The Mechanics of Breathing

Created by Jiří Kofránek

Elastic forces and lung volumes

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Key points

Inward elastic recoil of the lung opposes outward elastic recoil of the chest wall, and the balance of these forces determines static lung volumes.
Surface tension within the alveoli contributes significantly to lung recoil and is reduced by the presence of surfactant, though the mechanism by which this occurs is poorly understood.
Compliance is defined as the change in lung volume per unit change in pressure gradient and may be measured for lung, thoracic cage or both.
Various static lung volumes may be measured, and the volumes obtained are affected by a variety of physiological and pathological factors.

The Nature of the Forces Causing Recoil of the Lung

For many years it was thought that the recoil of the lung was due entirely to stretching of the yellow elastin fibres present in the lung parenchyma. In 1929 von Neergaard (see section on Lung Mechanics in online Chapter 2: The History of Respiratory Physiology) showed that a lung completely filled with and immersed in water had an elastance that was much less than the normal value obtained when the lung was filled with air. He correctly concluded that much of the ‘elastic recoil’ was due to surface tension acting throughout the vast air/water interface lining the alveoli.

Lung compliance in water and in air

Poddajnosr

Laplace law: P=2T/R

Surface tension

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Surface tension measurements

Wilhelmy-Plate-Method

Effect of surfactant on surface tension

Surfactant and hysteresis

Alveolar surfactans

Static plot of lung volume against transmural pressure gradient (transoesophageal pressure relative to atmospheric at zero air flow).

Scarpelli’s foam model of alveolar structure. Surfactant (red) lines the alveoli and forms films that span both the alveolar openings and the alveolar ducts.

Laplace's law and surface tension

A: Pressure relations in two alveoli of different sizes but with the same surface tension of their fluids on their surfaces.

B: The changes in surface tension in realation of the area of the alveolar lining film.

C: Pressure relations in two alveoli of different sizes when allowance is made for the probable changes in surface tension.

Hysteresis

Static plot of lung volume against transmural pressure gradient (transoesophageal pressure relative to atmospheric at zero air flow).

Transmural pressure gradient and intrathoracic pressure

Relationship between lung volume and the difference in pressure between the alveoli and the intrathoracic space (transmural pressure gradient). The relationship is almost linear over the normal tidal volume range. The calibre of small air passages decreases in parallel with alveolar volume. Airways begin to close at the closing capacity, and there is widespread airway closure at residual volume. Values in the diagram relate to the upright position and to decreasing pressure. The opening pressure of a closed alveolus is not shown.

Intrathoracic pressures: static relationships in the resting end-expiratory position

An elastic balloon inside a cylinder with a piston

Static properties of the lungs

Respiratory muscles during inspiration and expiration

Inspiration and expiration muscles

Pressures in the lungs and chest

P_A - alveolární tlak, P_B barometrický tlak, P_pl intrapleurální tlak

Intrapleural pressures at the end of a quiet expiration (at FRC)

Spirometer

Static lung volumes

Effect of Posture on Some Aspects of Respiratory Function

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The effect of position and obesity on static lung volumes.

Functional residual capacity in various body positions. BTPS, Body temperature and pressure, saturated.

Changes in static lung volumes with age

Static Pressure-Volume curves for lungs and chest

Static and dynamic compliance

Schematic diagrams of alveoli to illustrate conditions under which static and dynamic compliance may differ. (A) Represents a theoretically ideal state in which there is a reciprocal relationship between resistance and compliance resulting in gas flow being preferentially delivered to the most compliant regions, regardless of the state of inflation. Static and dynamic compliance are equal. This situation is probably never realized even in the normal subject. (B) Illustrates a state that is typical of many patients with respiratory disease. The alveoli can conveniently be divided into fast and slow groups. The direct relationship between compliance and resistance results in inspired gas being preferentially delivered to the stiff alveoli if the rate of inflation is rapid. An end-inspiratory pause then permits redistribution from the fast alveoli to the slow alveoli.ddajnost