514 CIRCULATION OF BLOOD AND LYMPH. 



say that four chief factors co-operate in producing the conditions 

 of pressure and velocity as we find them. These factors are: (1) 

 The heart beat. (2) The resistance to the flow of blood through the 

 vessels, and especially the peripheral resistance in the region of the 

 small arteries, capillaries, and small veins. (3) The elasticity of 

 the arteries. (4) The quantity of blood in the system. The 

 way in which these factors act may be pictured as follows : Suppose 

 the system at rest with the definite quantity of blood distributed 

 equally throughout the vascular system. The internal or side 

 pressure throughout the system will be everywhere the same, 

 probably zero (atmospheric) pressure, since the capacity of the 

 vascular system is sufficient to hold the entire quantity of blood 

 without distension of its walls. If, now, the heart begins to beat 

 with a definite rhythm and discharges a definite quantity of blood at 

 each beat the whole mass will be set into motion. The arteries 

 receive the blood more rapidly than it can escape through the capil- 

 laries into the veins, and consequently it accumulates upon the 

 arterial side until an equilibrium is reached, that is, a point at 

 which the elastic recoil of the whole arterial tree suffices to force 

 through the capillaries in a unit of time as much blood as is received 

 from the heart during the same time. In this condition of equilib- 

 rium the flow in capillaries and veins is constant, and the side 

 pressure in the veins increases from the right auricle back to the 

 capillaries. In the arteries there is a large side pressure throughout, 

 owing to the resistance between them and the veins and especially 

 to the great resistance offered by the narrow capillaries. This 

 pressure rises and falls with each discharge from the heart, and the 

 pulse waves, both as regards pressure and velocity, are most marked 

 in the aorta and diminish farther out in the arterial tree, failing 

 completely in the last small arterioles, since if taken together these 

 arterioles constitute a large and distensible tube of much greater 

 capacity than the aorta. 



General Conditions Influencing Blood-pressure and Blood- 

 velocity. Alterations in any of the four chief factors mentioned 

 above must, of course, cause a change in pressure and velocity. 



I. An increase in the rate or force of the heart beat will increase 

 the velocity of the flow throughout the system, although, of course, 

 that general difference in velocity in the arteries, capillaries, and 

 veins which depends upon the variations in width of bed will remain. 

 Such a change will also cause a rise of pressures throughout the 

 system. The energy exhibited in the vascular system as side pres- 

 sure, velocity pressure, etc., comes, in the long run, mainly from 

 the force of contraction of the heart muscle. This force is what is 

 represented in the model, Fig. 208, as the total head of pressure ( H). 

 An increase in rate or force of heart-beat is equivalent, therefore, 



