Thin and Strong! The Bioengineering Dilemma in the Structural and Functional Design of the Blood-Gas Barrier

Abstract

In gas exchangers, the tissue barrier, the partition that separates the respiratory media (water/air and hemolymph/blood), is exceptional for its remarkable thinness, striking strength, and vast surface area. These properties formed to meet conflicting roles: thinness was essential for efficient flux of oxygen by passive diffusion, and strength was crucial for maintaining structural integrity. What we have designated as "three-ply" or "laminated tripartite" architecture of the barrier appeared very early in the evolution of the vertebrate gas exchanger. The design is conspicuous in the water-blood barrier of the fish gills through the lungs of air-breathing vertebrates, where the plan first appeared in lungfishes (Dipnoi) some 400 million years ago. The similarity of the structural design of the barrier in respiratory organs of animals that remarkably differ phylogenetically, behaviorally, and ecologically shows that the construction has been highly conserved both vertically and horizontally, i.e., along and across the evolutionary continuum. It is conceivable that the blueprint may have been the only practical construction that could simultaneously grant satisfactory strength and promote gas exchange. In view of the very narrow allometric range of the thickness of the blood-gas barrier in the lungs of different-sized vertebrate groups, the measurement has seemingly been optimized. There is convincing, though indirect, evidence that the extracellular matrix and particularly the type IV collagen in the lamina densa of the basement membrane is the main stress-bearing component of the blood-gas barrier. Under extreme conditions of operation and in some disease states, the barrier fails with serious consequences. The lamina densa which in many parts of the blood-gas barrier is <50 nm thin is a lifeline in the true sense of the word.

Role of the GapA and CrmA Cytadhesins of Mycoplasma gallisepticum in Promoting Virulence and Host Colonization

“…In consideration of the cell invasion capabilities of M. gallisepticum (7,10), it is tempting to speculate that GapA and CrmA enable the mycoplasma to efficiently colonize the avian air sacs and the adjacent parabronchial system. Consisting of intermingling air and blood capillaries which are separated by extremely thin membranes ranging around 100 nm in thickness (32), this might be a suitable site for entering the avian bloodstream. We will address this topic in future experiments.…”

Biofluid Flow and Heat Transfer

“…An average partial pressure difference of 57 Pa supports the transfer of oxygen across the air-blood barrier at a rate of 2.3 × 10 −5 cm 2 /s that is completed in 250-500 ms [84].…”

“…However, the air compressibility (m 2 /N) is much smaller (4 order of magnitude) than the bronchus distensibility (m 2 /N). Therefore, the contribution to the propagation speed of tiny pressure waves that results from air compressibility can 35 In many parts of the air-blood barrier, the thickness of the lamina densa, a component of the basement membrane, located in the center of the intertitium is smaller than 50 nm [84]. Forced expiration is an usual test carried out to quantify volumes and flow rates in humans.…”

Membrane mediated development of the vertebrate blood‐gas‐barrier

“…The basic structure of the BGB has been well conserved through evolution, and comprises an epithelium, an interstitium, and an endothelium (Maina and West, 2005).…”

“…While controlling molecules may be similar to those in mammals and birds at the inaugural stages of lung development, remarkable differences would be expected in later stages of lung maturation. Indeed many of the controlling factors have been highly conserved through evolution (Maina and West, 2005;Warburton et al, 2010).…”