Does the pressure in the thoracic cavity increase when the diaphragm contracts?

No, when the diaphragm contracts, the pressure in the thoracic cavity actually decreases. This contraction expands the thoracic cavity, allowing air to flow into the lungs, as the pressure becomes lower than the atmospheric pressure. As a result, inhalation occurs. Conversely, when the diaphragm relaxes, the pressure increases, pushing air out of the lungs.