CHAPTER XL VII 



THE CIRCULATORY AND RESPIRATORY CHANGES ACCOM- 

 PANYING MUSCULAR EXERCISE* 



During activity the muscles require many times more blood than dur- 

 ing rest. When the activity is widespread the greater blood supply is 

 provided by increased heart action accompanied by dilatation of the 

 muscular arterioles and constriction of those of the splanchnic area, so 

 that the entire available blood supply of the body is made to circulate 

 more rapidly. When, on the other hand, the activity is confined to a 

 limited group of muscles, the increased blood supply is mainly provided 

 by a local dilatation of the blood vessels of the active muscles accom- 

 panied by a reciprocal constriction of those of inactive parts. Under 

 these conditions there may therefore be no quickening of the bloodflow 

 as a "whole. In order that this accurate adjustment of blood supply to 

 tissue demands may be promptly and adequately brought about, all 

 available types of coordinating mechanism are called into play; that is 

 to say, mechanical, nervous and hormone factors cooperate to an extent 

 which is dependent upon the type of work being performed. 



Besides the changes in pulse rate and blood pressure which are evi- 

 dently designed to supply more blood to the acting muscles, changes 

 dependent upon a secondary effect of the muscular movements have also 

 to be considered. Although the various factors work together and are 

 more or less interdependent, the final effect can be understood only after 

 we have studied the relative influence of each separately. 



The Mechanical Factor. — It is particularly with regard to this factor 

 that the circulatory changes may be an unavoidable consequence of, 

 rather than a useful adjustment to, the muscular effort. The effects vary 

 with the type of exercise performed. In repeatedly lifting and lowering 

 dumbbells from the floor to above the head, the contracting muscles of 

 the back and extremities and of the abdomen compress the veins and 

 cause the blood to flow more rapidly into the heart, so that the arterial 

 pressure suddenly rises. So long as this compression exists, the veins 

 remain relatively empty and the arteries overfilled, but whenever it 

 ceases and the muscles relax, the veins fill up again and the arterial pres- 



*This chapter is placed here rather than following circulation because of the interdependence of 

 the circulatory and respiratory adjustments. 



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