Which principle states that air goes in and out while blood circulates?

• A. Pascal's Law
• B. Fick Principle
• C. Bernoulli's Principle
• D. Boyle's Law

Answer: (B)

Gas exchange rate is governed by the Fick principle: the amount of gas that moves from air into blood per unit time depends on the blood flow (perfusion) and the difference in gas content between the blood entering and leaving the lungs. In the lungs, ventilation maintains a gradient by bringing in fresh air with a high O2 and low CO2, while blood flowing through the pulmonary capillaries carries deoxygenated blood to be oxygenated and to release CO2. The transfer rate can be expressed conceptually as perfusion times the concentration (or partial pressure) difference between alveolar air and blood (for oxygen, CaO2 minus CvO2). This is why air moving in and out and blood circulating work together to achieve gas exchange.

The other named principles describe how fluids or gases behave under pressure or in motion (not the mechanism of gas exchange across the alveolar-capillary membrane), so they don’t address the coupling of ventilation and perfusion that drives gas transfer.