Lung mechanics

Question 1

Measuring compliance

Answer 1

• Distending pressure (transmural) is measuring difference between pressure inside (alveolar) and pressure outside (pleural)
• Positive distending pressure: leads to inflation, even at FRC lungs are inflated due to positive distending pressure
• Increasing lung distending pressure, increases lung volume so pressure volume cure is equal to the compliance curve
• Compliance is the slope at any point of the curve of the pressure-volume curve and so gradient is required
• Gradient = change in lung volume/change in distending pressure

Question 2

Factors effecting lung compliance

Answer 2

• ‘Stretchability’ of tissue (elastic connective tissue forces): thickening lung tissue decreases compliance and surface tension at air-water interfaces within alveoli
• Structure of lungs: alveoli exist as a ‘frothy foam’ where alveolar have interdependence, so changes in one alveolus has an impact on the adjacent alveolus, this can aid in stability
• Walls of alveoli are made of type I cells (flat, squished, thin cells) so small diffusion distance between gas in alveolar space and blood in capillary
• Surface tension generated by fluid lining alveoli generates a collapsing force that increases work of breathing
• In normal compliance curve, lungs are filled with air and so there are elements of elastic recoil and surface tension
• Remove surface tension will cause compliance curve to move left and become steeper so lungs become more compliant
• Surfactant molecules interpose themselves between water molecules and reduce surface tension (type II cells make surfactant)

Question 3

How does surfactant reduce surface tension

Answer 3

• Surfactant molecules have strong attraction for each other but low attraction for other molecules so accumulate at the air-liquid interface to reduce surface tension, compliance, and work of breathing
• Area dependent effect of surfactant: smaller radius alveoli have smaller surface area, greater density of surfactant so more effective at reducing surface tension in smaller alveoli and during deflation of lungs
• lowers surface tension of water layer at alveolar surface, increasing compliance, making lungs easier to expand
• keeps lungs ‘dry’ to prevent surface tension from sucking fluid out of capillaries into alveolus
• aids alveolar stability as lack of surfactant results in collapse of small alveoli (Laplace’s law)
• in infant respiratory distress syndrome there is: decreased compliance, high elastic recoil, alveolar collapse, and hypoxia/hypercapnia
• Laplace’s law: P = 2T/r
• Laplace’s law states: the pressure inside an inflated elastic contained with a curved surface is inversely proportional to the radius (as long as the surface tension is presumed to change little)

Question 4

System compliance

Answer 4

• At FRC chest distending pressure is negative (tries to pull chest in)
• Breathing around FRC: just as easy to distend the chest wall from FRC as it is to distend the lungs from FRC
• Chest wall compliance at FRC is equal and opposite to the lung compliance at FRC and so by adding the two together to the total system, then it will equal 0
• Slope of the total system is not as great as the individual components showing that total respiratory system is not as easy to distend as the individual components
• With a pneumothorax, distending pressure is lost as air enter the pleural cavity, so the lung collapses and the chest wall expands
• FRC is determined by the position of chest wall and lung compliance curves

Question 5

Explain regional ventilation

Answer 5

• Effect of gravity means pleural pressure varies top to bottom of lung
• Difference in pleural pressure explains region ventilation, greater flow to bottom of lung means there is greater ventilation
• In upright position there is a gradient pressure from the apex to the base of the lung but in resting state, pleural pressure is less than atmospheric pressure (less sub-atmospheric at base)
• Region difference are attributed to two factors:
  1. weight of lung itself (considered to be semi-fluid)
  2. differences between shape of lung tissue and surrounding pleural space
• Transpulmonary pressure at the base is less, lung tissue less is expanded than at apex, so less expanded basal lung tissue has a greater compliance and greater relative ventilation, when inspiration starts from FRC and when these measurements are made under static or quasi-static conditions
• Effects of gravity on distribution of blood flow in lung are attributed to hydrostatic pressure difference between top and bottom of pulmonary arterial system, at upper parts of lung, pressure within vessels is less than alveolar pressure (these vessels collapse, accounting for physiological dead space)
• gravitational middle zone, pulmonary arterial pressure is greater and pulmonary artery pressure exceeds the alveolar pressure
• In lower zone pulmonary venous pressure also exceeds alveolar pressure