REGULATION OF ENVIRONMENT 69 



that during active secretion of saliva by a salivary 

 gland the venous blood issuing from the gland was of 

 a bright red colour, owing to quickening of the circu- 

 lation ; and Barcroft found that owing to the quantity 

 of liquid and C0 2 abstracted from the blood during 

 salivary secretion the absolute quantity of oxygen in 

 a given volume of the venous blood may be greater, 

 while that of C0 2 may be less, than in the arterial 

 blood. As one constituent or another assumes greater 

 or less importance in the exchange between blood 

 and tissues we must expect the circulation to vary 

 accordingly, and there is no doubt that it does so 

 vary. The gaseous exchange is, however, every- 

 where of such immediate importance that we may be 

 sure that the circulation is to a large extent regulated 

 with reference to the gaseous exchange. 



The flow of blood through any part of the body 

 depends partly on the difference in blood pressure be- 

 tween arteries and veins, and partly on the resistance 

 to the flow of blood from the arteries through the 

 capillaries to the veins. Now the difference between 

 the pressures in the main arteries and veins at any 

 given body level is nearly constant. This is so because, 

 if we neglect such part of the pressure as is accounted 

 for by the mere height above or below the heart, the 

 pressure in the larger arteries is high, and nearly con- 

 stant, while that in the veins is so low as to be insigni- 

 ficant in comparison with the arterial pressure. Hence 

 it is through variations in the resistance that variations 

 in the rate of flow are brought about. But variations 

 in the resistance depend almost entirely, so far as we 



