Compared with the non-pregnant state, which change in maternal respiratory physiology occurs during normal pregnancy?

• A Total lung capacity increases because rib cage dimensions enlarge permanently
• B Tidal volume remains unchanged and respiratory rate rises steadily throughout gestation
• C Functional residual capacity increases because diaphragmatic excursion is enhanced
• D Tidal volume increases by 30 to 40 percent under progesterone drive, while residual volume falls ✓

Explanation

Progesterone acts on the respiratory center to raise minute ventilation by about 40 percent, achieved almost entirely through a 30 to 40 percent rise in tidal volume rather than rate. The enlarging uterus elevates the diaphragm, so functional residual capacity and residual volume fall, and total lung capacity decreases slightly. Rate stays essentially unchanged and rib cage flare does not expand total lung capacity, which rules out the other options.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.