BLOOD-PRESSURE AND BLOOD-VELOCITY. 



461 



to the reservoir, and this in general is the picture presented by 

 the circulation. The reservoir, the head of pressure, is represented 

 by the aorta, the exit for the outflow by the opening of the venae 

 cavae into the right auricle, and the side pressure or internal tension 

 of the blood due to friction against the walls of the vessels increases 

 from the venae cavae back to the aorta. If from aorta to vena cava 

 the vessels were of the same diameter the increase would be in a 

 straight line, as in the case of the model. In this model it will be 

 noticed that the straight line showing the side pressure does not 

 strike the top of the column of liquid in the reservoir, but corre- 

 sponds to a certain height, h'. This expresses the fact that, of the 

 total head of pressure in the reservoir, which we may designate as 

 H, a certain portion only, but a large portion, h', is used in over- 



Fig. 193. Schema to illustrate the side pressure due to resistance, and the velocity pres- 

 sure (Tiger stedf) : H, A reservoir containing water; 1, 2, 3, 4, 5, the outflow tube with 

 gauges set at right angles to measure the side pressure ; h', the portion of the total pressure 

 used in overcoming the resistance to the flow ; A, the portion of the total pressure used in 

 moving the column of liquid the velocity pressure. 



coming the resistance along the tube. What is left that is, H-h', 

 represents the force that is employed in driving the liquid through 

 the tube with a certain velocity ; this portion of the pressure we may 

 speak of as the velocity pressure, h. If in measuring the side pressure 

 at any point the gauge were prolonged into the tube and bent so as 

 to face the stream, this velocity pressure would add itself to the 

 side pressure at that point and the water would rise to a higher 

 level in this particular tube. There are two important differences 

 between the circulation as it exists in the body and that repre- 

 sented by the model. In the body, in the first place, we have 

 the area of capillaries, small arteries, and veins, intercalated be- 

 tween the large arteries on one side and the veins on the other; 

 and, in the second place, the vessels, especially the arteries, are 

 extensible and elastic. The effect caused by the first of these 



