122 PRACTICAL PHYSIOLOGY 



the velocity of the pulse-wave. In the vascular system the pulse- 

 wave travels in the arteries 8 metres per second, while the blood 

 travels J-metre. 



The resistance to flow is chiefly situated in the arterioles, where the 

 velocity is high. It is due to the friction of the moving concentric 

 layers of blood against one another, arid against the stationary layer which 

 wets the wall of the blood vessels. It is proportional to the surface 

 area, to the viscosity of the blood nearly proportional to the square of 

 the velocity of flow, and inversely proportional to the sectional area of 

 the vessel. In the arterioles the velocity is high, the total wall surface 

 wet by the blood great, the sectional area of each arteriole very small. 



In the schema the resistance is increased by diminishing the sectional 

 area of the arterioles and increasing the velocity of flow. Owing 

 to the resistance to the outflow the arteries are expanded by each 

 systolic output, and the elasticity of their walls comes into play, causing 

 the outflow to continue during the succeeding diastole of the heart. 

 The larger part of the kinetic energy of the systolic outflow is stored 

 up as potential energy by the stretched arteries, and converted into 

 kinetic energy during diastole. 



Eemove the bottle A and connect the vein directly with the syringe 

 B. Stop the pump, the pressures in the manometers fall to the same 

 level, i.e. to a mean hydrostatic pressure. The amount of this depends 

 on the distension of the system with water. Start the pump again. 

 The fluid is taken from the vein and piled up in the artery, for at each 

 systole a greater quantity of blood is driven into the artery than can 

 escape through the capillaries. With each succeeding systole, there- 

 fore, the pressure in the artery rises, and the pressure in the vein 

 falls. Venous pressure cannot really sink below the atmospheric 

 pressure as in the schema, for the flaccid walls of the veins collapse. 

 It is not possible to measure a mean hydrostatic pressure in the 

 vascular system, for the arteries contract when the heart is inhibited 

 and drive the blood into the venous side of the system. The venous 

 side is capacious, and possesses .little elasticity. Thus the changes of 

 pressure in the venae cavae, when the heart is arrested or starts beating, 

 are insignificant. After the first few strokes of the pump, a condition 

 of equilibrium is established, as the capillary outflow during the period 

 of the cardiac cycle becomes equal in volume to the systolic output. 



The continuous flow of blood thus established through the capillaries 

 is due to the difference between the pressure in the arteries and 

 veins. This difference depends on three factors : (1) the energy of 

 the heart, (2) the elasticity of the arteries, (3) the peripheral resistance. 

 The energy of the heart is spent in overcoming the resistance, and is 

 dissipated into heat. 





