120 A MANUAL OF PHYSIOLOGY 



between this point and the outlet. In any system of tubes 

 the sum of the potential and kinetic energy must diminish 

 in the direction of the flow; and although the problem is 

 complicated in the vascular system by the branching of the 

 channel and the variation in the total cross-section, yet 

 theory and experiment agree that in the larger arteries the 

 lateral pressure diminishes but slowly from the heart to 

 the periphery, the resistance being small compared with the 

 resistance of the whole circuit. In the capillary region the 

 vascular resistance abruptly increases ; the velocity (and 

 therefore the kinetic energy) abruptly diminishes, and the 



FIG. 42. RELATION OF BLOOD-PRESSURE, VELOCITY, AND CROSS-SECTION. 



The curves P, V and S represent the blood-pressure, velocity of blood, and total 

 cross-section respectively in the arteries A, capillaries C, and veins V. 



lateral pressure falls much more steeply between the begin- 

 ning and the end of this region than between the heart and 

 its commencement. In the veins only a small remnant of 

 resistance remains to be overcome, and the lateral pressure 

 must sink again rather suddenly about the end of the capil- 

 lary tract. Fig. 42 shows by a rough diagram the manner 

 in which the pressure, velocity and cross-section probably 

 change from part to part of the vascular system. 



The Circulation in the Veins. The slope of pressure, as we 

 have just explained, must fall rather suddenly near the 

 beginning and near the end of the capillary tract. It con- 

 tinues falling as we pass along the veins, till the heart is 

 again reached. In the right heart, and in the thoracic 

 portions of the great veins which enter it, the pressure may 

 be negative that is, less than the atmospheric pressure. 



