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 consequence 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 hori- 

 zontal 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 pres- 

 sure 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 grea.t 

 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. 1015). In Fig. 421 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 

 the diagram on p. 1016 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 



1050 



