The venous system acts as a collecting system, returning blood from the capillary networks to the heart passively down a pressure gradient. The capillaries merge to form venules, which in turn unite to form larger, but fewer, veins which amalgamate finally into the venae cavae. The walls of veins consist of the same three layers as arteries, but the elastic muscle components are much less prominent; the walls in general are thinner and more distensible than those of arteries.

The vessels have a relatively large diameter (the vena cava is 2-3 cm in diameter) and thus offer low resistance to blood flow. Some veins, especially in the arms and legs, have internal folds of the endothelial lining that function as valves and allow blood to flow in one direction only, towards the heart. These valves can be damaged if over stretched by high venous pressures for long periods, for example during pregnancy or in people who stand for extended periods; the valves become incompetent, lose their function, and varicose veins develop. As a result of this, oedema and varicose ulcers can develop.

A major part of the blood volume, approximately 60%, is contained within the venous system and for this reason veins are sometimes referred to as capacity vessels. The capacity of the venous system can be modified by altering the lumen size of the muscular venules and veins; the changes are mediated by altering the venomotor tone, that is, the degree of contraction of the smooth muscle in the tunica media. Venomotor tone is mainly under the control of the sympathetic nervous system. Changes in the venomotor tone can increase or decrease the capacity of the venous circulation and therefore can partially compensate for variations in the effective circulating blood volume.