Answer
Both the skeletal muscle pump, and the respiratory pump aid venous return.
Work Step by Step
Venous return is the volume of blood flowing back to the heart . This flow is promoted initially by contractions of the left ventricle; however, the blood pressure in venules is very low -- about 16 mmHg ; by the time the blood reaches the right ventricle the blood pressure is even lower--close to 0.0 mmHg mmHg. The work of getting the blood back to the heart requires the generation of more force if one stands up. A standing position enhances the effect of gravity in pulling blood away from the heart --especially in the upper and lower extremities (free limbs). This means that the body needs help to reliably return venous blood to the heart. Fortunately, there are several physiological mechisms that assist the circulatory system in securing adequate venous return; These mechanisms include contractions of skeletal muscles during activities like walking, running, jumping, and playing sports. This skeletal muscle mechanism is called the skeletal muscle pump. Another kind of pump that aids in venous return is the respiratory pump. This mechanism which involves breathing movements--specifically inspiration-, generates negative intrathoracic pressure when the diaphragm contracts. One consequence of this is the increase venous return. Finally, systemic stimulation of veins and venules results in venoconstriction which increases venous blood pressure and aids venous return
