Veins - Circulation
The veins bring back the blood from the capillaries to the heart. They form a large volume low pressure system. The vessels have a larger internal diameter. In mammals 50% of the total blood volume is present in the veins and pressure is approximately 10-5 mm Hg. If there is any blood loss, the venous volume is decreased and not the arterial volume. So that arterial blood pressure and capillary blood flow is maintained. Flow of blood in veins is affected by several factors. Pressure exerted by the diaphragm on the gut and activity of the limbs both help to squeeze the veins of those regions.
This squeezing and action of pocket valves (that prevent back flow) help the flow of blood towards the heart. Breathing in mammals also helps in drawing the blood from the veins in the head and abdominal cavity. Smooth muscles in veins also help in regulating blood supply in the venous system. When a person changes his position from sitting to standing the change in the relative position of heart and brain with respect to gravity activates the nerve fibres that are present in the veins of the limb. This causes a contraction of the smooth muscles. The pooled blood is thus, redistributed.