CHAPTER XXVI. 



THE PHYSICAL FACTORS CONCERNED IN THE PRO- 

 DUCTION OF BLOOD-PRESSURE AND BLOOD- 

 VELOCITY. 



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In the preceding pages some of the essential facts have been 

 stated regarding the pressure and the velocity of the blood in the 

 different parts of the vascular system. We may now consider the 

 physical factors that are concerned in the production and main- 

 tenance of these peculiarities. The problem as it actually exists in 

 the circulation, with its elastic vessels varying in size from the 

 aorta, with an internal diameter of nearly 20 mms., to the capil- 

 laries, with a diameter of 0.009 mm., is extremely complex, but the 

 general static and dynamic principles involved are simple and easily 

 understood. 



Side Pressure and Velocity Pressure. When water flows 

 through a tube under, let us say, a constant head of pressure it 

 encounters a resistance due to the friction between the walls of the 

 vessel and the particles of water. This resistance will be greater, 

 the narrower the tube. A part of the head of pressure used to drive 

 the liquid along the tube will be used in overcoming this resistance 

 to its movement, and the volume of the outflow will be correspond- 

 ingly diminished. If we use an apparatus such as is represented in 

 Fig. 208, consisting of a reservoir, H, and a long outflow tube, 

 1, 2, 3, 4, 5, the outflow from the end and the pressure along the 

 tube may be measured directly. We must suppose that the head 

 of pressure that is, the height of the water in H is kept constant 

 by some means. The resistance or tension due to the friction in the 

 tube may be measured at any point by inserting a side-tube or 

 gauge (piezometer) at that point. The liquid will rise in this tube 

 to a level corresponding to the pressure or resistance offered to the 

 movement of the liquid at that point that is, the weight of the 

 column of liquid will measure the pressure at that point upon a 

 surface corresponding to the cross-area of the tube. The pressure 

 or tension at any point may be spoken of as the side pressure or 

 lateral pressure, and it expresses the amount of resistance offered 

 to the flow of the liquid because of the friction exerted upon the 

 water by the walls of the tube between that point and the exit. 

 This side pressure increases in a straight line from the point of exit 

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