Pulmonary ventilation is governed by Boyle's law: because pressure and volume are inversely proportional, changes in thoracic cavity volume produced by respiratory muscles and by the elastic recoil of the lungs generate pressure gradients between the intrapulmonary (alveolar) compartment and the atmosphere, and airflow follows those gradients according to flow = ΔP / resistance. Inspiration is always active — skeletal muscle contraction (diaphragm and external intercostals in quiet breathing, plus accessory muscles in forced breathing) expands the thorax, lowering intrapleural and intrapulmonary pressures below atmospheric; quiet expiration is passive, driven solely by lung elasticity and chest-wall recoil, while forced expiration recruits the internal intercostals and abdominal wall muscles. The topic belongs to human respiratory physiology within anatomy and physiology, sitting at the interface of gas-law physics, musculoskeletal mechanics, and medullary neural control of breathing.
Pulmonary ventilation is governed by Boyle's law: because pressure and volume are inversely proportional, changes in thoracic cavity volume produced by respiratory muscles and by the elastic recoil o…