CIRCULATION, RESPIRATION AND EXCRETION 137 



of an animal in the absence of any respiratory movements what- 

 ever. 



193. Respiration regulated by cell activity. It is apparent 

 from the foregoing that the amount of oxygen taken up by the 

 blood in the lungs depends in the first instance upon the amount 

 of this element consumed by the body cells. When they are 

 relatively inactive they take up correspondingly little oxygen 

 from the lymph and the tension of oxygen in the latter is low- 

 ered but little. As a consequence there is less dissociation of 

 the oxyhaemoglobin in the blood and the corpuscles tend to 

 return to the lungs still carrying more or less oxygen and there- 

 fore capable of taking up relatively less. On the other hand, as 

 the tissues become more active they consume more oxygen, the 

 oxyhaemoglobin in the corpuscles is more extensively dissociated 

 and the corpuscles tend to come back to the lungs relatively 

 exhausted of oxygen and ready to take up the maximum amount. 



Any considerable degree of tissue activity, however, calls for 

 a more rapid supply of oxygen than can be provided for in this 

 way and this need is met by a nerve stimulus to the heart, caus- 

 ing it to beat faster and more powerfully, thus increasing the 

 arterial pressure and therefore the amount of blood passing 

 through the capillaries in a given time. In these two ways 

 the amount of oxygen absorbed in the lungs is very accurately 

 adjusted to the needs of the organism. It is impossible to 

 stimulate the body oxidations by a free supply of air as, for 

 example, by deep and rapid breathing, as one might blow up a 

 fire with a bellows, or to get more intense combustion by re- 

 placing air with pure oxygen. In the body such additional air 

 or oxygen never reaches the fire. Each corpuscle is a recep- 

 tacle which can carry only a definite amount of oxygen and if 

 it comes back to the source still partly filled it takes up so much 

 the less on its next trip, or if it travels slowly it is less efficient 

 than if it returns more frequently. The respiration of the 

 tissues can no more be affected by increasing the ventilation of 

 the lungs than the amount of water delivered by a pump is by 

 the volume of the stream from which the water is taken. 



194. Regulation of the rhythm of breathing. The illus- 

 tration just used is true, of course, only on the condition that 

 the stream carries at least as much water as the pump can 

 handle. So, too, the amount of oxygen available in the lungs 



