THE VASCULAR MECHANISM. 1 87 



VELOCITY OF THE BLOOD FLOW. 



The average rate of flow in a river depends upon the pressure 

 gradient between its source and the sea; and in the same way the 

 actual velocity of the blood stream is determined by the driving force 

 of the heart pump, that is, upon the amount of blood expelled from it 

 at each beat. If the heart is beating feebly the velocity will be small, 

 whereas if the heart is beating strongly and rapidly, for example during 

 muscular exercise, the velocity may be considerable. These differences 

 are of importance, since the rate at which oxygen is carried from the 

 lungs to the various tissues of the body is largely determined by the 

 rapidity of the circulation. 



The relative velocity of the blood flow in the arteries, capillaries, 

 and veins is determined solely by the total width of the channels 

 through which the blood is flowing. Since the same quantity of blood 

 has to pass in a given time through each cross-section of the bed of 

 the vascular system, it is obvious that the smaller the cross section 

 the greater must be the velocity of the blood flow. For the same 

 reason the water flows rapidly in a river where the channel is narrow, 

 and slowly where the channel widens out into large pools. 



When an artery divides, each branch is smaller than the parent 

 artery, but the total cross section of the two branches is larger than 

 that of the parent artery. The total cross section of the vessels thus 

 increases with each branching, and in the capillaries it has been 

 estimated to be about 800 times as great as that of the aorta. The 

 sectional area of the veins gradually decreases as they unite to form 

 larger vessels, and that of the large veins entering the heart is approxi- 

 mately twice as great as that of the aorta. It has been found that 

 whereas the average velocity of the blood in the large arteries is about 

 400-500 mm. a second, it varies from J-l mm. a second in the capil- 

 laries, and is from 200-250 mm. a second in the large veins. 



In a small organ, such as the kidney or subinaxillary gland, the 

 velocity of the flow of blood is also modified by local changes in the 

 arterioles. Dilatation of the arterioles lessens the resistance to the 

 flow of blood through the organ without affecting the general arterial 

 blood pressure, and since the resistance is lessened in that organ as 

 compared with other organs in the body, a short-cut is provided between 

 the arteries and the veins ; hence the blood flows through the organ 

 with increased velocity and in increased amount. This result, which 

 furnishes an apparent exception to the general statement made above, 

 is only true when the organ is so small that alterations in the calibre 

 of its arterioles do not appreciably affect the general blood pressure. 



