BLOOD VESSELS 463 



the arterial pressure. The blood is not .Jriven through 

 these channels, and congestion of the capillaries and dropsy 

 may result. 



3rd. The influence of gravity plays a very important part on 

 the capillary pressure, since it has so inarked an influence on 

 the flow of blood in the veins. At the level of the heart 

 the pressure is about 20 mm. Hg. In the feet it is much 

 higher. When, through heart failure or want of exercise, 

 the blood is not properly returned from the legs, this 

 increased pressure becomes very marked, stagnation of blood 

 occurs, and swelling of the legs is apt to occur. 



■ith. Volume of the Blood. — The pressure in the capillaries 

 may also, to a certain extent, be varied by the ivithdrawal 

 of water from the body, as in purgation or in diuresis, or by 

 the addition of large quantities of fluid to the blood. In both 

 cases there is a rapid readjustment by the passage of water 

 from the tissues to the blood, or vice versa. The venous 

 system is so capacious that very great changes in the amount 

 of blood in the vessels may take place without materially 

 modifying the arterial or capillary pressure. 



III. Pressure in the Veins. 



The pressure in the veins is so low that it may best 

 be determined in the lower animals by a water manometer. 



In man it may be estimated in a prominent superficial vein 

 by stroking the vein downwards from the peripheral side of 

 a valve, applying the band of a Riva Rocci apparatus (p. 448), 

 and tlien relaxing the pressure and allowing the blood to 

 flow up into the emptied vein, and reading the pressure at 

 which this occurs. It may also be estimated in the veins of 

 the hand by finding at what level above the heart they 

 collapse. 



In the veins the force of inflow is small ; the resistance 

 to outflow is nil. Hence the pressure is low, and steadily 

 diminishes from the small veins to the large veins entering 

 the heart (fig. 183). 



The venous pressure may be modified by variations in these 

 two factors. Constriction of the arterioles tends to lower the 



