Final answer:
The breathing process involves inhalation, where contraction of the diaphragm and external intercostal muscles decreases pressure in the lungs causing air to enter, and exhalation, where relaxation of these muscles increases pressure, forcing air out. Transpulmonary pressure changes throughout the process facilitating lung volume changes.
Step-by-step explanation:
Inhalation (inspiration) begins when the diaphragm and external intercostal muscles contract, increasing the thoracic cavity volume and decreasing intrapleural pressure. As a result, lung volume expands, causing a decrease in alveolar pressure compared to atmospheric pressure, which allows air to enter the lungs.
Exhalation (expiration) is usually passive; the diaphragm and intercostals relax, reducing lung volume and increasing alveolar pressure. This pressure now becomes higher than atmospheric pressure, pushing air out of the lungs. In forced expiration, muscles like the internal intercostals and abdominal muscles contract to help expel the air.
Overall, transpulmonary pressure drives the change in lung volume during breathing, as it is the pressure difference between the intra-alveolar and intrapleural pressures. During inspiration, this pressure must be positive to expand the lungs, while during expiration it becomes less positive, allowing the lungs to recoil.