Fragility of pulmonary capillaries

Area of Science:

• Pulmonary physiology
• Vascular biology
• Microcirculation

Background:

• Pulmonary capillaries, discovered in 1661, have an ultrastructure revealed 60 years ago.
• Their wall, a mere 0.2 μm thick, separates endothelium from alveolar space, suggesting inherent fragility.
• This thin barrier is unique among systemic capillaries, yet its fragility was unrecognized for decades.

Purpose of the Study:

• To elucidate the unrecognized fragility of pulmonary capillaries.
• To explain the historical underappreciation of pulmonary capillary fragility.
• To present evidence for pulmonary capillary fragility across different physiological and pathological conditions.

Main Methods:

• Review of historical physiological data and ultrastructural findings.
• Analysis of evidence from physiological, pathological, and laboratory observations.
• Consideration of evolutionary adaptations and disease-induced changes.

Main Results:

• Pulmonary capillary fragility is unique and was historically underestimated due to comparisons with systemic capillaries and misconceptions about pulmonary pressure.
• Evidence from elite athletes and animals with extreme aerobic capacity demonstrates fragility under exceptional physiological stress.
• Lung and heart diseases frequently lead to pulmonary capillary disruption, and remodeling occurs with sustained elevated pressure.

Conclusions:

• Pulmonary capillaries exhibit unique fragility, adapting their structure to function, particularly under high-demand or pathological conditions.
• Neonatal capillaries are particularly fragile, likely due to lack of prior exposure to increased pressures.
• The study underscores the importance of understanding pulmonary capillary structure-function relationships in health and disease.