HAEMODYNAMICS 363 



iimch the saiiK' wuv as any other cciiiallv viscous (luid drixfu 

 through a series oi" tubes. In order to understand many of the 

 prol)lenis wliich one meets in the study of physiological phenomena, 

 it is necessary to obtain some insight into the movement of fluid 

 under an external driving force. As Servetus says, " In order to 

 learn how the blood is formed it is necessary to ascertain how it 

 moves." First of all, let us consider the flow of liquid from a 

 reservoir through a series of tubes. 



(1) Gravity. In a liquid the molecular forces are in equili- 

 brium ; the kinetic forces characteristic of matter in the gaseous 

 state are exactly balanced by the Newtonian forces predominant in 

 solids. As Soddy would put it, the processes of iDellation and 

 tractation woidd not be manifest. Gravitation alone has to be 

 reckoned with. In common parlance, liquids seek their own level 

 and so always tend to flow to the lowest possible position. It is a 

 well-known fact that the speed attained by a body falling in 

 vacuo through the distance (h) equals V'igh, g being the accelera- 

 tion produced by gra\'ity. 



(2) Resistance at Outlet. This formula cannot be used to 

 estimate the velocity of fluid escaping from a reservoir. As every 

 boy knows, when the waste water is being run out from the bottom 

 of a wash-hand basin, the fluid tends to rotate round the orifice 

 and to assume a conical form. This is due to the attempt of the 

 water particles to rush the exit (so to speak). Only a limited 

 number of them lie in the column vertically above the opening. 

 The majority, occupying more lateral positions, tend to escape 

 along with the minority in the queue and so exert a force applied 

 at an angle to the line of exit. Consequently, the total energy 

 cannot be used to produce velocity. Some of it has to be spent 

 in overcoming the resistance at the outlet. 



(3) Resistance to Flow. Still further modification of the formula 

 is required if the orifice is fitted with an exit tube. It must be 

 evident that the presence of this passage imposes a greater resist- 

 ance to outflow and materially reduces the rate. Let us consider 

 the effect produced on rate of flow by attaching a rigid cylindrical 

 tube of uniform bore to the lower orifice of the reservoir. In order 

 to simplify matters, we will place this pipe horizontally. Two 

 causes tend to reduce the kinetic energy of fluid flowing through a 

 tube, viscosity or internal friction and external friction on the walls 

 of the tube. 



(a) Friction. On account of the latter, the outermost layers of 

 the fluid adhere to the walls of the tube and become more or less 

 stationary. The molecules of the layers of fluid next to the outer- 

 most tend to cohere to the stationary layer on one hand and are 



