CHANGES ACCOMPANYING MUSCULAR EXERCISE 439 



of the adjustment which might be responsible for this. Since we know 

 that lactic aid is produced in vigorous exercise at such a rate that it 

 accumulates in the blood, but that it does not do so when the oxygen sup- 

 ply to the muscles is commensurate with the rate of production of the 

 acid, it is likely that "second-wind" coincides with a readjustment of 

 the chemical processes in the muscles leading to a more thorough elim- 

 ination of this metabolic product. The readjustment may depend, first, 

 on an increase in temperature in the muscles, stimulating the chemical 

 processes, and secondly, on increased bloodflow due to the opening up 

 of capillaries. The appearance of sweating is another effect of the rising 

 temperature. Beside the more adequate elimination of lactic acid, it is 

 also possible that changes occur in the blood, increasing its alkaline re- 

 serve by migration of basic radicles into the plasma from the erythro- 



cytes and tissues (see page 40). This will, of course, enable the plasma 



TT r*r\ -i 



to take up more acid without change of the normal ratio xr ^ r/s ,^ -= 



20 



The Influence of Oxygen Inhalations on the Effects of Muscular Exercise 



The most important work in this connection is that of Leonard Hill 

 and his pupils 46 who have found that the inhalation of pure oxygen for 

 a few minutes renders a person capable of greater exertion, and decidedly 

 lessons the degree of breathlessness and the various symptoms of cardiac 

 distress. That the improvement of the circulation does actually occur 

 is indicated objectively, by the fact that the pulse is slower, and the 

 blood pressure higher for a given degree of exercise after oxygen than 

 without it. Symptoms of cerebral anemia such as dizziness, blurring of 

 vision, etc., are also much less common during very strenuous work, if 

 oxygen has been inspired prior to the effort. There are at least two 

 ways by which the excess of oxygen may bring about these effects: either 

 it becomes stored and prevents incompletely oxidized acids from accu- 

 mulating or retards a fall in 2 -tensioii, or it increases the power of the 

 blood to carry away the oxidation products (C0 2 ). Regarding the sec- 

 ond possibility it is now well known that increase in oxygen in the blood 

 lowers the dissociation curve for C0 2 because the oxygen displaces CO 2 from 

 hemoglobin (page 404). The preliminary inhalations of O 2 will drive out 

 C0 2 from the blood and therefore make more room, as it were for the extra 

 load of this gas which the blood must carry during exercise. Experimental 

 evidence that these changes actually occur is afforded by measurement of 

 the breaking point when the breath is held, that is the time during which 

 the breath can be held before an irresistible stimulus to breathe is expe- 

 rienced. After inhalation of oxygen the breaking point is materially pro- 

 longed because the oxygen, by removing C0 2 from the hemoglobin, has 

 enabled it to take up more of the gas while the breath was held. 



