Microanatomy of the Lung: Alveoli, Surfactant, and the Respiratory Membrane

Microanatomy of the Lung: Alveoli, Surfactant, and the Respiratory Membrane

The Alveolus: Structure and Cell Types

The alveolus is the fundamental gas-exchange unit of the lung. An adult lung contains hundreds of millions of alveoli, providing an enormous surface area for oxygen and carbon dioxide exchange. Understanding what lines each alveolus — and why — is essential knowledge for every respiratory therapist.

The alveolar wall is composed of two primary epithelial cell types, plus immune cells that patrol the airspace.

Type I Pneumocytes (Type I alveolar cells) cover approximately 95% of the alveolar surface area. They are extremely thin, flat cells that permit rapid diffusion of gases across the respiratory membrane. Because they are so thin, they are vulnerable to injury from high oxygen concentrations, barotrauma, and toxic inhalants. They cannot replicate themselves.

Type II Pneumocytes (Type II alveolar cells) cover only about 5% of the alveolar surface but are critically important. They are cuboidal cells with a granular appearance under the microscope due to lamellar bodies — the organelles that synthesize and store pulmonary surfactant. Type II cells are also the progenitor cells of the alveolar epithelium: when Type I cells are damaged, Type II cells divide and differentiate to repair the surface.

Alveolar Macrophages reside in the alveolar airspace and on the epithelial surface. They are the primary defense against inhaled particles, bacteria, and other foreign material. They phagocytose debris and signal the immune system when pathogens are detected. Respiratory therapists must appreciate that cigarette smoke and chronic irritants impair macrophage function, reducing the lung's first line of defense.

The Respiratory Membrane

Gas exchange does not happen in the alveolus alone — it happens across the respiratory membrane, which separates alveolar air from capillary blood. From alveolus to blood, the layers are:

1. Surfactant layer (liquid film)

2. Type I pneumocyte cytoplasm

3. Alveolar basement membrane

4. Thin interstitial space (may be fused with the capillary basement membrane)

5. Capillary basement membrane

6. Capillary endothelial cell

The total thickness of this membrane is extremely small — on the order of less than a micrometer in the thinnest regions — which allows oxygen and carbon dioxide to diffuse rapidly by Fick's law. Any condition that thickens this membrane (pulmonary edema, fibrosis, pneumonia) impairs diffusion and can cause hypoxemia.

Pulmonary Surfactant

Surfactant is a lipoprotein mixture produced by Type II pneumocytes. Its main lipid component is dipalmitoylphosphatidylcholine (DPPC). Surfactant coats the alveolar surface and dramatically reduces surface tension at the air-liquid interface.

Without surfactant, the surface tension of the liquid lining the alveolus would cause the alveolus to collapse inward (atelectasis) during exhalation. Surfactant lowers surface tension as alveolar radius decreases during exhalation, which is precisely when surface tension forces are greatest. This property keeps small alveoli from collapsing into large ones.

Surfactant production in the fetus matures during the third trimester, peaking around 35 weeks gestation. Infants born before adequate surfactant is present develop Infant Respiratory Distress Syndrome (IRDS), characterized by widespread alveolar collapse, stiff lungs, and severe hypoxemia. Treatment includes exogenous surfactant replacement therapy and respiratory support — a cornerstone intervention that respiratory therapists administer and monitor.

Safety note: Surfactant replacement is a high-alert medication procedure. Proper patient positioning, airway management, and monitoring for adverse effects (airway obstruction, desaturation, bradycardia) are critical responsibilities of the respiratory therapist.

Laplace's Law and Alveolar Stability

Laplace's law describes the relationship between the pressure inside a sphere, its radius, and the surface tension of its wall. For a sphere with a single surface (such as an alveolus):

P = 2T / r

Where:

• P = distending pressure (cmH₂O or other pressure units)
• T = surface tension (dynes/cm)
• r = radius of the alveolus

This equation reveals a fundamental problem: smaller alveoli (lower r) generate higher inward pressure, which would cause them to empty into larger alveoli — a phenomenon called alveolar instability. Surfactant solves this by reducing T proportionally more in smaller alveoli, equalizing pressure across alveoli of different sizes.

Compliance and Its Relationship to Surface Tension

Compliance (C) is the change in volume per unit change in pressure: C = ΔV / ΔP. A lung with high surface tension has low compliance — it is stiff and requires more work to inflate. When surfactant is absent or depleted, surface tension rises, compliance falls, and the patient must exert far greater effort to breathe. This is why infants with IRDS tire rapidly and require ventilatory support. Respiratory therapists use compliance monitoring as a key parameter when managing mechanically ventilated patients.

Clinical Relevance for Respiratory Therapists

Understanding alveolar microanatomy directly informs clinical practice. Recognizing that Type II cells produce surfactant explains why prematurity, acute lung injury, and near-drowning can all compromise surfactant function. Knowing the layers of the respiratory membrane helps you anticipate why pulmonary edema worsens oxygenation. Applying Laplace's law explains why positive end-expiratory pressure (PEEP) recruits collapsed alveoli by increasing radius and reducing the pressure needed to keep them open. These are not abstract concepts — they underlie every ventilator setting and airway intervention you will perform.