Several modes of ventilation exist, spontaneous pulmonary ventilation being one of them. Waugh and Grant (2018) define it as breathing involving air moving into the lungs and out of them during inspiration and expiration, respectively. The action depends on such processes as the contraction and relaxation of intercostal muscular tissues. Other mechanisms involve atmospheric pressure, air pressure within the alveoli (intrapulmonary), and pleural cavity pressure (intrapleural). Spontaneous pulmonary ventilation is a complex phenomenon connected to various processes.
The lungs belong to the lower respiratory system and are attached to the trachea. When a person inhales, the diaphragm contracts air and moves downwards; simultaneously, the lungs’ external intercostals move upwards; their volume increases, and intrapulmonary pressure decreases. During the process, atmospheric air enters the airway and, subsequently, the trachea, which leads to the bronchi. They expand into the bronchiole connected to microscopic pockets called alveoli, which serve as the site for an important phenomenon.
The gaseous exchange involves oxygen diffusing into blood and carbon dioxide being released from it into the alveoli. According to Hedenstierna (2020), they are connected to the atmosphere through the airway, which makes atmospheric and pulmonary pressures equal. Intagliata et al. (2020) claim no exchange occurs in the respiratory tract, which explains the latter’s role as dead space. Intrapleural pressure is considered negative due to being lower than the atmospheric one throughout the process. Thus, a gaseous exchange involves atmospheric and pulmonary pressures and occurs in the alveoli.
The air exchanged during breathing is measured with a spirometer. Thibodeau and Patton (2019) indicate that 500 mL of air enters the lungs when an individual is at rest. As this amount enters and leaves them similarly to sea tides, it is referred to as the tidal volume. The lungs can stretch and return to their normal shape after each breath, which is called compliance.