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Alveolar Septum – Structure, Function and Diseases

The alveolar septum is the thin wall separating two adjacent air sacs (alveoli) in the lungs and plays a key role in the exchange of oxygen and carbon dioxide.

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Things worth knowing about "Alveolar Septum"

The alveolar septum is the thin wall separating two adjacent air sacs (alveoli) in the lungs and plays a key role in the exchange of oxygen and carbon dioxide.

Definition

The alveolar septum (plural: alveolar septa) is the thin partition separating two adjacent alveoli – the microscopic air sacs of the lungs. These walls form the structural basis of the lung parenchyma, meaning the functional lung tissue itself. Within the alveolar septum run fine blood capillaries, connective tissue fibers, and various cell types that together enable the vital gas exchange between inhaled air and the bloodstream.

Structure and Composition

The alveolar septum is composed of several layers and cell types:

Type I pneumocytes: Flat epithelial cells covering approximately 95% of the alveolar surface, primarily responsible for gas exchange.
Type II pneumocytes: Cuboidal cells that produce surfactant – a surface-active substance that keeps the alveoli open and prevents their collapse.
Capillary endothelium: The inner lining of the tiny blood vessels running through the septum, across which oxygen and carbon dioxide diffuse.
Interstitium: The connective tissue layer between the epithelial cells and the capillaries, composed of collagen and elastin fibers as well as interstitial cells.
Alveolar macrophages: Immune cells that defend against inhaled foreign particles and pathogens.

Function

The primary function of the alveolar septum is to facilitate pulmonary gas exchange. Oxygen from inhaled air diffuses through the septum into the blood capillaries, while carbon dioxide moves from the blood into the alveoli to be exhaled. The extremely small thickness of the alveolar wall – often less than 0.5 micrometers – ensures rapid and efficient diffusion of respiratory gases.

Clinical Significance

Diseases affecting the alveolar septum can significantly impair gas exchange. Key clinical conditions include:

Pulmonary emphysema: The alveolar septa are destroyed, causing adjacent alveoli to merge into enlarged air spaces, reducing the surface area available for gas exchange.
Interstitial lung diseases: Chronic inflammation or fibrosis within the alveolar septum leads to thickening of the wall and impairs gas diffusion.
Alveolitis: Inflammation of the alveolar walls, often triggered by allergens, infections, or autoimmune processes, damages the cells of the septum.
Pneumonia: During a lung infection, the septa may thicken and become infiltrated with fluid or inflammatory cells.
ARDS (Acute Respiratory Distress Syndrome): Severe damage to the alveolar-capillary membrane causes fluid accumulation in the septum and life-threatening failure of gas exchange.

Diagnosis

Changes to the alveolar septum can be detected using various diagnostic methods:

High-resolution computed tomography (HRCT): Allows visualization of septal thickening and interstitial changes.
Bronchoalveolar lavage (BAL): Analysis of cells and substances from the alveolar space.
Lung biopsy: Histological examination of tissue to precisely diagnose septal changes.
Pulmonary function tests (spirometry, diffusion capacity): Measurement of functional impairment caused by septal abnormalities.

References

Junqueira, L.C. & Carneiro, J. – Basic Histology: Text and Atlas, 11th Edition, McGraw-Hill, 2005.
West, J.B. – Respiratory Physiology: The Essentials, 10th Edition, Wolters Kluwer, 2016.
World Health Organization (WHO) – Chronic Respiratory Diseases, available at: https://www.who.int/health-topics/chronic-respiratory-diseases

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