During which phase of breathing does intrapulmonary pressure become more positive?

During expiration, intrapulmonary pressure becomes more positive. This occurs as the diaphragm and intercostal muscles relax, allowing the lungs to recoil and the volume of the thoracic cavity to decrease. As the volume inside the lungs decreases, the pressure within the lungs (intrapulmonary pressure) increases relative to the atmospheric pressure outside. This positive pressure gradient drives air out of the lungs and into the atmosphere.

Inhalation is characterized by a decrease in intrapulmonary pressure as the thoracic cavity expands, allowing air to flow into the lungs. The resting phase is a neutral point where pressures are balanced, and forced inspiration further decreases the pressure in the lungs as air is drawn in intensely. Therefore, expiration is the phase where intrapulmonary pressure is positively elevated relative to the surrounding atmosphere as air is expelled from the lungs.