IV infusion of normal saline increases pulmonary lymph flow by increasing capillary hydrostatic pressure. Which of the following best describes this effect?

• A. Increases plasma oncotic pressure
• B. Increases capillary hydrostatic pressure
• C. Decreases capillary hydrostatic pressure
• D. Decreases capillary permeability

Answer: (B)

The main idea is how forces across capillary walls control fluid movement and how the lymphatic system responds to excess interstitial fluid. Infusing normal saline expands blood volume, which raises venous return and central venous pressure, especially in the pulmonary circulation. This raises capillary hydrostatic pressure, pushing more fluid out of the capillaries into the interstitium. When interstitial fluid increases, the pulmonary lymphatics upregulate drainage to clear the excess fluid and maintain balance. So the effect is best described as an increase in capillary hydrostatic pressure driving more filtration and, consequently, greater lymph flow to remove the added interstitial fluid.

Putting it in context of the other possibilities: increasing plasma oncotic pressure would pull fluid into capillaries and reduce interstitial fluid and lymph flow; decreasing capillary hydrostatic pressure would lessen filtration; decreasing capillary permeability would also reduce filtration.