Why does inspiration increase venous return to the right heart?

• A. Positive intrathoracic pressure during inspiration increases venous return.
• B. Negative intrathoracic pressure during inspiration increases the pressure gradient from peripheral veins to the right atrium.
• C. Venous return is unchanged during inspiration.
• D. Expiration, not inspiration, increases venous return to the right heart.

Answer: (B)

During inspiration the chest expands and the diaphragm moves downward, which makes the intrathoracic (and right atrial) pressure more negative. This lowers the pressure in the right atrium while peripheral veins stay at higher pressure, increasing the gradient from the veins toward the right atrium. Blood is thus pulled into the right heart more readily. The diaphragm’s movement also helps push blood from abdominal veins into the thorax via the inferior vena cava. So, the negative intrathoracic pressure during inspiration enhances venous return to the right heart. Expiration does not produce this same effect, and positive intrathoracic pressure during inspiration would actually hinder, not help, venous return.