THE BLOOD FLOW 403 



in the arteries, least in the capillaries, and intermediate in the veins. 

 (Fig. 217). These changes in the flow, as we shall see later, are in no 

 way different from those displayed by water while traversing a tube 

 of varying diameter. Provided, therefore, that the quantity of the 

 circulating blood remains the same, its speed of flow must be inversely 

 proportional to the size of the blood-bed. It has been stated that the 

 cross-area of the capillaries is from 600 to 800 times larger than that 

 of the aorta. Thus, Tigerstedt estimates the capillary expanse of 

 man at 800 to 2200 sq. cm., while Nikolai, upon the basis of a ventricu- 

 lar output of 75 C.C., gives the value of 1500 sq. cm. It need not 

 surprise us, therefore, to find that a most profound reduction in the 

 speed of the blood flow results as soon as the capillaries have been 

 reached. 



In the second place, the velocity of the flow in any tube is dependent 

 upon the friction to which the constituents of the fluid are exposed. 



Fig. 217. — Diagram to Illustrate the Relationship Between the Size of the 

 Blood-bed and the Velocity of the Flow. 

 B, cross-section; S, speed of flow in {A) arteries; C, capillaries and (F) veins; Z„ 

 zero line. 



Thus, we recognize two types of friction, namely the one produced by 

 the fluid in coming in contact with the wall of the tube and the one 

 produced by its molecular constituents when thrown against one 

 another. The former is called ''external" friction and the latter "in- 

 ternal" friction or viscosity. For this reason, the blood does not speed 

 onward as a uniform column, but is separated into layers, the outer- 

 most of which remains stationary, while the central one, forming the 

 core of the stream, moves ahead with the greatest possible speed. The 

 red corpuscles and heavier elements are thus forced into the central 

 stream, while the lateral zone is filled chiefly with plasma. Hence, in 

 attempting to determine the speed of the blood flow under the micro- 

 scope, we reaUy measure the rate of progression of the cellular elements 

 in the axial stream. If these could be removed, the speed of the 

 plasma-blood would thereby be much augmented. Obviously, there- 

 fore, the solids tend to retard the flow, because they heighten the in- 

 ternal and external frictions. 



If these two factors are now united under the general term of 

 peripheral resistance, the further conclusion may be drawn that, every- 



