THE CIRCULATION OF THE BLOOD AND LYMPH 109 



and the kinetic energy diminish only slowly, and the lateral 

 pressure and velocity are not much less in the femoral artery 

 than in the aorta or carotid. But in the capillary region 

 the friction increases so much that although the velocity, 

 and therefore the kinetic energy, is greatly less than in the 

 arteries, the amount of kinetic energy lost is not upon the 

 whole equivalent to the energy consumed in overcoming the 

 extra resistance ; the potential energy of the blood is also 

 drawn upon, and the lateral pressure falls sharply in the capil- 

 lary region, as well as the velocity. Where the capillaries 

 open into the veins, the lateral pressure again sinks abruptly, 

 while the velocity begins to increase, till in the largest veins 

 it is probably about half as great as in the aorta. 



Where does the extra kinetic energy of the blood in the 

 veins come from ? To say that the vascular channel again 

 contracts as the blood passes from the capillaries into the 

 veins, and that, since the same quantity must flow through 

 every cross-section of the channel, the velocity must neces- 

 sarily be greater in the narrower than in the wider part, does 

 not answer the question. The greater portion of the kinetic 

 energy of the arterial blood is, as we have seen, destroyed, 

 or, rather, changed into an unavailable form, into heat, in 

 the capillary region. The mean velocity of the blood in the 

 capillaries is not more than -^ to -^ of the velocity in the 

 aorta ; the kinetic energy of a given mass of blood in the 

 capillaries cannot therefore be more than O^) 2 , or -^\-^ of 

 its kinetic energy in the aorta. In the veins, taking the 

 velocity at half the arterial velocity, the kinetic energy of the 

 mass would be one-fourth of that in the aorta, or at least 

 10,000 times as great as in the capillary region. This extra 

 kinetic energy comes partly from the transformation of some 

 of the potential energy of the blood. The resistance in the 

 veins is very small compared with that in the capillaries ; less 

 of the potential energy represented by the lateral pressure 

 at the end of the capillary tract is required to overcome this 

 resistance, and some of it is converted into the kinetic 

 energy of visible motion, the lateral pressure at the same 

 time falling somewhai abruptly. Contributory sources of 

 kinetic energy in the veins are the aspiration caused by the 



