Explanation

During pregnancy, both anatomical and physiological changes occur to the respiratory tract. As the uterus grows, the diaphragm rises and the thoracic circumference increases. As the diaphragm rises, the residual volume (RV) and expiratory reserve volume (ERV) decrease, which results in decreased functional residual capacity (FRC). There is an increased tidal volume (TV) and inspiratory capacity (IC). There is a small decrease to no change in the total lung capacity (TLC). The respiratory rate is unchanged, thus the increased tidal volume results in an increased minute ventilation. This is attributed primarily to progesterone, which increases respiratory effort by increasing the sensitivity of the chemoreceptors to carbon dioxide. The increased minute ventilation decreases PaCO₂, resulting in a state of respiratory alkalosis. The decreased partial pressure of carbon dioxide facilitates transfer of CO₂ away from the fetus, while the alkalotic state, through 2,3-diphosphoglycerate, shifts the oxygen dissociation curve to the right, enhancing oxygen release to the fetus.

Increased FEV1/FVC (A) is incorrect because neither parameter is changed in pregnancy. Peak expiratory flow rate (PEFR) is also unchanged. Increased residual volume (C) is incorrect because the rising diaphragm results in a decreased residual volume. Increased respiratory rate (D) is incorrect because the respiratory rate remains unchanged in pregnancy.