Pulmonary Surfactant: Physiology and What Research Reports

What pulmonary surfactant is

Pulmonary surfactant is a complex mixture of lipids and specialised proteins that coats the inner surface of the alveoli — the tiny air sacs where gas exchange happens. Its defining physical job is to lower surface tension at the air–liquid interface. Without it, the water lining each alveolus would pull the walls together, and the smallest air spaces would collapse at the end of every breath.

The mixture is predominantly lipid, with phosphatidylcholine species making up the bulk of the surface-active material, alongside a smaller protein fraction that includes the hydrophobic surfactant proteins B and C and the hydrophilic collectins surfactant protein A and D. Reviews of surfactant biology in airway disease have described this lipid–protein architecture and the way its composition shifts in inflamed lungs (review of pulmonary surfactant in asthma).

Where it is produced

Surfactant is synthesised by alveolar type II epithelial cells, packaged into lamellar bodies, secreted into the alveolar lining fluid, and then recycled or cleared largely by those same cells and by alveolar macrophages. Because production depends on an intact biosynthetic pathway inside type II cells, injuries that hit that pathway change the surfactant pool. A 2024 hazardous-materials study reported that polyhexamethylene guanidine disinfectant exposure blocked pulmonary surfactant biogenesis and that this blockage mediated the development of pulmonary fibrosis in the models used (Kim and colleagues' surfactant biogenesis blockage study).

What it does in the body

Two broad functions appear repeatedly in the literature: biophysics and host defence.

• Surface tension and lung mechanics. The film compresses and expands with each breath, keeping alveoli stable across a range of volumes. Researchers examining surfactant behaviour under liquid ventilation conditions reported that the biophysical function of the film could be assessed in a liquid-filled environment rather than only at an air–liquid interface (biophysical function in liquid ventilation).
• Immune modulation. Surfactant is not inert. One 2020 study reported that pulmonary surfactant phosphatidylcholines induced an immunological adaptation of alveolar macrophages, shaping how those cells responded to subsequent stimuli (surfactant phosphatidylcholines and alveolar macrophages).
• Interaction with microbes. The relationship runs both ways. A 2018 infection-and-immunity study reported that pulmonary surfactant promoted virulence gene expression and biofilm formation in Klebsiella pneumoniae, indicating that the same lining fluid that supports host defence can also be sensed by pathogens (surfactant and Klebsiella pneumoniae).

How pulmonary surfactant is measured and studied

Because the film sits deep in the lung, most measurement is indirect. The methods that appear across the cited literature fall into a few categories.

What the literature reports in disease states

Acute respiratory distress and viral infection

A 2022 study in Scientific Reports reported reduced levels of pulmonary surfactant in patients with COVID-19 acute respiratory distress syndrome, linking a clinical syndrome of stiff, poorly recruitable lungs to a measurable surfactant deficit (reduced surfactant in COVID-19 ARDS). Earlier in the pandemic, a hypothesis paper argued that pulmonary surfactant itself should be considered a strong defender against SARS-CoV-2 at the alveolar interface, and set out the reasoning for testing that idea (surfactant as a defender against SARS-CoV-2). The two papers illustrate a common pattern in this field: a mechanistic proposal followed by measurement in patients.

Fibrosis

Surfactant lipids have been studied as modifiers rather than bystanders in fibrotic lung disease. A 2025 study reported that pulmonary surfactant-associated phosphatidylethanolamine modulated pulmonary fibrosis in both in vitro and in vivo models (phosphatidylethanolamine and pulmonary fibrosis), while the disinfectant study described above reported fibrosis arising downstream of blocked surfactant biogenesis (2024).

Airway disease and sleep-disordered breathing

A review of surfactant in asthma summarised how surfactant composition and function relate to airway obstruction and inflammation in asthmatic lungs (pulmonary surfactant in asthma). In obstructive sleep apnoea, researchers examined surfactant-associated proteins together with inflammatory factors and reported associations between these markers and the disorder (surfactant proteins and inflammatory factors in OSA).

Why surfactant matters to peptide research

Surfactant is relevant to peptide science in two ways. First, two of its four named proteins — SP-B and SP-C — are small hydrophobic peptides whose sequences have inspired synthetic analogues used in interfacial research. Second, and more prominently in recent work, the surfactant layer is being explored as a route and a vehicle for delivering molecules to the lung.

Researchers investigating surfactant-assisted delivery reported that surface-associated structures beneath the interfacial film influenced how well material was carried and released, arguing that delivery models should look "beyond the interface" (surface-associated structures in surfactant-assisted delivery). An earlier controlled-release study characterised vehiculization, release and targeting of surfactant/tacrolimus formulations, using surfactant as the carrier matrix for an immunosuppressant (surfactant/tacrolimus formulations). On the vaccine side, a 2025 report described inhalable vaccines that hitchhiked on pulmonary surfactant protein and reported augmented mucosal and systemic antiviral immunity in the models tested (surfactant protein-hitchhiking inhalable vaccines). None of this work establishes a human protocol; it maps what the alveolar lining can do as a biological interface.

Signals of harm and complexity: What Studies Report

The cited literature does not describe surfactant as uniformly protective. The Klebsiella work reported that surfactant exposure increased virulence gene expression and biofilm formation in that bacterium (2018), and the disinfectant study reported that interference with surfactant biogenesis contributed to fibrosis (2024). Studies of macrophage adaptation to surfactant phosphatidylcholines reported altered immune responsiveness rather than simple suppression or activation (2020). Readers encountering surfactant in a delivery or vaccine context should note that these papers were mechanistic and preclinical or observational, and that none of the verified reports defined tolerability in healthy humans.

This page is for educational purposes only and is not medical advice; consult a licensed physician about any respiratory symptom, diagnosis or treatment decision. The summaries above describe what the study authors reported in the cited publications and nothing more.

References

• Pulmonary Surfactant Protein-Hitchhiking Inhalable Vaccines Augment Mucosal and Systemic Antiviral Immunity (ACS Nano, 2025)
• Pulmonary surfactant itself must be a strong defender against SARS-CoV-2 (Medical Hypotheses, 2020)
• Modulation of Pulmonary Fibrosis by Pulmonary Surfactant-Associated Phosphatidylethanolamine In Vitro and In Vivo (International Journal of Molecular Sciences, 2025)
• Pulmonary surfactant biogenesis blockage mediated polyhexamethylene guanidine disinfectant induced pulmonary fibrosis (Journal of Hazardous Materials, 2024)
• Pulmonary surfactant-associated proteins and inflammatory factors in obstructive sleep apnea (Sleep & Breathing, 2018)
• Pulmonary surfactant phosphatidylcholines induce immunological adaptation of alveolar macrophages (Molecular Immunology, 2020)
• Reduced levels of pulmonary surfactant in COVID-19 ARDS (Scientific Reports, 2022)
• Pulmonary surfactant in asthma (European Journal of Pharmacology, 2025)
• Biophysical function of pulmonary surfactant in liquid ventilation (Biophysical Journal, 2023)
• Pulmonary surfactant and drug delivery: Vehiculization, release and targeting of surfactant/tacrolimus formulations (Journal of Controlled Release, 2021)
• Pulmonary Surfactant Promotes Virulence Gene Expression and Biofilm Formation in Klebsiella pneumoniae (Infection and Immunity, 2018)
• Beyond the Interface: Improved Pulmonary Surfactant-Assisted Drug Delivery through Surface-Associated Structures (Pharmaceutics, 2023)