no A MANUAL OF PHYSIOLOGY 



any part of the vascular channel really implies. To say that when 

 the channel widens the velocity diminishes, is not to explain the 

 meaning of this diminution. A diminution of velocity implies a 

 diminution of kinetic energy, and it is necessary to know what 

 becomes of the energy that disappears. The stock of energy im- 

 parted by the contraction of the heart to a given mass of blood 

 constantly diminishes as it passes round from the aorta to the right 

 side of the heart, for friction is constantly being overcome and heat 

 generated. This energy, as we have seen, exists in a moving liquid 

 in two forms, potential and kinetic, the former being measured by the 

 lateral pressure, the latter varying directly as the square of the 

 velocity. Whenever the velocity, and therefore the kinetic energy, 

 of a given mass of the blood is diminished without a corresponding 

 increase in the potential energy, some of the total stock of energy 

 must have been used up to overcome resistance (p. 76). 



In a uniform, rigid, horizontal tube, as has been already remarked, 

 the velocity (and consequently the kinetic energy) is the same at 

 every cross-section of the tube, while the potential energy, repre- 

 sented by the lateral pressure, diminishes regularly along the tube. 

 When the calibre of the tube varies, it is different. Suppose, for 

 instance, that the liquid passes from a narrower to a wider part, the 

 velocity must diminish in the latter. The kinetic energy of visible 

 motion which has disappeared must have left something in its room. 

 Here there are three possibilities : ( i ) The kinetic energy that has 

 disappeared may be just enough to overcome the extra friction 

 in the wider part of the tube due to eddies and consequent change of 

 direction of the lines of flow ; in this case the potential energy of a 

 given mass of the liquid will be the same at the beginning of the 

 wider part as in the narrower part. The lost kinetic energy will 

 have been transformed into heat. (2) The kinetic energy which has 

 disappeared may be greater than is enough to overcome the extra 

 resistance ; a portion of it must, therefore, have gone to increase the 

 potential energy, and the lateral pressure will be greater in the wide 

 than in the narrow part. (3) The lost kinetic energy may be less 

 than enough to overcome the extra resistance ; in this case both the 

 lateral pressure and the velocity will be less in the wide than in the 

 narrow part. It has been experimentally shown that when a narrow 

 portion of a tube is succeeded by a considerably wider portion, and 

 this again by a narrow part, case (2) holds ; and the liquid may, 

 under these conditions, actually flow from a place of lower to a place 

 of higher lateral pressure. 



In the vascular system the conditions are not the same. The 

 widening of the bed which takes place as we proceed in the 

 direction of the arterial current is not due to the widening of a 

 single trunk, but to the branching of the channel into smaller 

 and smaller tubes. In the larger arteries the increase of resist- 

 ance is so gradual that both the potential 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 artericles the friction increases so greatly that although the 

 velocity, and therefore the kinetic energy, in the capillary region 

 is much less than in the arteries, the amount of kinetic energy lost 

 is not upon the whole equivalent to the energy consumed in over- 



