364 TEXT-BOOK OF PHYSIOLOGY 



THE VELOCITY OF THE BLOOD 



From the number of heart-beats per minute, 72, and the amount of blood 

 discharged from the left ventricle at each beat, 80 c.c., it is evident that the 

 blood must be flowing through the vascular apparatus with a certain velocity, 

 for during the minute the entire volume of blood, 3684 grams, must have 

 passed one and a half times through the heart. Direct observation of the 

 escape of blood from the central end of a divided artery, and from the pe- 

 ripheral end of a divided vein, as well as of the flow through the capillaries 

 as seen with the microscope, shows that the velocity of the flow varies in dif- 

 ferent parts of the vascular apparatus. In the arteries, moreover, the flow 

 is not quite uniform, but experiences an alternate acceleration and retarda- 

 tion with each heart-beat. In the capillaries and veins the flow is continu- 

 ous and uniform, as the conditions of the arterial walls are such as to 

 completely overcome the intermittency. 



If the systemic vascular apparatus be conceived of as a system of tubes 

 which have symmetrically divided and subdivided, and have again united 

 and reunited in a corresponding manner, it is clear that the total sectional 

 area will steadily increase from the beginning to the middle of the system, and 

 then as steadily decrease from the middle to the end of the system. In 

 such a system the same volume of blood must pass through any given section 

 in a unit of time if the balance of the circulation is to be maintained. As the 

 velocity of a fluid is inversely as the sectional area of the tubes through which 

 it flows, it follows that the initial mean velocity of the blood in the aorta will 

 steadily decrease as it flows into the steadily enlarging stream-bed until it reaches 

 a minimal value in the middle of the capillary system; and that it will again 

 steadily increase as it flows into the narrowing stream-bed until it reaches the 

 heart. The initial mean velocity of the blood in the aorta will not be attained 

 in the venae cavae, for the reason that the total sectional area of the latter is 

 somewhat greater than that of the former. The same facts hold true for the 

 pulmonic vascular system. 



The Mean Velocity in the Aorta. From the well-known fact that the 

 velocity with which a fluid is flowing through a tube may be determined 

 by dividing its sectional area into the quantity discharged in a unit of time, 

 attempts have been made to determine the mean velocity of the blood at the 

 beginning of the aorta. If it be assumed that the volume discharged at each 

 contraction is 80 c.c., and the number of heart-beats per minute is 72, the 

 total volume discharged per minute would be 5760 c.c., or 96 c.c. per 

 second. The sectional area of the aorta at its origin is 6.15 sq. cm. On 

 the principle above stated, these two factors would show a velocity of 156 

 mm. per second. This being the case the velocity in the aortic arch at 

 least would be considerably less than in the carotid artery as will be stated 

 later, a fact which may however be explained on the assumption that owing 

 to the curvature of the aorta and the extensibility of its walls the lateral 

 pressure becomes very great ; as a result the sectional area is increased and 

 the velocity diminished. With the cessation of the heart's activity, the elastic 

 recoil gives an impetus to the blood and increases its velocity. 



The Mean Velocity in the Arteries. The mean velocity of the blood 

 in the larger and more superficially lying arteries has been determined by 

 Volkmann with the hemodromometer, by Ludwig and Dogiel with the 

 Stromuhr, and by other investigators with different forms of apparatus. 



