Simulate breathing mechanics and gas exchange in the alveoli.
During inhalation, the diaphragm contracts and lowers, expanding the thoracic cavity and drawing air in. Exhalation is passive — the diaphragm relaxes. In the alveoli, O₂ diffuses into blood capillaries while CO₂ diffuses out (partial pressure gradients). Higher activity increases both breathing rate and tidal volume to meet oxygen demand.