208 THE CIRCULATION OF THE BLOOD 



centimeters the amount of blood that has flowed through the instrument 

 in a definite unit of time. This gives us the volume flow and, if the 

 result is divided by the cross section of the vessel in square centimeters, 

 we obtain what is known as the mean lineal velocity. Many modifica- 

 tions have been made of this instrument, but it is unnecessary to go into 

 them here. 



The general result of such measurements has been to show that the 

 lineal velocity is inversely proportional to the cross section of the ves;sei 

 at the point of observation. It is obvious that the volume of blood 

 flowing out of the heart to the aorta in a given time is exactly equal 

 to that flowing into it by the vena cava, and likewise that the volume 

 flowing into an organ is exactly equal to that which flows out. Conse- 

 quently the lineal velocity will be inversely proportional to the sec- 

 tional area of the vessel. The principle is the same as that which gov- 

 erns the velocity of flow of a stream: when the bed is narrow, the cur- 

 rent is swift, but it becomes sluggish when the bed is wide. If the 

 arteries were of the same caliber as the veins, the mean velocity of the 

 bloodflow through the two would be the same, but actually it is much 

 greater in the arteries because the lumen of these at a given point in the 

 circulation is only from one-third to one-half that of the corresponding 

 vein. 



It must be understood that we are dealing above with the mean 

 velocity in a unit of time, and that there must be considerable alteration 

 with each systole and diastole, constituting the velocity pul&e (page 200). 

 The degree of this alteration with each velocity pulse is much less at 

 the periphery of the circulation than near the heart. As the periphery 

 is reached, the flow becomes more uniform. It must further be re- 

 membered that, although the mean velocity depends essentially upon 

 the area of the vascular bed, yet it is subject to considerable variations 

 as a result of changes either in the force or rate of the heartbeat or 

 in the facility of outflow from the ends of the arterial system that is, 

 changes in peripheral resistance. 



It is usually stated that the mean lineal velocity in the carotid artery 

 is about 300 millimeters per second; and in the jugular vein, about 150 

 millimeters; whereas in the capillaries, where the total area of the 

 vascular bed has become enormously increased, being perhaps some 800 

 times that of the aorta, the velocity of flow is only about half a milli- 

 meter per second. 



MASS MOVEMENT OF THE BLOOD IN A VASCULAR AREA 



Methods. In considering the bloodflow or mass movement of the blood 

 in the different regions of the body, it is usually more practical to 



