72 ORGANISM AND ENVIRONMENT 



ately with the consumption of oxygen or production of 

 CO 2 . 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 2 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 2 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. 



