Question

a. What factors determine lung compliance?

b. Describe the role of pulmonary surfactants in stabilizing the alveoli to ensure that smaller ones do not collapse into larger ones.

c. At the end of normal respiration, a person has a lung volume of 3 L, alveolar pressure of 0 mmHg and trans-chest-wall pressure of -2 mmHg. At the end of inspiration, this person has inhaled 500 mL of fresh air into the alveoli, and the alveolar pressure is 0 mmHg while the trans-chest-wall pressure changes to - 4 mmHg. Calculate the lung compliance of this person, assuming an anatomical dead space of 150 mL (show the math)

Answer 1

A. Factors determining lung compliance:

i) Distensibility and elastic recoil:

• More distensibility and elasticity accounts for more compliance.
• A stiff lung has less compliance.

ii) Lung volume:

• Compliance is not equal at all lung volumes.
• Compliance is high at low lung volumes and is low at high lung volumes.

iii) Lung size:

• Compliance is proportionate lung size.
• Children lung is less compliant than that of adult lung.
• Compliance is less in advancing age due to increased stiff-ness of lung tissue.

​​​​​​​iv) Surface tension inside alveoli:

• It actually accounts for more than half of the lung compliance.

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​​​​​​​B. Role of pulmonary surfactant:

• Pulmonary surfactant is a thin film of alveolar lining that decreases surface tension at gas-liquid interface of alveoli and alters surface tension with change in alveolar diameter.
• It prevents lung collapse. During deflation of lung, compression of surfactant molecules decreases surface tension and during inflation of lung, new surfactant is extruded onto alveolar surface that forms a new film on alveolar epithelial lining.
• It promotes alveolar stability. By law of laplace, smaller alveoli could drain into larger ones. But that doesn't happen and alveoli co-exist which is maintained by surfactant. Surfactant actually achieves this by lower surface tension proportionately more in smaller alveoli.
• Helps to prevent edema in lung by reducing surface forces.
• Surfactant decreases work of breathing and makes it easier.
• IgA and apoproteins like SP-A and SP-D in surfactant provide innate immunity by acting as opsonins and promote phagocytosis of bacteria and viruses by alveolar macrophages.

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