In pregnancy, the normal physiological changes in respiratory function include:

• A Increased functional residual capacity (FRC) and residual volume (RV) due to diaphragm elevation
• B Increased tidal volume and minute ventilation with decreased PaCO2 (~30 mmHg) and mild respiratory alkalosis ✓
• C Decreased respiratory rate with compensatory increase in tidal volume maintaining constant minute ventilation
• D Increased total lung capacity due to progesterone-mediated bronchodilation

Explanation

Pregnancy causes a progesterone-mediated increase in central respiratory drive, raising tidal volume (not rate) and minute ventilation by about 40-50%. This produces a mild chronic respiratory alkalosis with PaCO2 of approximately 28-32 mmHg (normal ~40 mmHg). The kidneys compensate by excreting HCO3, maintaining pH near normal. FRC and RV decrease (not increase) due to uterine elevation of the diaphragm, which is clinically important in anaesthesia (reduced apnoea reserve).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.