1056 



PHYSIOLOGY 



above that obtaining during rest. This margin in a normal individual can 

 be increased by training, the essential features in which are graduated 

 exercise and healthy diet, so that the muscle grows and becomes free from 

 interstitial fat, while the fluid parts of the body and of the blood are 

 diminished so that a larger amount of oxygen can be carried per unit 

 volume of blood. The well-trained individual may have a margin of as 

 much as 120 per cent. Disease is marked by a diminution of the margin. 

 In Fig. 484 is given diagrammatically the response of the circulation and 



respiration of a man with heart disease 

 affecting the aortic and mitral valves. This 

 man had no discomfort and was able to do 

 ordinary work without ill effects. On testing 

 him on measured muscular tasks, it will 

 be seen that, although at first he reacts like 

 the normal individual, his margin is dimin- 

 ished, and when doing only 315 kilogram- 

 metres of work per minute, the rise in the 

 oxygen intake and in the heart output fails 

 to keep pace with the increase in the work 

 and loses also the parallelism which is so 

 marked a feature in normal individuals. 

 With increasing disease the time would finally 

 come when the margin was reduced to 50 

 per cent, or 100 per cent., so that even the 

 act of changing from a recumbent to an 

 erect position might be too much for the 

 enfeebled adaptive mechanisms of the body 

 and the patient would have to keep his bed. 

 There is thus no definite dividing line 

 between health and disease, the change from 

 one to the other being but a progressive 

 FIG. 485. Curves showing the in- diminution of margin or extent of adaptation. 



fluence of exercise on the circula- , lr , ,1,1 i i i 



tion. The exercise was a six-mile We nave seen tnat tne physiological 



run. Ordinates = mm. Hg. pres- condition of the heart is measured by 



sure and rate per minute. ^0. S. , , _._ , . ... 



LOWSLEY.) the degree of dilatation of its cavities, i. e. 



the length of its muscle fibres, required 



in order that in its beat it may set up a contractile stress adequate 

 to expel its contents against the arterial resistance. Thus a degree of 

 filling of the heart, which in a well-trained man may be adequate to excite a 

 contraction sufficient entirely to empty its cavities, in a weaker heart would 

 be inadequate, so that blood would accumulate at each diastole until the 

 stretching of the fibres was sufficient to ensure that the amount entering 

 during diastole was expelled at each systole. The trained man i. e. with 

 a heart in good condition will therefore have a considerable range over 

 which the output per beat caji be increased with increasing inflow without 

 alteration of rhythm. In the untrained man this margin will be smaller, 



