

CIRCULATION 289 



in transparent structures by means of a microscope with an 

 eye-piece micrometer. The velocity of the blood is 



Carotid of the dog about . . 300 mm. per sec. 

 Capillaries about . . . 0'5 to 1 m. 

 Vein (jugular) about . .150 mm. 



It is not so easy to give definite figures for the velocity of 

 the lymph stream. 



Disturbance of any of the factors which govern the rate 

 of flow will bring about alterations in the velocity of the 

 blood in arteries, capillaries, and veins. Thus, an increased 

 venous pressure, by leading to a diminution in the difference 

 of pressure between arteries and veins, will materially slow 

 the blood stream. Great dilatation of the arterioles will slow 

 the blood stream in them; and increased viscosity of the 

 blood by increasing friction with the vessel wall will also slow 

 the stream. 



Special Characters of Blood Flow 



(a) Arteries. The flow of blood in an artery is rhythmi- 

 cally accelerated with each ventricular systole. This is due 

 to the pulse wave. As the wave of high pressure passes along 

 the vessels, the blood tends to flow forwards and backwards 

 from it so that in front of the wave there is an acceleration 

 of the stream and behind it a retardation. In a wave at sea 

 the same thing happens, and a cork floating on the surface is 

 moved forward in front of the wave and again backwards after 

 the wave has passed. 



(b) Capillaries. In the capillaries the flow is uniform. 



(c) Veins. In most veins, too, it is uniform, but in the great 

 veins near the heart it undergoes accelerations 



1st. With each diastole of auricle and ventricle. 

 2nd. With each inspiration. 



3rd. By muscular action squeezing the blood out of the 

 small veins. 



In all vessels the blood in the centre of the stream moves 

 more rapidly than that at the periphery on account of the 

 friction between the blood and the vessels. An "axial" 

 rapid and "peripheral" slow stream are, therefore, described. 



19 



