Which phenomenon explains the decrease in the residual volume of the lungs during pregnancy?

The correct answer highlights how the growing uterus exerts pressure on the diaphragm, which is a significant factor in the physiological changes of the respiratory system during pregnancy. As the uterus expands, particularly during the later stages of pregnancy, it pushes upward against the diaphragm, limiting its ability to descend fully during inhalation. This restriction results in a decrease in tidal volume and residual volume of the lungs.

By pressing against the diaphragm, the growing uterus alters the mechanics of breathing, affecting lung capacity. Consequently, the residual volume, which is the amount of air remaining in the lungs after exhalation, decreases as the diaphragm cannot move as freely. This is a normal adaptation that allows for more efficient gas exchange, despite the physical limitations imposed by the uterus.

Other options, while related to changes in respiration during pregnancy, do not accurately account for the primary reason for the decrease in residual volume. Therefore, the phenomenon of increased pressure on the diaphragm from the growing uterus effectively explains this aspect of respiratory physiology in pregnant individuals.