Lung Mechanics

Question 1

What is cohesion? What are cohesive forces responsible for?

Answer 1

• Attraction of molecules for other molecules of the same kind.
• Surface tension.

Question 2

Why do water molecules have strong cohesive forces?

Answer 2

Ability to form hydrogen bonds (attraction between more electronegative oxygen atom and more positive hydrogen atoms) with one another.

Question 3

What is surface tension and why does this occur?

Answer 3

• Tendency of a liquid’s surface to resist rupture when placed under tension/stress.
• Water molecules at the surface (air-water interface) will form bonds with their neighbour molecules. But as they are exposed to air on one side, they will have fewer neighbouring water molecules to bond with - will form stronger bonds with the neighbours they do have.

Question 4

What are lungs surrounded by?

Answer 4

Pleura: continuous membrane that folds over itself.

• parietal pleura (lines the chest wall)
• visceral pleura (covers the lung)

Question 5

What are the parietal and visceral pleurae separated by?

Answer 5

Lubricating pleural fluid (approx. 25ml/lung) in the pleural space.

• Reduces friction - allows pleura to slide easily during breathing.
• Provides surface tension - prevents lung recoiling and collapsing (as lung contains much elastin)

Question 6

What keeps the 2 pleurae together and prevents lungs collapsing?

Answer 6

Vacuum (negative pressure) in the pleural space - intrapleural pressure.

Question 7

What is the intrapleural pressure during inspiration and exhalation?

Answer 7

• Inspiration: -8 mmHg (below atmospheric pressure)

- Exhalation: -4 mmHg (below atmospheric pressure)

Question 8

What is Boyle’s Law?

Answer 8

• When the volume of a container increases, the pressure decreases.
• When the volume of a container decreases, the pressure increases.
• If 2 areas of different pressures communicate, gas will move from the area of higher pressure to the area of lower pressure.

Question 9

Describe the changes in intrapulmonary pressure at rest, during inhalation and during exhalation, and the effect this has on air movement.

Answer 9

1. At rest
   - P outside = P inside
   - No air movement occurs
2. Inhalation
   - Diaphragm contraction increases volume of thoracic cavity - P inside increases
   - P outside > P inside
   - Air flows into lungs.
3. Exhalation
   - Diaphragm relaxes (moves up in chest), reducing volume of thoracic cavity.
   - P outside < P inside
   - Air flows out of lungs.

Question 10

What happens if air enters the pleural space?

Answer 10

• Pneumothorax
• The -4 mmHg pressure gradient that normally keeps the lungs against the chest wall disappears so lung recoils and collapses.

Question 11

What is the difference between open and closed pneumothorax?

Answer 11

• Open: opening in the chest (with or without lung puncture) allows atmospheric air to enter the pleural space. Caused by penetrating trauma, e.g. Stab, gunshot, impalement, surgery.
• Closed: chest wall is intact but rupture of the lung and visceral pleura (or airway) allows air into the pleural space.

Question 12

What is LaPlace’s Law and how does this relate to alveoli size?

Answer 12

• The pressure inside a sphere is equal to 2x the tension within divided by the radius.
• Lower pressure in larger alveoli, higher pressure in smaller alveoli.

Question 13

What are the possible consequences of differences in alveoli size (and therefore pressure)?

Answer 13

Upon inhalation, air would preferentially travel to the larger alveoli, and could even flow back from the higher pressure alveoli to the lower pressure alveoli.

• Large alveoli risk becoming overdistended
• Small alveoli risk collapsing (atelectasis)

Question 14

What does the body do to overcome differences in alveoli size/pressure and the associated consequences?

Answer 14

• Type II pneumocytes lining the alveoli produce pulmonary surfactant: complex amphiphilic lipoprotein.
• Contains DPPC which greatly reduces the surface tension of the fluid coating the inside of alveoli.
• Inspiration: alveoli expand and surfactant molecules move apart; expiration: lungs contract and surfactant molecules become concentrated - surface tension is reduced.
• Effects of pulmonary surfactants are greater in smaller alveoli (have even lower surface tension than large alveoli) so offsets effect of smaller radius - air enters both small and large alveoli.

Question 15

What are the 3 consequences of pulmonary surfactant?

Answer 15

1. Reduced atelectasis.
2. More even distribution of ventilation amongst alveoli.
3. Improvement of lung compliance (ability of lungs and thorax to expand - the volume change per unit of pressure change across the lung).

Question 16

Why does infant respiratory distress syndrome occur and how can this be treated?

Answer 16

• Surfactant not reliably produced until ~34 weeks gestation - premature infants born prior may have significant difficulty with oxygenation and ventilation.
• Treatment = positive pressure ventilation and exogenous surfactant.
• If a mother is considered at high risk of preterm delivery, she may be given antenatal corticosteroids - speeds up infant’s synthesis and release of surfactant.