Final answer:
In the breath cycle, intrapleural pressure becomes more negative as the diaphragm flattens during inhalation, aiding lung expansion, and less negative during exhalation as the lungs recoil.
Step-by-step explanation:
In the breath cycle, intrapleural pressure gets more negative as the diaphragm flattens down. This is due to the diaphragm contracting and moving downward, increasing the volume of the thoracic cavity and decreasing the pressure inside the lungs to less than atmospheric pressure.
As a result, the pressure between the chest wall and the lungs, which is the intrapleural pressure, becomes even lower or more negative to overcome the positive pressure created by surface tension in the lungs. This change facilitates the expansion of the lungs during inhalation.
Intrapleural pressure is always negative relative to intra-alveolar pressure, and this negative pressure is essential for keeping the lungs inflated.
During exhalation, the diaphragm and intercostal muscles relax, and the elasticity of the lung tissue causes the lung to recoil. This decrease in volume leads to an increase in intrapulmonary pressure, causing air to leave the lungs. At this stage, intrapleural pressure becomes less negative but remains lower than intra-alveolar pressure to maintain the lungs inflated.