Overview
This lecture explains how the lungs expand during inhalation, focusing on the roles of transmural pressure gradient, lung compliance, and pulmonary surfactant.
Lung Expansion Mechanics
- The lungs are only attached to the body at the root, so external forces must expand them.
- Lung tissue is elastic and tends to recoil, opposing expansion.
- Water molecules in alveoli also promote lung shrinkage by creating surface tension.
Transmural Pressure Gradient
- The transmural pressure gradient is the difference between intra-alveolar and intrapleural pressures (about -4 mm Hg).
- Intrapleural pressure is always lower than intra-alveolar pressure, creating a small vacuum.
- This negative pressure keeps the lungs expanded against the thoracic wall.
- If the pressure gradient is lost, such as in pneumothorax (chest puncture), the lungs collapse.
- A collapsed lung cannot be used for breathing until the pressure gradient is restored.
Lung Compliance
- Compliance measures how easily the lungs expand (C = ΔV / ΔP).
- Low compliance means less air can be inhaled with a normal pressure change; inspiratory muscles must work harder.
- Compliance mainly depends on the surface tension of the fluid in the alveoli.
Surface Tension and Surfactant
- Alveoli are lined with a thin water layer to keep cells moist and aid gas diffusion.
- Water creates surface tension, pulling alveolar walls inward and reducing compliance.
- Pulmonary surfactant, produced by alveolar type II cells, reduces surface tension and increases compliance.
- Without surfactant, alveoli would collapse and compliance would be very low.
Surfactant in Premature Infants
- Fetal surfactant production is low until about two weeks before birth.
- Premature infants may lack enough surfactant, leading to breathing difficulties.
- Artificial surfactant can be administered to help premature infants' lungs expand.
Key Terms & Definitions
- Transmural Pressure Gradient — difference between intra-alveolar and intrapleural pressure, keeping lungs expanded.
- Compliance — the ease with which lungs expand; calculated as change in volume divided by change in pressure.
- Surface Tension — force created by water molecules pulling together in the alveoli, opposing expansion.
- Pulmonary Surfactant — a substance made by alveolar type II cells that decreases surface tension and increases lung compliance.
- Pneumothorax — collapse of a lung caused by loss of negative intrapleural pressure.
Action Items / Next Steps
- Review the formula for lung compliance (C = ΔV / ΔP).
- Study the effects of surface tension and surfactant on alveolar stability.
- Understand the consequences and treatment of pneumothorax.