510 THE OUTGO OF ENERGY 



section of the vascular system some of the pres- 

 sure is lost in overcoming resistance ; hence the 

 pressure gradually falls. The blood flows from 

 the area of higher pressure, near the ventricle, to 

 the area of lower pressure. Thus the contrac- 

 tions of the ventricle establish a difference of 

 pressure in the blood vessels, which causes a 

 movement of the contained liquid. 



At the two points at which the vascular 

 tube joins the ventricle membranous valves are 

 placed. One of these valves opens into the 

 ventricle. It is an inflow valve. The inflow 

 valve closes when the ventricle contracts. Con- 

 sequently the contractions cannot drive the 

 blood through this orifice. The ventricle can 

 drive the blood only through the remaining 

 orifice. Thus the ventricle becomes a pump 

 and its contractions move the blood always 

 in one direction. The vessels by which the 

 blood is carried from the ventricle to the cap- 

 illaries are called arteries ; those which bring 

 the blood from the capillaries back to the ven- 

 tricle are called veins. Adjoining the ventricle 

 the great veins meet in a common enlarge- 

 ment called the auricle. It is at the junction of 

 the auricle with the ventricle that the inflow 

 valve is placed. 



The outflow valve is placed at that orifice of 



