990 



PHYSIOLOGY 



the individual and the physiological activity of the various parts and 

 organs of the body : 



Large arteries (e.g. carotid) 



Medium arteries (e.g. radial) 



Capillaries 



Small veins of arm 



Portal vein . 



Inferior vena cava 



Large veins of neck 



90 mm. mercury (65-110). 

 . 85 mm. 

 .about 15 to 40 mm. mercury. 



9 mm. mercury. 

 . 10 mm. 

 3 mm. ,, 

 . from to -8 mm. mercury 



The cause of these peculiarities in the circulation in different parts 

 of the vascular system will be rendered clearer by a study of a flow of 





P 3 



B 



FIG. 378. 



fluid through a tube of uniform bore (Fig. 378). If the tube AG be 

 connected with the reservoir R, fluid will flow from A to G under the 

 influence of the pressure difference between the fluid in the reservoir 

 and that at D. The pressure on the fluid at each part of the tube can 

 be measured by attaching at a series of points e.g. at B, c, D, E, F 

 vertical tubes in which the fluid will rise to a height corresponding to 

 the lateral pressure existing at these several points. When fluid is 

 flowing from A to G it will be found that the heights of the fluid in the 

 tubes show a continuous descent, so that the line joining the tops of the 

 fluid in the various tubes is a straight one. The movement of the 

 fluid from B to c can be regarded as due to the difference of the pressure 

 between B and c., i.e. P 2 -P 3 . It will be noticed in the diagram that 

 the straight line joining the tops of the fluid does not strike the surface 

 of the fluid in R, but falls a little below it. Of the total pressure in R, H, 

 the large portion Ji is employed in overcoming the resistance of the 

 tube AG, while a small portion h represents the force necessary to give 

 to the fluid as it leaves the reservoir at A a certain velocity. If the 

 flow of fluid be diminished by partially clamping the end at G the rate of 

 fall of the pressures will be diminished. The same effect will be 



