Histology · Respiratory system
The alveolus: type I and II pneumocytes, surfactant, the blood–air barrier !
- Organ
Alveoli are tiny air sacs with walls so thin that oxygen passes straight into the blood; a soap-like film keeps them from collapsing.
Updated 2026-11-01
What it is
The alveolus is where gas exchange happens. Its wall is mostly flat type I pneumocytes, a few cuboidal type II pneumocytes that make surfactant and repair the lining, and resident macrophages. Oxygen crosses a barrier made of the type I cell, a shared basement membrane and the capillary endothelium. Damage to these cells explains neonatal respiratory distress, ARDS and much of pulmonary pathology.
Structure
- Type I pneumocytes: very thin squamous cells covering the vast majority of the alveolar surface; they cannot divide.
- Type II pneumocytes: cuboidal cells with foamy cytoplasm, often at alveolar corners; under the electron microscope they contain lamellar bodies.
- Alveolar macrophages: in the alveolar space and septa; those loaded with hemosiderin in heart failure are called heart failure cells.
- Interalveolar septum: capillaries, elastic and reticular fibres, and small openings (pores of Kohn) between neighbouring alveoli.
Function
- Gas exchange by simple diffusion across a barrier well under a micrometre thick.
- Surfactant (mostly phospholipids, chiefly dipalmitoylphosphatidylcholine, plus surfactant proteins A, B, C and D) lowers surface tension so small alveoli do not collapse at the end of expiration.
- Type II cells act as the stem cells of the alveolar lining after injury.
Step by step
- Air reaches the alveolus through respiratory bronchioles and alveolar ducts.
- Oxygen dissolves in the thin fluid film and its surfactant layer.
- It crosses the type I pneumocyte.
- It crosses the fused basement membranes of epithelium and endothelium.
- It crosses the capillary endothelium into plasma and binds hemoglobin in the red cell.
Cells of the alveolar wall
| Cell | Look on H&E | Job | When it fails |
|---|---|---|---|
| Type I pneumocyte | Flat, thin nucleus barely visible | Forms the thin side of the blood–air barrier | Destroyed early in ARDS |
| Type II pneumocyte | Cuboidal, foamy, at corners | Makes surfactant; divides and replaces type I cells | Too few or immature in preterm newborns |
| Alveolar macrophage | Free in the alveolus, may hold dust or hemosiderin | Clears particles and microbes | Hosts Mycobacterium tuberculosis |
| Capillary endothelium | Thin cells of the septal capillaries | Blood side of the barrier | Leaks fluid in pulmonary edema |
Clinical notes
- Neonatal respiratory distress syndrome: preterm lungs lack surfactant; alveoli collapse and hyaline membranes line them.
- ARDS: diffuse alveolar damage with type I cell loss, protein-rich exudate forming hyaline membranes, then type II cell proliferation.
- Emphysema destroys alveolar walls and so shrinks the exchange surface.
Test yourself
Think of the answer, then tap the question to see it.
Which alveolar cell stores surfactant in lamellar bodies?
The type II pneumocyte, a cuboidal cell that also renews the alveolar lining after injury.
What three layers form the blood–air barrier?
The type I pneumocyte, the fused basement membranes, and the capillary endothelium.
Why do very preterm newborns develop respiratory distress?
Their type II cells have not yet made enough surfactant, so surface tension collapses the alveoli at the end of each breath.
What are heart failure cells?
Alveolar macrophages filled with hemosiderin from red cells that leaked into the alveoli because of pulmonary congestion.
Related
Related conditions
Microorganisms
Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.
Sources: 22.1 Organs and Structures of the Respiratory System - Anatomy and Physiology 2e | OpenStax (OpenStax (Rice University), 2022); On Top of the Alveolar Epithelium: Surfactant and the Glycocalyx (International Journal of Molecular Sciences (MDPI), via PubMed Central, 2020); Neonatal respiratory distress syndrome: MedlinePlus Medical Encyclopedia (MedlinePlus (U.S. National Library of Medicine), 2026); Bronchi, Bronchial Tree, & Lungs | SEER Training (National Cancer Institute (SEER Training), 2026).
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