518 CIRCULATION OF BLOOD AND LYMPH. 



to an increase in this head of pressure, and along with the increase 

 in velocity thus caused there is an increased friction or resistance. 



II. An increase or decrease in the width of the vessels will 

 influence both the resistance to the flow and the velocity. Under 

 normal conditions it is the small arteries that are constricted or 

 dilated (vasoconstriction and vasodilatation) . A constriction of 

 these arteries causes an increase in arterial pressures and a decrease 

 in venous pressure. The velocity of the blood-flow is decreased. 

 A dilatation has the opposite effects. Numerous instances of this 

 relation will be referred to in describing the physiology of the vaso- 

 motor nerves. 



III. A diminution in elasticity of the arteries will tend to 

 interfere with the constancy of the flow from the arteries into the 

 capillaries, and in the arteries themselves the swings of pressure 

 from systolic to diastolic during the heart beat will be more 

 extensive. This latter fact can be shown upon elderly individuals 

 whose arteries are becoming rigid, but whether a change of this 

 character is ever so advanced in human beings as to seriously modify 

 the capillary circulation does not appear to have been investigated. 



IV. A loss of blood, other conditions remaining the same, 

 will also cause a fall in blood-pressures and velocity. As a matter 

 of fact, however, a considerable amount of blood may be lost with- 

 out any marked permanent change in arterial blood-pressure. The 

 reason for this result is found in the adjustability or adaptability 

 of the vascular system. It is in such respects that the system differs 

 greatly from a rigid schema such as we use for our models. When 

 blood is withdrawn from the vessels the loss may be offset by an 

 increased action of the heart and by a contraction of the arterioles, 

 the two effects combining to give a normal or approximately normal 

 arterial pressure. To carry out the analogy with the model (Fig. 

 208) if by chance some of the store of water was lost we might sub- 

 stitute a narrower reservoir, so that with a diminished supply we 

 could still maintain the same level of pressure. In the body, 

 moreover, a loss of blood by hemorrhage may be compensated in 

 part, so far as the bulk of the liquid is concerned, by a flow of 

 liquid from the tissues into the blood-vessels. 



The Hydrostatic Effect. In the living animal, especially in 

 those, like ourselves, that walk upright, the actual pressure in the 

 arteries of the various tissues must vary much also with the position. 

 For instance, in standing erect the small arteries in the hands or 

 feet are, in addition to other conditions noted above, exposed to the 

 weight of the column of arterial blood standing over them. In 

 the pendent arm the skin of the fingers is congested; if, however, 

 the arm is raised above the head the skin may become blanched 

 because now the column of blood from fingers to shoulder exercises 



