The rate of blood flow back to the heart is the venous return. It normally limits cardiac output.
Factors That effect Venous return:
1. Pumping Action Of the heart: During the cardiac cycle atrial pressure changes alter central venous pressure (CVP). The central venous pressure is altered because there is no valve between the hearts atrials and the veins. Because there are no valves atrial pressure alters CVP. CVP reflects changes in atrial pressure. Atrial pressure changes venous pressure and therefore it also alters venous return.
2. Venous Tone:
When the smooth muscles around the veins are innervated by the sympathetic nervous system they contract. Venoconstriction causes the lumen diameter of the veins to decrease. The contraction of the smooth muscles also decreases the destenasability of the walls of the veins. The above only has a little effect on the blood pressure in the veins.
When inspiration occures the pressure inside the thoracic cavity is reduced. This venous pressure decreases and therefore venous return is greater. The diaphragm also pushes on the abdominal contents during inspiration. This also increases the venous return as it pushes the blood out of the organs.
4. Muscle Pump
When active the muscles put pressure on veins and venuoles. The valves in the venules prevent backflow and therfore ensure flow is only in the direction towards the heart.