MCAT Biology Question 79: Answer and Explanation
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Question: 79
4. The intrapleural pressure is necessarily lower than the atmospheric pressure during:
- A. inhalation, because the expansion of the chest cavity causes compression of the intrapleural space, decreasing its pressure.
- B. inhalation, because the expansion of the chest cavity causes expansion of the intrapleural space, decreasing its pressure.
- C. exhalation, because the compression of the chest cavity causes compression of the intrapleural space, decreasing its pressure.
- D. exhalation, because the compression of the chest cavity causes expansion of the intrapleural space, decreasing its pressure.
Correct Answer: B
Explanation:
During inhalation, the chest cavity expands, causing expansion of the intrapleural space. According to Boyle's law, an increase in volume is accompanied by a decrease in pressure. When the intrapleural pressure (and, by extension, the alveolar pressure) is less than atmospheric pressure, air enters the lungs. During exhalation, these pressure gradients reverse; thus, during exhalation, intrapleural pressure is higher than atmospheric pressure, not lower.