Chap. XII] GENERAL CIRCULATION OF BLOOD 219 



heart. The valves of the veins also render assistance in this 

 respect. 



Rate of blood flow. — The rate at which the blood flows is 

 highest in the arteries, about twelve inches to a second. It is 

 lower in the veins, about eight inches to a second. In the capil- 

 laries it is exceedingly slow, about an inch to a minute. When 

 it is remembered that the actual service of the blood to the tissues 

 is rendered in the capillaries (since the walls of the arteries and 

 veins are too thick to permit of diffusion) the value of the slow 

 passage is obvious. 



Distribution of blood. — We have quoted the estimate of blood 

 that the body contains as four quarts. During a period of rest 

 about one-fourth of this may be assumed to be in the thorax, one- 

 fourth in the skeletal muscles, one-fourth in the liver, and the 

 remaining fourth elsewhere. Activity of any part of the body will 

 increase the supply to that part, and lessen the supply in another 

 part. When the digestive organs are active, other parts of the 

 body are kept short of blood, but any condition that dilates the 

 vessels of the skin will lessen the amount of blood sent to the 



digestive organs. 



BLOOD PRESSURE 



By blood pressure is meant the pressure the blood exerts against 

 the walls of the vessels in which it is contained. The term includes 

 arterial, capillary, and venous pressure. 



Arterial pressure. — When an artery is severed, the flow of 

 blood from the proximal end (that on the heart side) comes in 

 jets corresponding to the heart-beats, provided the artery be near 

 the heart. The larger the artery, and the nearer to the heart, 

 the greater the force with which the blood issues, and the more 

 marked the intermittence of the flow. This indicates that the 

 blood in the arteries is under a high pressure, and the factors which 

 produce and maintain this pressure are : — 



1. The force of the contraction of the ventricles. 



2. The extra supply of blood which the heart forces into arteries 

 that are already full. 



3. The semilunar valves which prevent regurgitation of the blood 

 into the ventricles, when the ventricles relax. 



4. The elasticity of the arteries, which enables them to dilate to 

 receive the extra supply of blood, then to contract. 



