Final answer:
Pulmonary ventilation involves inspiration and expiration. The diaphragm and external intercostal muscles contract during inspiration, causing the lung volume to increase and air to be drawn in. During expiration, the diaphragm and intercostals relax, causing the lung volume to decrease and air to be forced out.
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 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. However, during forced exhalation, the internal intercostals and abdominal muscles may be involved in forcing air out of the lungs. In the context of pulmonary ventilation, forced exhalation like that during a cough involves the internal intercostal and abdominal muscles, which help in forcefully expelling air from the lungs. This is distinct from normal exhalation, which is a passive process and typically does not involve these muscles. The glottis, which is the opening between the vocal cords, temporarily closes to build pressure before the air is expelled. A cough is a protective reflex that serves to clear the airways.