432 THE RESPIRATION 



have passed through the lungs without all of the excess of C0 2 being re- 

 moved from it. Hill believes that the diffusion of C0 2 out of the blood 

 into the alveolar air may be depressed in muscular exercise, and that 

 this, rather than the appearance of lactic acid in the blood, is responsi- 

 ble for the low C0 2 tensions usually found present as illustrated by the 

 results given in the table on page 336. He points out in support of 

 this view that a person after exercise can hold his breath for a much 

 shorter time than is usual, and the C0 2 meanwhile mounts in the alveo- 

 lar air very rapidly. 



In view of the fact that the respiratory center also becomes excited 

 when there is a lowering of the tension of oxygen in the plasma (page 

 374) a contributory cause for its maintained stimulation during exer- 

 cise may depend on the great demands of the active muscles for this 

 gas. In its passage through the lungs the blood under these circumstances 

 may not succeed in taking on its full load of oxygen. That such is 

 the case during muscular exercise has been suggested by Barcroft. 



It is well known that an animal under emotional stress may perform an 

 amount of muscular work that is much greater than the usual, and Can- 

 non has brought forward evidence to show that this may be associated with 

 an increase in the concentration of adrenin in the blood. The adrenin as- 

 sists in facilitating the action of the autonomic nervous system and perhaps 

 by improving muscular contraction (see page 778). 



The Effects of the Hormone. These may be classified as follows: (1) 

 strictly local effects on the muscles themselves; (2) effects on the heart; 

 and (3) effects on the nerve centers. The local production of acids in 

 the muscles will cause dilatation of the arterioles, for it has been shown 

 by various observers that acids cause relaxation of vascular muscle. 

 Independently of changes in the arterioles, the capillaries themselves are 

 also altered in tone during muscular activity (see page 252). For the 

 maintenance of capillary tone an adequate supply of oxygen is essential 

 so that when this is rapidly used up by exercise capillary dilatation 

 occurs. This is no doubt further assisted by the appearance of the blood 

 of certain products of the metabolism of the muscles. These are proba- 

 bly in part related to histamine (-see page 307) and in part are acids, 

 such as C0 2 and lactic. The effects produced by changes in H-ion con- 

 centration of the blood on the heart have been particularly studied by 

 Starling and Patterson, 38 who, working on isolated heart-lung prepara- 

 tions, have shown that the heart relaxes more and more and discharges 

 less blood as the H-ion concentration of the perfusion fluid is increased 

 by adding C0 2 to the air ventilating the lungs. 



It is unlikely that C H in the blood is raised to the extent of causing 

 these changes in the heart during muscular exercise. It is possible, how- 



