CH. XXII.] BLOOD-PRESSURE 267 



there is a diminution of pressure from the heart onwards through 

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



Fig. 235 represents roughly the fall of pressure along the systemic 

 vascular system. 



It falls slowly in the great arteries and manifests oscillations 

 corresponding with the alternate systole and diastole of the heart ; 

 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 

 of blood-pressure is thus very 

 different from one of velocity ; 

 the velocity like the pressure 



falls from the arteries to the LV~ A c v 



capillaries but unlike it, rises Fm 235 ._ Height of blood . pressure (BP) in LV , ^ 



again in the Veins. ventricle. A, arteries ; C, capillaries ; V, veins ; 



We must now study the 



methods by which blood-pressure is measured and recorded, and the 

 main causes that produce variations in its amount. 



In order that we may understand the methods that are used for 

 this purpose, it will be first necessary for us to consider some of the 

 general laws of fluid pressure, and then to study the methods that 

 are employed in an artificial schema of the circulation. 



Fluid pressure is a different thing from the pressure of a solid, 

 and is exercised equally in all directions. If a cylindrical vessel, 

 placed vertically, is filled with a cylinder of ice, the pressure of the 

 ice will be exercised on the bottom of the cylinder, but not on its 

 sides. When the ice melts, the water presses on the sides also, and 

 if a hole is made in the cylinder below the level of the upper surface 

 of the water, the water will flow out of the hole, and the force with 

 which it escapes will be proportional to the depth of the hole beneath 

 the surface. If we take a square centimetre as the unit of area, the 

 actual pressure exerted on it is h x d x g, where h is the height of the 

 free surface above the level where we are measuring the pressure, d 

 the density of the fluid, and g the acceleration of gravity (981). 

 Suppose a gramme of water to flow out, we may consider that this 

 gramme has fallen through a height or head h in centimetres from 

 the free surface to the opening ; it comes practically from the top, 

 because it is there that the liquid disappears from inside the vessel. 

 In falling the height k, it gives out hg ergs of work. 



The unit of force is called a dyne ; a moving body is said to possess 

 momentum : this is measured by the product of its mass and its velocity ; thus the 

 effective quantity of motion of a body may be large on account of its having a large 



