SECTION VI 

 THE FLOW OF BLOOD IN THE VEINS 



IN the veins there is a constant decrement of pressure as we pass from the 

 periphery towards the heart. This decrement of pressure is the conse- 

 quence of the pumping action of the heart, so that the flow through the 

 veins must be ascribed to the same force as that which determines the 

 flow through the arteries, viz. the heart-beat. Owing to the fact that no* 

 appreciable resistance lies between the veins and the heart, the difference 

 of pressure necessary to maintain a constant flow through these vessels 

 is very small. Thus in the horizontal position the pressure in the femoral 

 veins may be from 5 to 10 mm. Hg., and in the inferior vena cava from 

 1 to 5 mm. The pressure in the great veins near the heart is generally 

 negative owing to the aspiration of the thorax, and this negative pressure 

 is naturally increased during inspiration. Opening the thorax therefore 

 causes a rise of pressure in all the large veins. In the latter the pressure 

 depends chiefly on the heart activity, being lowered by vigorous action 

 of the heart pump and raised when this fails in any way. In the peri- 

 pheral veins the pressure is more dependent on the 'flow through the 

 corresponding arteries. If an artery of a limb be ligatured the pressure in 

 the small veins of the limb sinks until it is reduced to the pressure in the 

 nearest large trunk in which a flow of blood continues. 



Each cardiac cycle causes variations in the pressure in the great veins 

 next the heart in two ways : 



(1) By the transmission along the veins of the alterations in the intra- 

 auricular pressure. 



(2) By the diminution in the volume of the heart in consequence of the 

 expulsion of its blood along the arteries with each heart-beat. 



On this account the jugular veins show pulsations with each heart-beat 

 which are somewhat complex in character and resemble closely those 

 occurring in the auricle (vide p. 201). In Fig. 419 a tracing from the 

 wall of the jugular vein is given. It will be seen that each heart- beat 

 gives rise to three variations in pressure within the veins. These three 

 undulations are evidently exactly analogous to those given in Fig. 400 

 as occurring in the auricular tracing. We should therefore regard a as the 

 auricular contraction, c as the elevation due to the closure of the auriculo- 

 ventricular valves, v as the elevation due to the accumulation of blood 

 in the auricles during the ventricular systole. The curve c is often spoken 



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