THE CIRCULATION OF THE BLOOD 



81 



III. THE FLOW OF LIQUID THROUGH TUBES UNDER CONSTANT 



PRESSURE. 



The problems presented by the circulation of the blood through 

 the bloodvessels involve some of the general principles of hydraulics. 



The supply of blood to the various glands 

 and other active tissues of the body is FIG. 45 



analogous to the supply of water to the 

 buildings of a city. 



The blood-circulatory system differs from 

 the water-circulatory system in possessing 

 elastic tubes instead of inelastic ones, and 

 an intermittent initial force instead of the 

 constant force furnished by the "head" of 

 water in the reservoir or stand-pipe. 



It will be profitable for the student to 

 make a few simple experiments in hydraulics 

 in order to make himself familiar with those 

 physical laws which he will apply later. 



1. Appliances. Each table is provided 

 with a reservoir (Fig. 45) consisting of a 

 galvanized-iron reservoir about 10 cm. in 

 diameter and 70 cm. in height, with a sup- 

 ply tank above. At the bottom of the 

 reservoir there is a faucet, to which may 

 be screwed a 6-mm. nozzle or a 3-mm. 

 nozzle, thus varying the radius of the outlet 

 stream. 



The reservoir is supplied with a gauge 

 which indicates the height of the water above 

 the middle of the outlet nozzle. 



Each table will need besides the reservoir 

 three 6-mm. T-tubes and five 6-mm. glass 

 tubes 50 cm. long; also ten rubber connec- 

 tors, a screw clamp, and centimetre rule. 

 Provide a large flask or jar for catching 

 discharge and a 500-c.c. graduated cylinder 

 for measuring _the discharge. 



2. Preparation. We have thus a means Of trating principles underlying 



varying the radius of the outlet and the circulation of the blood, 

 height of the water above the outlet. These 



are the two factors upon which the quantity of the discharge 

 depends, viz., area of a cross-section of the stream and velocity 

 of flow of the stream. The velocity of flow is determined by the 

 law of Torricelli: "The rate at which a fluid is discharged through 



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