SECTION I.— PHYSIOLOGY. 



THE CONTROL OF THE 

 CIRCULATION OF THE BLOOD 



ADDRESS BY 



PROF. R. J. S. McDOWALL, M.D., D.Sc, F.R.C.P. (Edin.), 



PRESIDENT OF THE SECTION. 



It is now more than 300 years since William Harvey discovered the 

 circulation of the blood, but we are yet far from understanding its control — - 

 a fact which is brought home to us when we realise that each year thousands 

 of people die from failure of the circulation other than heart disease. 

 Indeed it can fairly be said that certain diseases of the circulation are 

 definitely diseases of civilisation and are on the increase. 



The purpose of the blood circulation is to supply the tissues with 

 nourishment and particularly with oxygen, and since the different parts of 

 the body vary enormously in their activity from time to time, their needs 

 vary also. 



In this address I shall endeavour to indicate the various kinds of 

 mechanisms which work together in order to provide adequate blood 

 supply to any part of the body, whatever its activity or whatever the 

 posture of the body. 



For the sake of simplicity I shall confine myself to the effects of physical 

 exercise, since most of the mechanisms which I shall describe are brought 

 into operation thereby, although they are also used for other purposes. 



When a tissue, say a muscle, increases its activity, it needs more oxygen 

 and fuel and therefore more blood supplied to it per minute, and this increase 

 is brought about in two ways : (1) by the same blood being pumped 

 round the body more rapidly — that is, by increased activity of the heart, 

 or pump ; and (2) by utilising blood which previously went to other less 

 active and for the moment less important parts of the body — that is, by re- 

 distributing the blood. This is accomplished by varying the calibre of 

 the blood vessels and has two effects. It alters the resistance to the 

 blood flow to any particular region, and it alters the capacity of any organ 

 or part of the body, but since there is only a limited amount of blood in 

 the body, it is evident that, if the circulation is to be maintained, vessels 

 opened up must not exceed the capacity of those closed down. 



Variations in the Activity of the Heart. 



This I shall summarise rapidly, as much of it is sufficiently old to be 

 in most of the text-books, and perhaps I should say that throughout I shall 



