During inspiration, how does intrathoracic pressure change and how does this affect venous return?

• A. Intrathoracic pressure increases and intraabdominal pressure decreases
• B. Intrathoracic pressure decreases and intraabdominal pressure increases
• C. Intrathoracic pressure decreases and intraabdominal pressure decreases
• D. Intrathoracic pressure increases and intraabdominal pressure increases

Answer: (B)

During inspiration the chest expands and the diaphragm descends, so intrathoracic pressure becomes more negative. That negative pressure creates a stronger gradient pulling blood from the systemic veins into the right atrium, increasing venous return. At the same time, the downward movement of the diaphragm increases pressure in the abdomen, helping to push blood from the abdominal veins toward the heart. The net effect is an increase in venous return with inspiration. Expiration reverses this: intrathoracic pressure rises and abdominal pressure falls, reducing venous return.