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[Histology](https://durgical.com/en/histology/) · [Respiratory system](https://durgical.com/en/histology/#a-respiratory)

# 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

## Key points

- Type I = gas exchange, most of the surface, cannot divide.
- Type II = surfactant from lamellar bodies, and progenitor of type I.
- Blood–air barrier: type I cell + fused basement membranes + endothelium.
- Surfactant deficiency in preterm newborns causes respiratory distress syndrome with hyaline membranes.
- Hyaline membranes in an adult point to diffuse alveolar damage (ARDS).

## 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](https://durgical.com/en/conditions/acute-respiratory-distress-syndrome/) | 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](https://durgical.com/en/conditions/tuberculosis/) | 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.

## Memory aid

Type I is the thin window, type II is the window cleaner who also replaces broken panes.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/histology/the-alveolus-type-i-and-ii-pneumocytes-surfactant-the-blood-air-barrier/)

## Related

- [The airway: from nose to bronchioles](https://durgical.com/en/histology/the-airway-from-nose-to-bronchioles/)
- [Serous membranes: pleura and peritoneum](https://durgical.com/en/histology/serous-membranes-pleura-and-peritoneum/)

## Related conditions

- [Acute respiratory distress syndrome (ARDS)](https://durgical.com/en/conditions/acute-respiratory-distress-syndrome/)
- [Chronic obstructive pulmonary disease (COPD)](https://durgical.com/en/conditions/chronic-obstructive-pulmonary-disease/)
- [Tuberculosis](https://durgical.com/en/conditions/tuberculosis/)

## Microorganisms

- [Streptococcus pneumoniae](https://durgical.com/en/microbiology/streptococcus-pneumoniae/)
- [Mycobacterium tuberculosis](https://durgical.com/en/microbiology/mycobacterium-tuberculosis/)
- [Pneumocystis jirovecii](https://durgical.com/en/microbiology/pneumocystis-jirovecii/)

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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](https://openstax.org/books/anatomy-and-physiology-2e/pages/22-1-organs-and-structures-of-the-respiratory-system) (OpenStax (Rice University), 2022); [On Top of the Alveolar Epithelium: Surfactant and the Glycocalyx](https://pmc.ncbi.nlm.nih.gov/articles/PMC7246550/) (International Journal of Molecular Sciences (MDPI), via PubMed Central, 2020); [Neonatal respiratory distress syndrome: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/001563.htm) (MedlinePlus (U.S. National Library of Medicine), 2026); [Bronchi, Bronchial Tree, & Lungs | SEER Training](https://training.seer.cancer.gov/anatomy/respiratory/passages/bronchi.html) (National Cancer Institute (SEER Training), 2026).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/histology/the-alveolus-type-i-and-ii-pneumocytes-surfactant-the-blood-air-barrier/