what is the major function of the valves found in human veins?
1 Veins have valves which maintain the unidirectional flow of blood, even against gravity. 2  Since the venous pressure is low (7 mm Hg) the valves are of utmost value in the venous return. However, the valves are absent in: 3  The veins of less than 2 mm diameter.  The venae cavae.