Overview
This lecture explains the concepts of surface tension and surfactant in the alveoli, covering their effects on lung mechanics, their molecular basis, and clinical implications.
Surface Tension in the Alveoli
- Surface tension is caused by cohesive forces between water molecules lining the alveoli.
- Water molecules interact more strongly with each other than with air, creating tension at the air-water interface.
- Surface tension causes alveoli to recoil, shrink, and promotes their collapse.
- Collapsing pressure of alveoli is described by Laplace’s law: ΔP = 2T/r (T = surface tension, r = radius).
- Increased surface tension or decreased alveolar radius increases the collapsing pressure.
Alveolar Cell Types
- Type I alveolar cells: squamous epithelial cells primarily responsible for gas exchange.
- Type II alveolar cells: cuboidal epithelial cells that produce surfactant.
Effects of Surface Tension
- High surface tension can cause alveolar collapse (atelectasis), unequal ventilation, and pulmonary edema due to water being pulled into alveoli.
- Pores of Kohn connect adjacent alveoli, allowing air equilibration and preventing collapse.
- Ventilation-perfusion mismatch occurs if some alveoli are under- or over-ventilated, impacting gas exchange.
Surfactant: Structure, Production, and Function
- Surfactant is a lipid-protein complex (90% lipid, 10% protein), mainly produced by type II alveolar cells.
- Lipid component: dipalmitoylphosphatidylcholine (hydrophobic tails, hydrophilic head).
- Protein component: includes albumin, IgA, and surfactant proteins (SP-A, SP-B, SP-C, SP-D).
- Surfactant reduces surface tension by disrupting water molecule cohesion at the interface.
- Surfactant production begins at 24 weeks gestation, increases near 34–35 weeks, stimulated by fetal cortisol.
- Deficiency (e.g., in preemies) causes Infant Respiratory Distress Syndrome, requiring mechanical ventilation.
Surfactant Mechanisms and Clinical Importance
- Surfactant more densely coats smaller alveoli, reducing their tendency to collapse and evening out collapsing pressures.
- Without surfactant, small alveoli collapse, causing gas exchange issues and fluid accumulation.
- SP-A and SP-D aid in immune defense (opsonization); SP-B and SP-C help spread surfactant across alveolar surfaces.
Key Terms & Definitions
- Surface tension — Cohesive force at the air-water interface in alveoli, promoting their collapse.
- Alveoli — Tiny lung sacs where gas exchange occurs.
- Laplace’s Law — Formula (ΔP = 2T/r) linking pressure, surface tension, and radius in a sphere.
- Surfactant — Lipid-protein substance that reduces alveolar surface tension.
- Type I alveolar cells — Flat cells for gas exchange.
- Type II alveolar cells — Cuboidal cells that produce surfactant.
- Infant Respiratory Distress Syndrome — Condition caused by insufficient surfactant in premature infants.
Action Items / Next Steps
- Review Laplace’s law and its implications for alveolar stability.
- Study surfactant structure and function, including clinical consequences of deficiency.
- Prepare for discussion on ventilation-perfusion relationships and their physiological significance.