Ventilation is the bulk movement of air between the atmosphere and the alveoli, and it is governed by the principle that airflow occurs only down a pressure gradient: gas moves from a region of higher pressure to one of lower pressure, with the direction and magnitude of flow set by the difference between alveolar pressure and pressure at the mouth (atmospheric). The pleural space maintains a persistently sub-atmospheric (negative) pleural pressure because the chest wall's outward elastic recoil and the lungs' inward elastic recoil oppose one another across the pleural membranes; active inspiratory muscle contraction (diaphragmatic descent) makes pleural pressure more negative, expands the lung, and drops alveolar pressure below atmospheric so air flows inward, while relaxation reverses the gradient and drives expiration. This is a core topic of respiratory physiology within human/mammalian systems physiology, and it supplies the mechanical foundation for the derived quantitative measures of ventilation (minute and alveolar ventilation), airway resistance, work of breathing, compliance, and surface tension, and for the classification of restrictive versus obstructive lung disease.
Ventilation is the bulk movement of air between the atmosphere and the alveoli, and it is governed by the principle that airflow occurs only down a pressure gradient: gas moves from a region of highe…