Surfactant function and synthesis

Surfactant Basics - The Tension Tamer

• Function: A complex of lipids & proteins that ↓ surface tension in alveoli, preventing collapse (atelectasis) and ↑ pulmonary compliance.
• Mechanism: Disrupts the hydrogen bonds between water molecules. According to the Law of Laplace ($P = 2T/r$), by decreasing tension (T), surfactant lowers the pressure (P) needed to keep small alveoli open.
• Synthesis:
  • Produced by Type II pneumocytes and stored in lamellar bodies.
  • Key component: Dipalmitoylphosphatidylcholine (DPPC).
  • Begins at ~24-28 weeks gestation; mature levels by ~35 weeks.

⭐ Fetal lung maturity is assessed via the Lecithin-to-Sphingomyelin (L/S) ratio in amniotic fluid; a ratio >2:1 indicates maturity. Corticosteroids can accelerate production.

Synthesis & Secretion - The Type II Hustle

• Producers: Synthesized by Type II pneumocytes.
• Composition: Primarily phospholipids, with Dipalmitoylphosphatidylcholine (DPPC) being the most crucial component. Also contains surfactant proteins (SP-A, B, C, D).
• Synthesis & Storage:
  • Precursors (choline, fatty acids, glycerol) are assembled in the ER.
  • Packaged into lamellar bodies (storage granules), which appear foamy on microscopy.
• Secretion & Formation:
  • Lamellar bodies are released into the alveolar space via exocytosis.
  • They unravel to form tubular myelin, which then creates the final surfactant film at the air-liquid interface.
• Regulation:
  • Stimulated by: Corticosteroids, Thyroxine, β-adrenergic agonists.
  • Inhibited by: Insulin.

⭐ Fetal lung maturity is assessed by the Lecithin-to-Sphingomyelin (L/S) ratio in amniotic fluid. A ratio > 2:1 indicates maturity. Corticosteroids (e.g., Betamethasone) are administered to mothers at risk of premature delivery to accelerate fetal surfactant production.

Function & Physics - Bubble Trouble Buster

• Reduces Surface Tension: A lipoprotein complex that disrupts the cohesive forces between water molecules lining alveoli.
• Prevents Atelectasis (Collapse): By lowering surface tension, it equalizes pressure between small and large alveoli, preventing collapse during expiration. Follows the Law of Laplace: $P = 2T/r$. Surfactant ↓ T (tension), stabilizing P.
• Increases Compliance: Lungs inflate more easily.
• Keeps Alveoli Dry: Opposes fluid transudation from capillaries.

• Synthesis & Composition
  • Produced by Type II pneumocytes; stored in lamellar bodies.
  • Stimulated by fetal cortisol & thyroxine.

⭐ High-Yield: The primary component is dipalmitoylphosphatidylcholine (DPPC). Production begins ~20 weeks gestation but is only sufficient after ~35 weeks, a critical factor in Neonatal Respiratory Distress Syndrome (NRDS).

Clinical Correlates - Code Blue Babies

• Neonatal Respiratory Distress Syndrome (NRDS): Surfactant deficiency in premature infants (born < 35 weeks gestation) due to immature Type II pneumocytes.
• Pathophysiology: ↓ Surfactant → ↑ alveolar surface tension → widespread atelectasis (alveolar collapse) → ↓ lung compliance & functional residual capacity (FRC) → V/Q mismatch & severe hypoxemia.
• Clinical Findings: Presents within minutes to hours of birth with tachypnea, expiratory grunting, and nasal flaring.
• Management:
  • Antenatal: Corticosteroids (e.g., betamethasone) to accelerate fetal lung maturity.
  • Postnatal: Exogenous surfactant administered via endotracheal tube.

⭐ The lecithin-to-sphingomyelin (L/S) ratio in amniotic fluid assesses fetal lung maturity. An L/S ratio < 2.0 is predictive of NRDS.

High‑Yield Points - ⚡ Biggest Takeaways

• Surfactant, primarily dipalmitoylphosphatidylcholine (DPPC), is synthesized by Type II pneumocytes.
• It reduces alveolar surface tension, which prevents atelectasis (collapse) and increases lung compliance.
• Per LaPlace's Law (P=2T/r), its effect is greater in smaller alveoli, preventing their collapse.
• Synthesis is stimulated by cortisol and thyroxine; steroids are often administered before premature delivery.
• Deficiency is the cause of Neonatal Respiratory Distress Syndrome (NRDS).