CIRCULATION OF BLOOD 73 



blood to the arteries of the limb, as the outflow of blood into the 

 veins is uniform. 



3. The resistance to moving blood due to friction. 



The energy of the moving blood must overcome the resist- 

 ance due to the friction of the particles of blood upon each 

 other and upon the walls of the vessels. The friction is 

 greater the smaller the cross-section of the vessel. 



In a given cross-section of a blood vessel, all the parts of 

 the blood do not move with the same velocity, but those in 

 the middle of the vessel move faster, while those touching 

 the walls move slowest; this is due to the resistance caused 

 by friction. In the swifter axial current float the specifically 

 heavier particles of the blood, the red blood corpuscles; in 

 the slower peripheral stream are found the lighter leucocytes. 



4. Relation between fall in pressure, rate of flow and 

 resistance. The energy is used for producing movement 

 and for overcoming resistance ; its consumption is the 

 greater, the greater the movement produced and the greater 

 the resistance overcome. The amount of energy consumed 

 in a given length of vessel is measured by the decrease in 

 pressure. The decrease in pressure in a unit of distance is 

 called the fall. 



In a tube of uniform diameter, in which the resistance offered 

 by every cross-section is the same, and in which the rate of flow 

 is the same at all cross-sections, the decrease in pressure is pro- 

 portional to the distance traversed, i.e. theyfr// is uniform through- 

 out. But if the fluid flows through a tube of non-uniform bore, 

 the fall in the wider portions will be less than in the narrower 

 parts, because in the wider parts the resistance is less. 



The question where the fall is greatest cannot be definitely 

 answered. In the vascular system the total cross-section 

 increases as we proceed from the arteries to the capillaries, 

 but the cross-section of the individual vessel decreases ; going 

 from the capillaries to the veins, it is the reverse. Now the 

 resistance is, on the one hand, the smaller the greater the 

 total cross-section; but, on the other hand, the greater the 

 smaller the individual cross-section. Of these two opposing 



