Pulmonary Capillaries

1. Overview

Pulmonary capillaries are microscopic blood vessels that form a dense network around the alveoli in the lungs. They are the primary site of gas exchange, where oxygen from inhaled air enters the blood and carbon dioxide is removed. These capillaries represent the transition between the pulmonary arteries (carrying deoxygenated blood from the heart) and the pulmonary veins (carrying oxygenated blood back to the heart). Their thin walls and close proximity to alveolar surfaces enable rapid and efficient gas diffusion.

2. Location

Pulmonary capillaries are located within the interalveolar septa of the lungs. They:

• Surround each alveolus in a fine meshwork

• Are embedded in the connective tissue between alveolar epithelial cells

• Are part of the pulmonary circulation, which originates from the right ventricle and returns blood to the left atrium

Because alveoli are clustered throughout the lung parenchyma, pulmonary capillaries are widely distributed across both lungs.

3. Structure

Pulmonary capillaries are specialized for rapid diffusion of respiratory gases. Their structural features include:

• Endothelium: Lined by a single layer of thin, flattened endothelial cells with tight junctions and minimal basement membrane

• Capillary diameter: Approximately 5–10 μm—just wide enough to allow passage of red blood cells in single file

• Shared basement membrane: Fused with the alveolar epithelium in many places, forming the respiratory membrane

• Close proximity to alveoli: Gas travels only ~0.5 μm across the alveolar-capillary interface

This minimal barrier thickness maximizes diffusion efficiency and minimizes gas exchange time.

4. Function

The primary function of pulmonary capillaries is gas exchange:

• Oxygen uptake: O2 from alveolar air diffuses into capillary blood and binds to hemoglobin in red blood cells

• Carbon dioxide elimination: CO2 diffuses from blood into alveoli to be exhaled

Other functions include:

• Regulation of blood filtration and pressure: Pulmonary capillaries help modulate pressure between the pulmonary arteries and veins

• Barrier protection: Selectively allows solute and gas passage while excluding large or harmful substances

5. Physiological Role(s)

Pulmonary capillaries play vital roles in overall respiratory and cardiovascular homeostasis:

• Ventilation-perfusion matching (V/Q): Capillary perfusion is regulated to match alveolar ventilation for efficient gas exchange

• Blood filtration: Act as a filter for small emboli and clots before they reach systemic circulation

• Metabolic activity: Endothelial cells help metabolize vasoactive substances (e.g., converting angiotensin I to angiotensin II via ACE)

• Adaptation to exercise: Capillaries dilate and recruit reserve capacity to accommodate increased cardiac output

6. Clinical Significance

Pulmonary Edema

Occurs when fluid leaks from pulmonary capillaries into the alveolar spaces due to:

• Increased hydrostatic pressure (e.g., left heart failure)

• Increased capillary permeability (e.g., ARDS)

Leads to impaired gas exchange, dyspnea, and hypoxemia. Visible on chest X-ray as “bat-wing” opacities.

Acute Respiratory Distress Syndrome (ARDS)

A condition where diffuse alveolar damage leads to capillary leakage, severe inflammation, and impaired oxygenation. Common triggers include:

• Sepsis

• Trauma

• COVID-19

Treatment includes mechanical ventilation and supportive care.

Pulmonary Hypertension

Elevated pressure in pulmonary capillaries and arteries increases right ventricular strain and may cause capillary remodeling or rupture, reducing gas exchange efficiency and leading to right heart failure.

Pulmonary Embolism

A blood clot lodged in pulmonary arteries reduces downstream capillary perfusion, causing ventilation-perfusion mismatch and hypoxemia. Large emboli may lead to sudden cardiovascular collapse.

Emphysema

A subtype of COPD where alveolar walls and associated capillaries are destroyed, resulting in decreased surface area for gas exchange. Symptoms include chronic dyspnea and reduced oxygenation.

High-Altitude Pulmonary Edema (HAPE)

Occurs at high altitudes due to exaggerated pulmonary capillary pressure, causing fluid leakage into alveoli. Prevented with acclimatization or medication (e.g., nifedipine).