EFFECT OF EXERCISE ON CIRCULATION 



1007 



vessels, and that there is an active constriction of the vessels, especially of 

 the splanchnic area, so that the greater part of the blood in circulation is 

 available for the use of the actively contracting muscles. On this account 

 hard exercise is not easily carried out after a meal, and, if forced, seriously 

 interferes with digestion, by the diversion of the current of blood needed for 

 the carrying out of this function. 



It has been shown by Cannon that every state of excitement, and probably 

 also the effort of concentration which precedes muscular effort, is attended 

 with increased secretion of adrenaline into 

 the blood. At the same time the contracting 

 muscles are producing carbon dioxide in 

 large quantities, and also if the supply of 

 oxygen is not sufficient for their needs, lactic 

 acid. Both these, substances co-operate in 

 increasing the hydrogen ion concentration 

 (the acidity) of the blood. Where they are 

 present in greatest concentration they will 

 produce local dilatation of the blood-vessels, 

 i.e. in the contracting muscles. But as they 

 are carried by the blood- stream to the medul- 

 lary centres, they will cause a general 

 vaso- constriction especially marked in the 

 splanchnic area. On the heart, as already 

 mentioned, the detrimental effect of increased 

 acidity will be more than counterbalanced by 

 the adrenaline which enters the circulation 

 at the same time. But the nervous irritation 

 of effort probably sets all these mechanisms 

 in action at the same time. The quickening 

 of the pulse, which is a normal concomitant 

 of muscular effort, as well as the quickening 

 of the respiration, and the rise of pressure, 

 may be observed to begin before the actual 

 muscular contractions, so that the brain, 

 while sending impulses along the pyramidal 

 tracts to the skeletal muscles, sends also 

 impulses to the medullary centres which 



quicken the respiration and the pulse and sends up the blood-pressure by 

 constriction of the splanchnic area. Under these circumstances, therefore, 

 there is an abrogation of the normal rule that a rise of blood -pressure is 

 attended with a slowing of the pulse. Chemical mechanisms come in as a 

 sort of second line in maintaining the conditions favourable for exercise, 

 which are initiated by the direct action-of the central nervous system. 



Slight acceleration of the heart is observed, after division of all its nervous connec- 

 tions, on tetanising the lower limbs. Mansfeld has shown that probably the chief, if not 

 the only, factor in this case is the rise of temperature in the blood flowing to the heart. 



FIG. 472. Curves showing the in- 

 fluence of exercise on the circula- 

 tion. The exercise was a six-mile 

 run. Ordinates = mm. Hg. pres- 

 sure and rate per minute. (0. S. 

 LOWSLEY.) 



