During inspiration when the diaphragm contracts, the intrapleural pressure becomes:

The intrapleural pressure is always negative to the atmospheric pressure. During inspiration it is -6mHg and during expiration it is -2mmHg. Intrapleural pressure prevents collapse of lungs. Intra alveolar pressure is -4mm during inspiration and +4mm Hg during expiration. It helps in flow of air in and out of alveoli and helps in exchange of gases.