During inspiration, which of the following occurs?

• A. Intrathoracic pressure decreases and intraabdominal pressure increases; leg veins are inhibited and upper-extremity vein flow velocity increases
• B. Intrathoracic pressure increases and intraabdominal pressure decreases; leg veins velocity increases
• C. Intrathoracic pressure decreases and intraabdominal pressure decreases; both leg and upper-extremity veins velocities decrease
• D. Intrathoracic pressure increases and intraabdominal pressure increases; leg veins velocity increases

Answer: (A)

During inspiration the intrathoracic pressure becomes more negative, which tends to pull venous blood toward the heart and increases venous return from the upper body. At the same time, the diaphragm moves downward and abdominal contents are compressed, raising intraabdominal pressure and reducing venous return from the lower body through abdominal and pelvic veins into the IVC. The net effect is increased flow velocity in upper-extremity veins and decreased (inhibited) flow in leg veins. This combination—decreased intrathoracic pressure, increased intraabdominal pressure, inhibited leg venous flow, and increased upper-extremity venous velocity—fits the physiological pattern seen with inspiration.