RESPIRATION 177 



12. The circulation as an essential part of the mechanism 

 of respiration. The consumption of oxygen and the produc- 

 tion of carbon dioxide thus involve an interchange of these 

 gases between the blood and the tissues (internal respiration) 

 on the one hand, and between the blood and the air in the 

 lungs (external respiration) on the other. But to carry out 

 these gaseous exchanges a third factor is obviously necessary, 

 namely, a means of communication between the two, so that 

 the oxygen absorbed in the lungs may be carried to the 

 tissues, and the carbon dioxide produced in the tissues be 

 carried back to the lungs. This communication is provided, 

 as has been shown in earlier chapters, by the circulation, 

 which thus becomes an essential part of the respiratory 

 mechanism. 



We have already seen that under the most varying con- 

 ditions 100 cc. of arterial blood always contain approxi- 

 mately 20 cc. of oxygen and 38 cc. of carbon dioxide and 

 that this is practically all the oxygen this amount of blood 

 can hold. From this it follows that so long as the amount 

 of blood pumped by the heart in a given time remains con- 

 stant, no more oxygen will be carried to the tissues, even 

 if we breathe more deeply. In other words, increased ventila- 

 tion of the lungs without any accompanying increase in the rate 

 and force of the heart beat will not supply more -oxygen to the 

 tissues. The beat of the heart is as important to proper tis- 

 sue respiration as are the breathing movements ; and we find 

 accordingly that these two events are closely coordinated. 

 Greatly increased tissue respiration invariably carries along 

 with it increased work on the part of the heart. 



A large number of measurements of the respiratory ex- 

 changes l under different conditions and activities of our 

 life has shown that these are increased by the taking of 

 food, by exposure to cold, by awaking from sleep, and, 

 above all, by muscular activity. Exposure to cold acts by 



1 That is, oxygen absorbed and carbon dioxide discharged in a given time. 



