The unloading of oxygen from the blood 167 



The data of this experiment are given in graphic form in Fig. 89. 

 There are two periods, the first in which the animal was breathing air, 

 the second in which the quantity of oxygen in the inspired mixture 

 swells to 8'4 per cent, or about 63 mm. The pressure of oxygen in the 

 arterial blood is indicated by the line A l A z , that in the venous 

 blood by V l V 2 . The arithmetic mean of these pressures is shown as 

 Ci C 2 and is taken to be the capillary pressure. The question is, 

 Can we draw a line T T. 2 representing the extra-capillary or tissue 

 pressure. Let us consider what the inferior limit of the tissue pres- 

 sure must be. At no time can it be less than zero. Let us suppose 



100 



Exp.I Exp.II Exp.III 



FIG. 89. A l A 2 pressure of oxygen in artery, F, V 2 in vein, C i (7 2 in capillary and 

 Tj T 2 in tissue, during the periods of each of three experiments on the submaxillary 

 gland. In period I the animal was breathing air, in period II a mixture of oxygen and 

 nitrogen poor in oxygen. Ordinate = mm. pressure. The numbering of the experi- 

 ments follow that in Verzar's paper. 



therefore that when the intra-capillary pressure p is at its lowest 

 the extra-capillary pressure p' is zero. Then the point C would repre- 

 sent p and T would represent p'. But C 2 = 16 mm., T 2 = 0. Therefore 

 p -p' = 16, or rather p -p' cannot be greater than 16. To find the 

 inferior limit of the extra-capillary pressure in the first period, we 

 need the following additional data. The quantity of oxygen used by 

 the gland Q 1 was '025 c.c. per g. per min. and in the second period 

 Q 2 = "022 c.c. Q therefore, so far from varying directly with p', 

 remained approximately constant in spite of the fact that p sank 

 from 61 mm. to 16 mm. 



