The Mechanics of Breathing: How Inhalation and Exhalation Drive Gas Exchange

Breathing (pulmonary ventilation) moves air into and out of the alveoli for gas exchange. It relies on pressure gradients created by changing thoracic volume.

1. Inhalation (Active Process)

• Diaphragm contracts → flattens, descends ~1.5–2 cm
• External intercostals contract → ribs elevate (bucket-handle) and sternum lifts (pump-handle)
• Thoracic volume ↑ → intrapulmonary pressure ↓ (below atmospheric)
• Air flows in along the pressure gradient: nose/mouth → pharynx → larynx → trachea → bronchi → bronchioles → alveoli

2. Exhalation (Passive at Rest)

• Diaphragm relaxes → recoils upward (elastic recoil of lungs/chest wall)
• Internal intercostals (quiet breathing: not active; forced exhalation: depress ribs)
• Thoracic volume ↓ → intrapulmonary pressure ↑ (above atmospheric)
• Air flows out via the same path

Gas Exchange

At the alveoli: O₂ diffuses into pulmonary capillaries (high → low partial pressure); CO₂ diffuses out. Driven by partial pressure gradients (PₐO₂ ~100 mmHg, PₐCO₂ ~40 mmHg).

Regulation

Medullary respiratory centers (dorsal/ventral respiratory groups) set the basic rhythm. Pons (pneumotaxic, apneustic centers) modulates rate and depth. Chemoreceptors (central: medulla—CSF pH/CO₂; peripheral: carotid/aortic bodies—O₂, CO₂, pH) provide feedback. Higher centers (cortex) allow voluntary control.

This precise biomechanical and neural control ensures O₂ delivery and CO₂ removal match metabolic demand—from sleep to sprint.