the intrapleural space is a cavity between the visceral pleural and the parietal pleural. the visceral pleural wraps around the lungs while the parietal pleural wraps around the entire cavity that the lungs are contained in. therefore, the diaphragm is connected to parietal pleural.
when the diaphragm contracts, it flattens downward and brings the connected parietal pleural down with it. this increases the volume of the intrapleural space. from boyle's law we know that Pressure and volume are inversely proportional. therefore, the IPP will decrease given the new increase in volume, and subsequently lead to the inflation of the lungs.
