ontract passively with changes in pressure) of the veins is high so most changes in blood volume occur in the venous system(11)
             Contraction of muscles in the limbs squeezes blood up to the heart since valves in the veins only allow blood flow in this direction, the increase in venous blood flow into the heart (venous return) causes an increase in cardiac output (8). Smooth muscle tone in the veins can be increased by the sympathetic nervous system which acts to ensure that blood returns to the heart. In the reclining position, the heart is lying along the same plane as the head, legs and feet, so that there is very little difference in hydrostatic pressure between these areas. Standing raises the heart above the legs but it is now lower than the head. Gravity causes an increase in venous volume and pressure of the lower limbs whilst pressure in the head decreases, as well as stroke volume. Central venous pressure (pressure in the thoracic vena cava near the right atrium) decreases, as does stroke volume and venous return decreases so that cardiac output and arterial blood pressure fall. The veins, which need to g
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