PHYSICAL FACTORS OF THE CIRCULATION 185 



velocity of flow; and is inversely proportional to the sectional area of 

 the vessels. Owing to the resistance to the capillary outflow, the 

 large arteries are expanded by each systolic output of the heart, and 

 the elasticity of their walls comes into play, causing the outflow to 

 continue during the succeeding diastole of the heart. The conditions 

 are such that the intermittent flow from the heart is converted into 

 a continuous flow through the capillaries. If the arteries were rigid 

 lubss, it would be necessary for the heart to force on the whole column 

 of blood at one and the same time ; but, owing to the elasticity of these 

 vessels, the heart is saved from such a prolonged and jarring strain, 

 and can pass into diastolic rest, leaving the elasticity of the distended 

 arteries to maintain the flow. Besides the saving of heart-strain, 

 there is the advantage of a greater outflow through elastic than through 

 rigid tubes. As a result of disease, the elastic tissue may degenerate 

 and the arteries become more rigid. 



The four chief factors which co-operate in producing the conditions- 

 of pressure and velocity in the vascular system are 



(1) The heart-beat; 



(2) The peripheral resistance ; 



(3) The elasticity of the arteries; 



(4) The quantity of blood in the system. 



Suppose the body to be in a horizontal position, and the vascular 

 system to be brought to rest by, say, arrest of the heart. A sufficiency 

 of blood to distend it collects within the venous cistern. The arterial 

 system, owing to its elasticity and contractility, empties. If the heart 

 now begin to beat, blood is taken from the venous system, and is 

 driven into the arterial system. The arteries receive more blood than 

 escapes through the capillary vessels, and the arterial side of the 

 system becomes filled until equilibrium is reached, when as much 

 blood escapes from the arterial into the venous side per unit of time 

 as is delivered into it by the heart. The flow in the capillaries and 

 veins now becomes a constant one, and if the side-pressure be 

 measured, it will be found to fall from the arteries to the capillaries, 

 and from the capillaries to the venae cavse. In the large arteries 

 there is a large side-pressure which rises and falls with the pulses 

 of the heart. The pulse-waves spread out over a wider and wider 

 area as the arteries branch. They finally die away in the arterioles. 

 An increase or decrease in the energy of the heart-beat will increase 

 or decrease respectively the velocity of flow and pressure of the 

 blood. An increase or decrease in the total width of the arterioles 

 respectively will lessen or raise the resistance, increase or decrease 

 the velocity, lower or raise the blood -pressure. A loss of blood, 

 other conditions remaining the same, would cause a decrease in pres- 

 sure and velocity. As a matter of fact, even a considerable loss is 

 compensated for by the adjustability of the vascular system. Tissue 

 lymph passes from the tissues into the blood, and the bloodvessels of 

 the limbs and abdomen constrict, and thus the pressure is kept up, 

 and an efficient circulation maintained through the brain, lungs, and 

 coronary vessels of the heart. 



