A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 35-year-old woman volunteers for a study on respiratory physiology. Pressure probes A and B are placed as follows:
Probe A: between the parietal and visceral pleura
Probe B: within the cavity of an alveoli
The probes provide a pressure reading relative to atmospheric pressure. To obtain a baseline reading, she is asked to sit comfortably and breathe normally. Which of the following sets of values will most likely be seen at the end of inspiration?
-
A
Probe A: 0 mm Hg; Probe B: -1 mm HgIncorrect. Intrapleural pressure is never 0 in a living person — it's always negative due to chest wall recoil outward and lung recoil inward. A pressure of 0 indicates pneumothorax.
-
B
Probe A: -4 mm Hg; Probe B: -1 mm HgIncorrect. Probe A at −4 mm Hg is closer to the resting (end-expiratory) intrapleural pressure, not end-inspiration. Probe B at −1 is mid-inspiration, not end-inspiration.
-
C
Probe A: -6 mm Hg; Probe B: 0 mm HgCorrect. At end-inspiration, intrapleural pressure is maximally negative (~−6 mm Hg) because the chest is fully expanded, while alveolar pressure has returned to 0 because airflow has just stopped.
-
D
Probe A: -6 mm Hg; Probe B: -1 mm HgIncorrect. Probe A at −6 mm Hg is correct, but Probe B at −1 mm Hg describes mid-inspiration when air is still flowing in; at END-inspiration, alveolar pressure equilibrates with atmosphere.
-
E
Probe A: -4 mm Hg; Probe B: 0 mm HgIncorrect. Probe A at -4 mm Hg reflects the resting (end-expiratory) intrapleural pressure, not the more negative end-inspiratory value; at end-inspiration, intrapleural pressure is about -6 mm Hg while alveolar pressure has equilibrated to 0.
↑ Tap an answer to reveal the reasoning
Answer: C. This question tests respiratory physiology at the end of normal inspiration. Two pressures are being measured: intrapleural pressure (probe A, between parietal and visceral pleura) and alveolar pressure (probe B, within an alveolus). Both are expressed relative to atmospheric pressure (which is set to 0).
During quiet breathing:
• At rest (end-expiration): intrapleural pressure is about −5 cm H2O; alveolar pressure is 0 (equal to atmosphere — no airflow).
• During inspiration: the diaphragm contracts and the chest expands, making intrapleural pressure MORE negative (approximately −7 to −8 cm H2O). This decreases alveolar pressure to about −1 cm H2O, driving air INTO the lungs.
• At END-inspiration: airflow has stopped, so alveolar pressure returns to 0 (equal to atmosphere). Intrapleural pressure remains maximally negative at about −6 to −8 cm H2O because the chest is still expanded.
So at end-inspiration: Probe A (intrapleural) ≈ −6 mm Hg, Probe B (alveolar) = 0 mm Hg. This matches option C.
**Why each option:**
**A.** Intrapleural pressure is never 0 in a living person — it's always negative due to chest wall recoil outward and lung recoil inward. A pressure of 0 indicates pneumothorax.
**B.** Probe A at −4 mm Hg is closer to the resting (end-expiratory) intrapleural pressure, not end-inspiration. Probe B at −1 is mid-inspiration, not end-inspiration.
**C.** Correct. At end-inspiration, intrapleural pressure is maximally negative (~−6 mm Hg) because the chest is fully expanded, while alveolar pressure has returned to 0 because airflow has just stopped.
**D.** Probe A at −6 mm Hg is correct, but Probe B at −1 mm Hg describes mid-inspiration when air is still flowing in; at END-inspiration, alveolar pressure equilibrates with atmosphere.