What effect does a distended uterus have on functional residual volume?

The correct choice highlights that a distended uterus decreases lung capacity and tidal volume. During pregnancy, as the uterus expands, particularly in the later stages, it exerts pressure on the diaphragm and lungs. This compression reduces the thoracic cavity's volume available for lung expansion, which directly affects functional residual volume (FRV) – the amount of air remaining in the lungs after a normal exhalation.

As the uterus becomes distended, particularly towards the end of pregnancy, the downward displacement of the diaphragm leads to a decrease in FRV. Additionally, tidal volume, which is the volume of air inhaled or exhaled during normal breathing, can also be impacted due to limited space for lung expansion. Therefore, both lung capacity and the amount of air exchanged with each breath can be diminished due to the physical constraints imposed by an enlarged uterus.

This physiological adjustment is crucial for understanding respiratory changes during pregnancy, as it highlights how various factors can influence a person's lung function.