CH. XXL] 



BLOOD-PRESSURE 



263 



found in the arterioles or small arteries, just before the blood passes 

 into what we have termed the vast capillary lake. These small 

 arteries with their relative excess of muscular tissue, are in health 

 always in a state of moderate tonic contraction. 



The large arteries contain a considerable amount of muscular as 

 well as elastic tissue. This co-operates with the elastic tissue in 

 adapting the calibre of the vessels to the quantity of blood they 

 contain. For the amount of blood in the vessels is never quite 

 constant, and were elastic tissue only present, the pressure exercised 

 by the walls of the containing vessels on the contained blood would 

 be sometimes very small, sometimes too great. The presence of a 

 contractile element, however, provides for a certain uniformity in the 

 amount of pressure exercised. There is no reason to suppose that 

 the muscular coat assists in propelling the onward current of blood, 

 except in virtue of the fact that muscular tissue is elastic, and there- 

 fore co-operates in the large arteries with the elastic tissue in keeping 

 up the constant flow in the way already described. 



The contractility of the arterial walls fulfils a useful purpose in 

 checking haemorrhage should a small vessel be cut, as it assists in the 

 closure of the cut end, and this in conjunction with the coagulation 

 of the blood arrests the escape of blood. 



Blood-pressure . 



The circulation of the blood depends on the existence of different 

 degrees of pressure in different parts of the circulatory system ; 

 there is a diminution of pressure from the heart onwards through 

 arteries, capillaries, and veins, back to the heart again. 



FIG. 266. Height of blood-pressure (BP) in LV, left ventricle. A, arteries ; c, capillaries ; v, veins ; RA, 

 right auricle ; oo, line of no pressure. (After Starling.) 



Fig. 266 represents roughly the fall of pressure along the systemic 

 vascular system. 



It falls slowly in the great arteries ; at the end of the arterial 

 system it falls suddenly and extensively in the course of the 

 arterioles ; it again falls gradually through the capillaries and veins 

 till in the large veins near the heart it is negative. Such a diagram 



