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Embryology · Respiratory system, diaphragm and body cavities

Surfactant and fetal lung maturity !

  • Topic

Surfactant is a film of fats and proteins that stops the air sacs collapsing when we breathe out. Without it, a preterm baby cannot breathe well.

Updated 2026-11-01

See it in 3D: Segmental bronchi

What it is

Surfactant is made by type II pneumocytes and stored in lamellar bodies; it is mostly phospholipid (dipalmitoylphosphatidylcholine) with the proteins SP-A, SP-B, SP-C and SP-D. It lowers surface tension in the alveolus, which would otherwise collapse at the end of each breath out. Its lack in preterm infants causes neonatal respiratory distress syndrome (hyaline membrane disease).

Key facts

  • Surfactant reaches the amniotic fluid with fetal lung fluid, which made it possible to measure lung maturity by amniocentesis.
  • Surfactant rises most in the last two weeks before birth.
  • SP-A and SP-D take part in the lung's immune defence.
  • Genetic defects of SFTPB, SFTPC or ABCA3 cause surfactant dysfunction even in term newborns.

Step by step

  1. Type II pneumocytes make surfactant phospholipids and proteins and store them in lamellar bodies.
  2. The lamellar bodies are secreted into the alveolus and spread as a film at the air–liquid interface.
  3. The film lowers surface tension, more so as the alveolus shrinks on breathing out.
  4. So the alveoli do not collapse at end-expiration and the next breath takes less effort.
  5. Without surfactant the alveoli collapse (atelectasis), oxygen falls and hyaline membranes form.

Markers

  • SFTPB
  • SFTPC
  • ABCA3
  • NKX2-1
  • Type II pneumocyte

Clinical notes

  • Respiratory distress syndrome: tachypnoea, grunting, nasal flaring, retractions and cyanosis in the first hours.
  • Chest X-ray: ground-glass pattern with air bronchograms and low-volume lungs.
  • Risk factors: prematurity, male sex, maternal diabetes and caesarean without labour.
  • Prevention and care: antenatal corticosteroids when preterm birth is expected; exogenous surfactant via the endotracheal tube and CPAP for the newborn.
  • Complications of oxygen and ventilation: bronchopulmonary dysplasia and retinopathy of prematurity.

Test yourself

Think of the answer, then tap the question to see it.

Which cell makes surfactant?

The type II pneumocyte, which stores it in lamellar bodies before secreting it into the alveolus.

Why is the infant of a diabetic mother at higher risk of respiratory distress syndrome?

Because fetal hyperinsulinaemia delays surfactant synthesis, even when the baby is large.

Which amniotic fluid tests indicate fetal lung maturity?

A lecithin/sphingomyelin ratio of 2 or more and the presence of phosphatidylglycerol.

What does the chest X-ray of a preterm infant with surfactant deficiency show?

A ground-glass pattern with air bronchograms and low-volume lungs.

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See it in 3D

The structures of Durgical's 3D anatomy where it is found.

← All of embryology

Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Sources: Newborn Breathing Conditions - Respiratory Distress Syndrome (RDS) | NHLBI, NIH (National Heart, Lung, and Blood Institute, 2022); Surfactant dysfunction: MedlinePlus Genetics (MedlinePlus, U.S. National Library of Medicine, 2017); Neonatal respiratory distress syndrome: MedlinePlus Medical Encyclopedia (MedlinePlus, U.S. National Library of Medicine, 2025); Langman's Medical Embryology, 13th ed., ch. 14: Respiratory System (Wolters Kluwer, 2015).

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