Medicine & USMLE


  1. Respiratory Overview
  2. Airway Anatomy
  3. Alveoli
  4. Breathing Mechanics
  5. Gas Exchange in Tissues
  6. Nervous Control of Breathing
  7. Respiratory Defense Systems
  • Alveoli
    • Function: gas exchange
      • Site of oxygen and carbon dioxide exchange between air and bloodstream 
      • Alveoli have high pO2 and low pCO2
        • Gases diffuse down their concentration gradients
          • High concentration → low concentration
        • O2 diffuses from lungs to pulmonary capillaries
        • CO2 diffuses from pulmonary capillaries to lungs
        • Hyperventilation
          • Equilibrium shifts towards CO2 (less CO2), less bicarbonate ions
        • Hypoventilation
          • Equilibrium shifts towards bicarbonate ions (more CO2)
    • Structure
      • Alveoli are tiny sacs of elastic lung tissue, surrounded by capillaries
      • Alveoli
        • Walls are one cell thick to allow for rapid gas exchange
        • Large surface area to volume ratio to maximize diffusion for small amount of gas
        • Surfactant coats alveolar surface to reduce surface tension
          • Prevent alveoli from collapsing
          • Composed of amphipathic phospholipids 
      • Capillaries
        • Extremely narrow that RBCs must pass through single file = maximizes surface area for gas exchange
        • Large total surface area to maximize gas exchange