

 a small oxygen pressure was detected in resting muscle (25 mm. or less) 



