[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Embryology](https://durgical.com/en/embryology/) › [Respiratory system, diaphragm and body cavities](https://durgical.com/en/embryology/#a-respiratory) › Surfactant and fetal lung maturity [Embryology](https://durgical.com/en/embryology/) · [Respiratory system, diaphragm and body cavities](https://durgical.com/en/embryology/#a-respiratory) # 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](https://durgical.com/en/anatomy/segmental-bronchus/) ## Key points - Made by type II pneumocytes; stored in lamellar bodies. - Main component: dipalmitoylphosphatidylcholine (lecithin). - Production starts around weeks 20 to 24 and is enough by about weeks 34 to 35. - Glucocorticoids increase its synthesis; fetal hyperinsulinaemia (diabetic mother) delays it. - Lung maturity: lecithin/sphingomyelin ratio of 2 or more and phosphatidylglycerol in amniotic fluid. - Laplace's law: small alveoli collapse first when surfactant is lacking. ## 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 - Type II pneumocytes make surfactant phospholipids and proteins and store them in lamellar bodies. - The lamellar bodies are secreted into the alveolus and spread as a film at the air–liquid interface. - The film lowers surface tension, more so as the alveolus shrinks on breathing out. - So the alveoli do not collapse at end-expiration and the next breath takes less effort. - 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. ## Memory aid Surfactant is the alveolus's soap: without it, the smallest bubbles burst first. ## 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. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/embryology/surfactant-and-fetal-lung-maturity/) ## See it in 3D The structures of Durgical's 3D anatomy where it is found. - [Segmental bronchi](https://durgical.com/en/anatomy/segmental-bronchus/) ## Related - [Lung development: the five stages](https://durgical.com/en/embryology/lung-development-the-five-stages/) - [Fetal lung, kidney and brain maturation: what preterm birth interrupts](https://durgical.com/en/embryology/fetal-lung-kidney-and-brain-maturation-what-preterm-birth-interrupts/) - [Amniotic fluid: polyhydramnios and oligohydramnios](https://durgical.com/en/embryology/amniotic-fluid-polyhydramnios-and-oligohydramnios/) - [Maternal diabetes and obesity as teratogens](https://durgical.com/en/embryology/maternal-diabetes-and-obesity-as-teratogens/) - [Fetal period: month 3 to birth](https://durgical.com/en/embryology/fetal-period-month-3-to-birth/) ## Related conditions - [Neonatal respiratory distress syndrome (hyaline membrane disease)](https://durgical.com/en/conditions/neonatal-respiratory-distress-syndrome/) - [Preterm labour and birth](https://durgical.com/en/conditions/preterm-labour-and-birth/) - [Prematurity and its complications](https://durgical.com/en/conditions/prematurity-and-its-complications/) - [Gestational diabetes mellitus](https://durgical.com/en/conditions/gestational-diabetes-mellitus/) - [Atelectasis](https://durgical.com/en/conditions/atelectasis/) [← All of embryology](https://durgical.com/en/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](https://www.nhlbi.nih.gov/health/respiratory-distress-syndrome) (National Heart, Lung, and Blood Institute, 2022); [Surfactant dysfunction: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/surfactant-dysfunction) (MedlinePlus, U.S. National Library of Medicine, 2017); [Neonatal respiratory distress syndrome: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/001563.htm) (MedlinePlus, U.S. National Library of Medicine, 2025); Langman's Medical Embryology, 13th ed., ch. 14: Respiratory System (Wolters Kluwer, 2015). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/embryology/surfactant-and-fetal-lung-maturity/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/embryology/surfactant-and-fetal-lung-maturity/