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 CO, 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 CO, 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 
