It is important for an increase in cardiac output to be accompanied by an increase in venous return. During exercise, the venous return is increased via three mechanisms: the muscle pump, the respiratory pump and venoconstriction.
The muscle pump is a result of the mechanical pumping action caused by repetitive muscular contractions. When the muscles contract, the veins are squashed together and the blood in them is forced towards the heart. Valves in the veins prevent the blood from flowing backwards. When the muscle relaxes, the veins fill with blood until the next contraction and the process continues, resulting in a 'pumping' action.
Respiratory pump: during inspiration (breathing in), the diaphragm increases abdominal pressure and veins in the thorax and abdominal are emptied towards the heart. Then during expiration (breathing out), the process is reversed-the veins fill with blood ready to be emptied again. Venous return is promoted simply through breathing. During exercise, when venous return venous return needs to increase, respiratory rate increases, which makes the pump more effective.
Venoconstriction: assist in venous return. It reduces the capacity of the venous system, forcing blood to be pushed out towards the heart.