Lung Compliance (W1)

Question 1

LO’s

Answer 1

1. Describe why intrapleural pressure varies in different parts of the thorax and during the respiratory cycle
2. Describe how lung elastic recoil (lung compliance) changes in emphysema and alveolar fibrosis
3. Describe the importance of alveolar surface tension forces to lung recoil
4. Describe the composition and functions of surfactin (surfactant)
5. Describe how thoracic wall elastic recoil (thoracic wall compliance) changes in disease

Question 2

Why does inspiration require energy?

Answer 2

• Body has resistance to overcome, in order to respire.
• 80% is overcoming the elastic recoil of the lungs and thorax.
• Resistance to airflow in the airways is 15%.
• Resistance due to deformation of tissue in lung and thorax (tissue resistance) is 5%.

Question 3

Lung Elastic Recoil

Answer 3

• The only way the lungs stay in the chest wall is because the visceral and parietal pleura have fluid between them sticking the lungs to the chest wall. Like sticking two surfaces together with water.
• Overall, intrapleural pressure is negative (less than atmospheric pressure). It is -0.4kPa or -3mmHg.
• But, intrapleural pressure varies in different parts of the lungs.

Question 4

What is lung compliance?

Answer 4

• Measurement of change in volume (ventilation) / change in intrepleural presssure.
• When working out compliance choose two points above and below each axis. The work out the change for each axis, and put in equation.
• If a lung has low compliance, need lots of chnge in pressure to cause a small change in volume.
• Impossible to get every bit of air out of the lungs so lung volume never reaches zero.
• Lung volume levels off, as lungs have an elastic limit.
• When inspire, lung pressure decreases (as the lung volume has increased.) So the less pressure in the lungs, the more atmospheric air flows in.

Question 5

What is specific compliance?

Answer 5

• Compliance for both lungs/ Lung Volume (one lung only).

Question 6

What does a higher and lower value for compliance compared to a normal value, mean clinically?

Answer 6

• High value: higher compliance, lungs are flabby and easier to expand. May have emphysema, have higher compliance than normal, because the alveoli are destroyed, so need less pressure to inflate them.
• Low value: low compliance, lungs are rigid and harder to expand. For example due to alveolar fibrosis, harder to inflate fibrosed alveoli.

Question 7

What Is Responsible For Elastic Recoil?

Answer 7

• The alveoli have elastin and collagen fibres.
• There is a liquid film lining the alveoli, causing surface tension (liquid molecules being more attracted to each other than air).
• This compresses the alveolar air. This makes it more difficult to inflate the alveoli.
• Type 2 alveolar cells secrete surfactant. This is made of phospholipids, and a tiny bit of proteins.
• It dissolves into the film of water, and because it is a phospholipid, reduces the attractive forces between water molecules, and less surface tension and it is easier to ventilate.
• Don’t want hydrostatic pressure in pulmonary capillaries around the alveoli to force water out. Surfactant stops water leaking from p.capillaries.

Question 8

How does air in lungs cause a collapsed lung?

Answer 8

• The visceral pleura on the lung surface and parietal pleura on chest wall, are no longer stuck together by a layer of liquid due to air.
• Lung collapses.