278 THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXII. 



decrease in the size of the arterioles of other areas may make its 

 effect felt in the special area under investigation. 



Venous pressure varies directly with the volume of the blood ; in 

 the arteries the effect of increase of fluid is slight and temporary, 

 owing to the rapid adaptability of the peripheral resistance; the 

 excess of fluid collects in and distends the easily dilatable venous 

 reservoir. With regard to the first and second factors in the fore- 

 going table, venous pressure varies in the opposite way to arterial 

 pressure. 



It is easy to understand how this is ; when the rate of the heart 

 increases, the total volume of blood discharged into the aorta per 

 second is increased ; similarly, an increase in the force of the beat 

 also results in an increase in the cardiac output, and in both cases 

 a more rapid and complete emptying of the auricle is produced. This 

 is felt throughout the whole of the pulmonary circulation, and the 

 accelerated flow therefore causes a fall in the venous pressure. If, 

 however, the rise of pressure is due to a contraction of the arterioles, 

 a stage may be reached in which the heart is no longer able to over- 

 come the high pressure produced. It then fails to empty itself, and 

 the bloo4 is dammed up on the venous side, i.e. the venous pressure 

 rises. 



With regard to the arterioles, contraction means a rise in arterial 

 pressure, because while the amount sent into the arteries remains the 

 same the outflow into the capillaries is cut down. More blood is 

 therefore retained in the arteries ; they become more distended and 

 the pressure rises. The first effect of this upon the venous pressure 

 will be to diminish it, because if more blood is retained in the arteries 

 there is less for the veins and capillaries. The flow into the veins is 

 thus decreased, and the venous pressure therefore falls. Moreover, 

 the heart usually responds to a rise in pressure by increasing its force 

 and rate, and consequently more blood is taken from the veins near 

 the heart. For both reasons, then, the venous pressure will fall, but 

 that fall is limited, as pointed out above, to such an increase only as 

 the heart is capable of overcoming successfully. 



Capillary pressure is increased by 



1. Dilatation of the arterioles; the blood-pressure of the large 

 arteries is then more readily propagated into them. 



2. The size of the arterioles remaining the same, increase of 

 arterial pressure from any other cause will produce a rise of capillary 

 pressure. 



3. By narrowing the veins leading from the capillary area ; com- 

 plete closure of the veins may quadruple the capillary pressure. 

 This leads -secondarily to an increased formation of lymph (dropsy) ; 

 as when a tumour presses on the veins coming from the legs. 



4. Any circumstance that leads to increased pressure in the veins 



