Final answer:
During inhalation, the diaphragm and external intercostal muscles contract, causing the rib cage to expand and move outward. This creates a lower pressure within the lung than the atmosphere, drawing air into the lungs. During exhalation, the diaphragm and intercostals relax, causing the thorax and lungs to recoil, forcing air out of the lungs.
Step-by-step explanation:
During inspiration, 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 expiration, 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.