How Respiratory Pump Affects Venous Return, Animation. 

Thoracic pump (or effect of breathing) on the rate of venous return.
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Venous return is the flow of blood from the periphery back to the heart’s right atrium. Blood from the upper body returns via the superior vena cava, blood from the lower body returns via the inferior vena cava.
The rate of venous return is determined by two factors: the pressure gradient between venous pressure and right atrial pressure; and venous resistance. A decrease in right atrial pressure leads to an increase in venous return, and vice versa.
Breathing is one of the mechanisms that facilitate venous return. This is known as thoracic pump, or respiratory pump.
During inspiration, the diaphragm moves down, expanding the thoracic cavity, resulting in a decreased intra-thoracic pressure and a subsequent expansion of the lungs. Part of this change in pressure is transmitted across the walls of the heart, lowering right atrial pressure and thus facilitating venous return. Another aspect of the diaphragmatic descent is the concomitant increase in abdominal pressure. As the inferior vena cava passes through both abdominal and thoracic cavities, an increase in abdominal pressure together with a decrease in thoracic pressure squeeze the blood upward - toward the heart.
On the other hand, left ventricular stroke volume is decreased during inspiration. This is because the expansion of the lungs causes pulmonary blood volume to increase and the blood flow from the lungs to the left atrium to decrease.
During expiration, the diaphragm moves up, the pressure in the thoracic cavity reverses. Venous return decreases. Pulmonary blood vessels shrink pumping more blood through the pulmonary veins into the left atrium. Stroke volume increases as a result.
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