Final answer:
During inhalation, the diaphragm and external intercostal muscles contract, expanding the thoracic cavity and drawing air into the lungs. During exhalation, the diaphragm and intercostals relax, causing the thorax and lungs to recoil and forcing air out of the lungs.
Step-by-step explanation:
Pulmonary ventilation consists of the process of inspiration (or inhalation), where air enters the lungs, and expiration (or exhalation), where air leaves the lungs. During inhalation, the diaphragm and external intercostal muscles contract, causing the rib cage to expand and move outward, and expanding the thoracic cavity and lung volume. This creates a lower pressure within the lung than that of the atmosphere, causing air to be drawn into the lungs.
During exhalation, the diaphragm and intercostals relax, causing the thorax and lungs to recoil. The air pressure within the lungs increases to above the pressure of the atmosphere, causing air to be forced out of the lungs. However, during forced exhalation, the internal intercostals and abdominal muscles may be involved in forcing air out of the lungs.