72 ORGANISM AND ENVIRONMENT 
ately with the consumption of oxygen or production of 
CO,. Accordingly the differences in gas contents be- 
tween arterial and venous blood vary far less than does 
the rate of metabolism. To judge from observations 
on myself, the venous gas pressures are practically 
constant during rest. The differences in gas pressures 
between the two kinds of blood differ only slightly 
with great differences in the consumption of oxygen. 
The gross regulation of the circulation is of such a 
nature as to keep the venous gas pressures nearly 
steady, while regulation of breathing keeps the arte- 
rial gas pressures nearly steady. Hence although the 
pressure of oxygen is lower, and that of CO, higher, 
in the venous than in the arterial blood, yet in each 
case the pressure is relatively steady. How the pecul- 
iar forms of the dissociation curves of oxyhaemoglobin 
and of the compounds which CO, forms in the circu- 
lating blood contribute toward this result has been 
explained in the previous lecture. 
The rate of the total circulation depends of course 
on the amount of blood pumped round by the heart; 
and it might seem at first as if the heart were the 
prime regulator of the circulation. This mistake has, 
in fact, been made by many physiologists through 
failure to look at physiological knowledge as a whole. 
Under normal conditions the heart simply maintains 
the pressure in the large arteries by pumping more, or 
less, blood according to the rate at which the blood- 
vessels allow blood to escape. It is thus the state 
of contraction of the blood-vessels in the various parts 
of the body that governs the rate of circulation. 
