Inspiration is an active, neurally driven process in which somatic motor output — via the phrenic nerve (C3–C5) to the diaphragm and the intercostal nerves (T1–T11) to the external intercostals, under both voluntary cortical control and involuntary drive from the medullary ventral respiratory group — contracts the primary inspiratory muscles and enlarges the thoracic cavity in the vertical, transverse ("bucket handle"), and anteroposterior ("pump handle") dimensions. By Boyle's law, this volume increase lowers intrapleural pressure (about −4 to −6 mmHg) and, through pleural coupling that transmits chest-wall expansion to the visceral pleura and alveoli, lowers intrapulmonary (intra-alveolar) pressure roughly 1–2 mmHg below the unchanged atmospheric pressure, so air flows down its pressure gradient until alveolar pressure re-equilibrates with atmospheric pressure at the peak of inspiration. The topic belongs to respiratory physiology within human physiology, providing the pressure-gradient framework — transpulmonary, transthoracic, and transrespiratory pressures — that also governs the reverse process of expiration.
Inspiration is an active, neurally driven process in which somatic motor output — via the phrenic nerve (C3–C5) to the diaphragm and the intercostal nerves (T1–T11) to the external intercostals, unde…