Which factor leads to decreased pulmonary blood flow?

• A. Left to right shunting
• B. Pulmonary hyperinflation
• C. Release of mediator substances (bradykinin, prostaglandins)
• D. Pulmonary edema

Answer: (B)

Pulmonary blood flow is driven by the pressure gradient from the pulmonary arteries to the left heart. When the lungs become hyperinflated, intrathoracic and alveolar pressures rise, which directly compresses the small pulmonary vessels and increases pulmonary vascular resistance. This makes it harder for blood to pass through the pulmonary circulation, lowering flow. Additionally, the higher intrathoracic pressure reduces venous return to the heart, decreasing left heart filling and overall cardiac output, which further diminishes pulmonary blood flow.

Left-to-right shunting actually increases blood flow to the lungs, not decreases. Mediator substances like bradykinin and prostaglandins tend to cause vasodilation, which can increase flow rather than decrease it. Pulmonary edema involves fluid accumulation that impairs gas exchange but does not primarily reduce pulmonary blood flow through vessel compression in the same way hyperinflation does.