ARTERIAL BLOOD PRESSURE. 173 



offer to the flow of blood prevents the passage of the entire sys- 

 tolic output of the heart into the veins during the actual ven- 

 tricular contraction. It is, therefore, necessary that the large 

 arteries expand in order to make room for the blood. A part of 

 the energy of the heart beat is stored up in the elastic coats of 

 the arteries, and after closure of the semilunar valves, which 

 guard the ventricular orifice, the blood in the distended arteries 

 is forced on through the capillaries by the pressure of the ar- 

 terial walls. 



Arterial Blood Pressure. From the foregoing description we 

 see that there are several factors which contribute to the main- 



Fig. 20. Diagram of experiment to show how a pulse (produced by com- 

 pivssing the bulb B) comes to disappear when fluid flows through an elastic 

 tube (F) when there is resistance <) to the outflow. A, basin of water; 

 B, bulb syringe ; C and E, stop cocks ; D, rigid tube ; F, elastic tube ; O, 

 bulb filled with sponge. 



tenance of a constant stream of blood through the capillaries: 

 viz., the pumping action of the heart, the resistance of the ar- 

 terioles and capillaries, the elastic recoil of the blood vessels, and 

 the amount of blood itself. That the velocity and the pressure 

 of the blood depend on these factors was first of all demonstrated 

 in 1732 by Rev. Stephen Hales, who in a book published in that 

 year reports having experimentally determined the blood pres- 

 sure in the femoral artery of a horse. He found that the pres- 

 sure was sufficient to raise the blood in a tube seven feet above 

 the level of the heart, and he also observed that each beat of the 

 1 it-art and each respiratory movement affected the pressure of the 

 blood. The pressure exerted by the blood on the vessel wall at 

 the height of the systole of the ventricle is known as the systolic 

 blood pressure, and that exerted by the elastic recoil of the 

 arteries 011 the blood during the diastole of the heart is known 



