Veins return blood to the heart. Over the course of a minute this volume is called the venous return (VR). By the time blood has reached the veins its pressure has been reduced to practically nothing. Because blood flows along its pressure gradient there are several mechanisms that assist the flow.
Veins are compliant (COM), that is, they stretch when filling with blood but, unlike elastic arteries, recoil is minimal. Compliance (COM) is shown as the dashed outline around the vein. The stretch is caused by the hydrostatic pressure ( HP block arrow) resulting from blood pressure (BP) entering veins.
Compliant vessels (COM) have large diameters which means they have low peripheral resistance (PR). This inverse relationship is shown by the dashed arrow between these two factors. Also, low peripheral resistance is inversely related to flow as indicated by the dashed arrow.
High compliance favors flow by providing little loss of pressure due to friction; the peripheral resistance is small. However, compliance simultaneously does not favor flow because the pressure gradient--flow arrow between the two BP acronyms--is kept small. In other words, since less energy is lost to friction, the pressure at the downstream end of the vessel will not have dropped much.
Many veins have one way valves that prevent the backflow of blood; a handy mechanism especially in light of the low pressure gradient in veins. These are not shown on the model.
Deep veins pass between skeletal muscles in the extremities. Contraction of these muscles (Csm) presses on the veins causing forward movement of blood through the one way valves. This is shown by the solid arrow (direct relationship) between Csm (i.e., contraction of skeletal muscles) and venous return (VR).
During inhalation, the pressure in the pleural cavities decreases causing the lungs to expand. The veins entering the heart are affected by this pressure drop in the same manner as the lungs; they expand. This pressure drop, due to decreased pleural pressure (Pp) at the end of the great veins, decreases blood pressure (BP) at this location. This direct relationship is shown by the solid arrow between these two factors. The term 'thoracic pump' is often applied to this phenomenon.
Last updated: 7/14/2005