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Properties of Gas Exchange Quiz
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Which statement best describes Boyle's law as it applies to ventilation at constant temperature?
- A As lung volume decreases, intrapulmonary pressure remains constant, so air leaves by diffusion. You picked ✕
- B As lung volume increases, intrapulmonary pressure remains constant, so no air flows. You picked ✕
- C As lung volume decreases, intrapulmonary pressure decreases, allowing air to flow into the lungs. You picked ✕
- D As lung volume increases, intrapulmonary pressure decreases, allowing air to flow into the lungs. You picked ✓
- E As lung volume increases, intrapulmonary pressure increases, forcing air out of the lungs. You picked ✕
Correct!
Incorrect
The answer is D: As lung volume increases, intrapulmonary pressure decreases, allowing air to flow into the lungs.
Explanation
Why D is correct
See Boyle's Law. Option D is correct. Boyle's law states that, for a fixed amount of gas at a constant temperature, pressure and volume are inversely related: when the volume gets larger the pressure falls, and when the volume gets smaller the pressure rises. Gas pressure is made by the molecules colliding with the walls of their container, so in a bigger space they collide less often and the pressure drops. In breathing in, the diaphragm and chest wall muscles contract and the chest enlarges. The lungs expand with it, so the pressure inside the air sacs (the intrapulmonary or alveolar pressure) falls below the pressure of the air outside. Air flows down a pressure gradient, that is from higher to lower pressure, so it moves into the lungs. In breathing out, the lung volume falls, the pressure rises above atmospheric pressure and air leaves. A simple way to remember it is that a bigger space means lower pressure and air flows in.
- Boyle's law: at constant temperature, the pressure and volume of a fixed amount of gas are inversely related.
- Breathing in: the chest enlarges, so the pressure in the air sacs falls below atmospheric pressure.
- Airflow: air flows down a pressure gradient, from higher to lower pressure, so it moves into the lungs.
Why the other options are incorrect
- A. As lung volume decreases, intrapulmonary pressure remains constant, so air leaves by diffusion.: During expiration the lung volume decreases, which raises intrapulmonary pressure above atmospheric pressure and forces air out along a pressure gradient. If pressure stayed constant, air would not leave by diffusion alone. Your answer
- B. As lung volume increases, intrapulmonary pressure remains constant, so no air flows.: Boyle's law requires intrapulmonary pressure to change when lung volume changes; if pressure remained constant, no pressure gradient would exist and air would not move. Constant pressure would make ventilation impossible. Your answer
- C. As lung volume decreases, intrapulmonary pressure decreases, allowing air to flow into the lungs.: When lung volume falls, the gas molecules have less space and collide more often, so intrapulmonary pressure rises, not falls. If pressure fell below atmospheric pressure during expiration, air would move in rather than out. Your answer
- E. As lung volume increases, intrapulmonary pressure increases, forcing air out of the lungs.: Boyle's law says that pressure and volume change in opposite directions, so a bigger lung volume lowers the pressure. If pressure rose with volume, breathing in would be impossible. Air leaves the lungs when volume falls and pressure rises. Your answer
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