MDCAT Gaseous Exchange & Respiration 2026

Respiratory System

• Path: nasal → pharynx → larynx → trachea → bronchi → bronchioles → alveoli
• Alveoli — 300 million; thin epithelium for gas exchange; surface area ~70 m²
• Inspiration: diaphragm contracts, downward; ribs up and out; intrathoracic pressure drops
• Expiration: passive (relaxation); active during exercise

Gas Transport

• O₂: 97% on hemoglobin (oxyhemoglobin); 3% dissolved in plasma
• CO₂: ~70% as bicarbonate; ~23% as carbaminohemoglobin; ~7% dissolved
• Bohr effect: low pH / high CO₂ shifts O₂ dissociation curve right (releases O₂)

Pair with Circulation.