414 THE RESPIRATION 



state. That the C0 2 tension of the alveolar air should be found to be 

 lowered by prolonged muscular exercise in no way detracts from this 

 explanation, for it is dependent upon the greatly increased rate of 

 movement of air into and out of the alveoli (see also page 366). 



One serious difficulty in accepting the HC0 3 ion as the exciting hor- 

 mone of the nerve centers during muscular exercise depends on the ob- 

 servation that the alveolar C0 2 after some time is lower than normal. 

 If we accept Ilaldane's teaching that there is accurate correspondence 

 between the tensions of C0 2 in arterial blood and alveolar air not only 

 during rest but also during muscular activity, then obviously we must 

 discard the HC0 3 hypothesis. Leonard Hill and Flack, 37 however, have 

 shown quite clearly both in experimental animals and in man that equi- 

 librium between the blood and alveolar tensions of C0 2 may fail to 

 occur. When blood with excess of C0 2 is injected into the jugular vein 

 of dogs, the respiratory center is stimulated, as shown by the increased 

 breathing, which indicates that the C0 2 -rich blood must have passed 

 through the lungs without the excess of C0 2 being removed 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 responsible for the low C0 2 ten- 

 sions usually found present (see page 369). 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 alveolar 

 air very rapidly. 



The only way by which progress may be made in a problem like that 

 under discussion is, however, to adopt some hypothesis and then to 

 gather evidence for or against it. At the present stage of our knowl- 

 edge, the hypothesis usually adopted is that a slight change in H-ion 

 concentration of the blood is the effectual hormone. It is an hypothe- 

 sis which is supported by the parallelism between the effects observed 

 during muscular exercise and those produced by experimental increase 

 in H-ion concentration. 



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. 

 Even the capillaries themselves are said to be dilated by carbonic acid 

 (Severini). The effects produced on the heart by changes in H-ion con- 

 centration of the blood have been particularly studied by Starling and 

 Patterson, 38 who, working on isolated heart-lung preparations, have 

 shown that the heart relaxes more and more and discharges less blood 



