270 HUMAN PHYSIOLOGY 



in its powerful systole drives out more blood, in its diastole 

 sucks in more. 



Compression of the thorax has a very marked effect in 

 retarding the flow of blood from the great veins into the 

 heart, and thus tends to raise the venous pressure and to 

 lower the arterial pressure. Venous pressure may be tem- 

 porarily modified by the loss or gain of water. 



V. LYMPHATIC PRESSURE. 



No exact determination of the lymph pressure in the 

 tissue spaces has been made, but since there is a constant 

 flow from these spaces through the lymphatic vessels and 

 through the thoracic duct into the veins at the root of the 

 neck, the pressure in the tissue spaces must be higher than 

 the pressure in the great veins. 



This pressure is kept up by the formation of lymph from 

 the blood, and from the cells of the tissues (see p. 207). 



B. Flow of Blood. 



The flow of blood, as already indicated, depends upon the 

 distribution of pressure, a fluid always tending to flow from 

 the point of higher pressure to the point of lower pressure. 

 Since a high pressure is maintained in the aorta and a low 

 pressure in the veins entering the heart and in the cavities 

 of the heart during its diastole, the blood must flow through 

 the circulation from arteries to veins. 



The velocity of the flow of a fluid depends upon the 

 width of the channel. Since in unit of time unit of 

 volume must pass each point in a stream if the fluid is 

 not to accumulate at one point, the velocity must vary 

 with the sectional area of the channel. In the case of a 

 river, in each second the same amount of water must pass 

 through the narrowest and through the widest part of its 

 channel. Now for a ton of water to get through any point 

 in a channel one square yard in sectional area in the same 

 time as it takes to pass a point in a channel ten square yards 

 in area, it must obviously flow with greater velocity. This 

 may be stated in the proposition that the velocity (V) 



