This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Medical, surgical and gynaecological illness complicating pregnancy, Subject name: Gynaecology & Obstetrics
Changes in the respiratory system in pregnancy; 
Select and explain. 
A. Vital capacity is increased, B. Subcostal angle remains unchanged, C. Tidal volume remains unaltered, D. Residual volume Is decreased
D: Residual volume Is decreased
Answer: D: Residual volume Is decreased 
Explanation: Anatomical changes:The lower ribs flare out,the subcoastal angles increases from 68 to 103 degree,transverse diameter of chest increases by 2 cm and the diaphragm rises about 4cm in pregnancy. Functional changes:Increase in ventilation in pregnancy is achieved by an increase in tidal volume by 40%.Minute ventilatory volume also increases.There is no change in vital capacity.The functional residual capacity and residual volume are decreased due to elevated diaphragm. TEXT BOOK OF OBSTETRICS,Sheila Balakrishnan,2nd edition,page no.80