Comprehensive Physiology Wiley Online Library

Micromechanics of the Lung

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Abstract

The sections in this article are:

1 Configuration of Terminal Air Spaces
1.1 Macroscopic Isotropy and Symmetry of Expansion
1.2 Alveolus–Alveolar Duct Interactions
1.3 Air‐Space Morphometry—Light Microscopy
1.4 Air‐Space Morphometry—Electron Microscopy
2 Mechanical Interactions of Tissue and Surface
2.1 Mechanical Models
2.2 Morphology in Presence and Absence of Surface Tension
2.3 Effects on Capillaries and Interstitial Fluid
3 Closure
3.1 Airways
3.2 Alveoli
3.3 Sequence of Closure
4 Opening
4.1 Airways
4.2 Alveoli
4.3 Mechanics of Opening
4.4 Septal Pleats
5 Gas Trapping
5.1 Foam
5.2 Mechanism
5.3 Trapping in Vivo
6 Instability
6.1 Negative Pressure‐ Volume Compliance
6.2 Configurational Stability
6.3 Surface Tension Elastance
6.4 Positive Tissue Elastance
6.5 Network Behavior (Interdependence)
Figure 1. Figure 1.

Possible acinar configuration changes and corresponding values of exponent n. A: isotropic expansion. B: alveolar septa change length by unfolding in an accordion‐like manner. Surface area is constant at all volumes. C: surface area is constant as shapes of alveoli change from “cup” to “saucer.” D: changes in lung volume result solely from recruitment or derecruitment of alveoli. Surface area and volume appear and disappear proportionately. E: alveoli bud from rigid alveolar duct and surface area increases disproportionately to volume.

Figure 2. Figure 2.

Schematic representation of the alveolar duct and its surrounding alveoli (top). The mechanical relationships of tissue (coiled lines) and surface tensions (wavy lines) are shown schematically (bottom). A, septa forming outer circumferential boundary; B, radially dispersed septa; C, alveolar entrance rings. Subscripts t and γ refer to tensions in the tissue elements and air‐liquid interfaces, respectively.

Figure 3. Figure 3.

Air‐liquid interfaces at sites of airway closure. A: peripheral airway (left) is closed by narrowing over a considerable length of airway. B: there is only a film across the lumen resulting in only minor narrowing. C: film has different surface tension on central and peripheral sides of an air bubble and is shown in straight airway (a) and at bifurcation (b).

Figure 4. Figure 4.

Theoretical pressure‐volume characteristics of alveolus showing region of negative compliance from B to C, causing an alveolus when inflated to critical opening pressure (PO) to jump from B to D and when deflated to critical closing pressure (PC) to jump from C to A.

Adapted from Mead
Figure 5. Figure 5.

Alveolar configuration during deflation from high (A) to middle (B) to low (C) volumes. With deflation septa shorten and, as tissue volume is conserved, they widen. Radius of curvature in the corners increases between high and middle volumes but decreases at low volumes, particularly in presence of some alveolar fluid.



Figure 1.

Possible acinar configuration changes and corresponding values of exponent n. A: isotropic expansion. B: alveolar septa change length by unfolding in an accordion‐like manner. Surface area is constant at all volumes. C: surface area is constant as shapes of alveoli change from “cup” to “saucer.” D: changes in lung volume result solely from recruitment or derecruitment of alveoli. Surface area and volume appear and disappear proportionately. E: alveoli bud from rigid alveolar duct and surface area increases disproportionately to volume.



Figure 2.

Schematic representation of the alveolar duct and its surrounding alveoli (top). The mechanical relationships of tissue (coiled lines) and surface tensions (wavy lines) are shown schematically (bottom). A, septa forming outer circumferential boundary; B, radially dispersed septa; C, alveolar entrance rings. Subscripts t and γ refer to tensions in the tissue elements and air‐liquid interfaces, respectively.



Figure 3.

Air‐liquid interfaces at sites of airway closure. A: peripheral airway (left) is closed by narrowing over a considerable length of airway. B: there is only a film across the lumen resulting in only minor narrowing. C: film has different surface tension on central and peripheral sides of an air bubble and is shown in straight airway (a) and at bifurcation (b).



Figure 4.

Theoretical pressure‐volume characteristics of alveolus showing region of negative compliance from B to C, causing an alveolus when inflated to critical opening pressure (PO) to jump from B to D and when deflated to critical closing pressure (PC) to jump from C to A.

Adapted from Mead


Figure 5.

Alveolar configuration during deflation from high (A) to middle (B) to low (C) volumes. With deflation septa shorten and, as tissue volume is conserved, they widen. Radius of curvature in the corners increases between high and middle volumes but decreases at low volumes, particularly in presence of some alveolar fluid.

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How to Cite

Ian A. Greaves, Jacob Hildebrandt, Frederic G. Hoppin. Micromechanics of the Lung. Compr Physiol 2011, Supplement 12: Handbook of Physiology, The Respiratory System, Mechanics of Breathing: 217-231. First published in print 1986. doi: 10.1002/cphy.cp030314