Which mechanism promotes venous return through intrathoracic pressure changes during breathing?

• A. Increasing abdominal pressure during expiration
• B. Decreasing thoracic pressure during inspiration
• C. Increasing thoracic pressure during inspiration
• D. Blocking venous valves

Answer: (B)

The key idea is the respiratory pump: venous return is helped when intrathoracic pressure becomes more negative during breathing. When you inspire, the diaphragm contracts and moves downward while the chest wall expands, making the intrathoracic (and pleural) pressure more negative than the surrounding atmospheric pressure. That negative pressure lowers the pressure in the large veins returning to the heart (the vena cavae) and in the right atrium, creating a pressure gradient that pulls blood back toward the heart. This suction effect during inspiration increases venous return.

The other options don’t fit this mechanism. Increasing abdominal pressure during expiration isn’t about the intrathoracic pressure changes that drive venous return. Increasing thoracic pressure during inspiration would push blood away from the heart, reducing return. Blocking venous valves would disrupt one-way flow rather than promoting return via breathing.