160 APPLIED PHYSIOLOGY 



which it is always employed in clinical work, it will be 

 used with that meaning in what follows. 



The advantages of having a certain amount of fluid- 

 pressure within the arteries are these : 



1. It ensures a ' head ' of pressure by which any 

 sudden demand for an increased supply of blood on the 

 part of an organ or tissue can be immediately met. 



2. It ensures an adequate degree of transudation of 

 lymph for the nourishment of the cells. 



3. It accommodates the amount of their contents to 

 the size of the arteries, so that a considerable loss of 

 blood can take place without the pressure falling to a 

 dangerously low level. 



It is obvious that the arterial pressure must always be 

 at least high enough to overcome the resistance of the 

 arterioles, and so ensure a steady flow of blood through 

 the capillaries. The fraction of the total arterial pressure 

 which is necessary for this purpose may be spoken of as 

 the ' essential ' element in blood-pressure, whilst the 

 margin above this, which maintains a * head of pressure ' 

 sufficient to meet the varying demands of the tissues, is 

 the ' functional ' element (Janeway). About 50 milli- 

 metres of mercury is the amount of the ' essential ' 

 pressure ; the remainder (70 millimetres or more) is 

 functional and variable. 



In considering arterial pressure, one has always to 

 distinguish between the pressure during the pulse wave 

 or systolic pressure, and the pressure between the waves 

 or diastolic pressure. Most clinical sphygmometers 

 record the former, and it is systolic pressure that is 

 ordinarily referred to when one speaks clinically of a 



