Lung Mechanics Flashcards Preview

ESA 1 - Body Logistics > Lung Mechanics > Flashcards

Flashcards in Lung Mechanics Deck (16)
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1
Q

What is cohesion? What are cohesive forces responsible for?

A
  • Attraction of molecules for other molecules of the same kind.
  • Surface tension.
2
Q

Why do water molecules have strong cohesive forces?

A

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

3
Q

What is surface tension and why does this occur?

A
  • 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.
4
Q

What are lungs surrounded by?

A

Pleura: continuous membrane that folds over itself.

  • parietal pleura (lines the chest wall)
  • visceral pleura (covers the lung)
5
Q

What are the parietal and visceral pleurae separated by?

A

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)
6
Q

What keeps the 2 pleurae together and prevents lungs collapsing?

A

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

7
Q

What is the intrapleural pressure during inspiration and exhalation?

A
  • Inspiration: -8 mmHg (below atmospheric pressure)

- Exhalation: -4 mmHg (below atmospheric pressure)

8
Q

What is Boyle’s Law?

A
  • 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.
9
Q

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

A
  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.
10
Q

What happens if air enters the pleural space?

A
  • Pneumothorax
  • The -4 mmHg pressure gradient that normally keeps the lungs against the chest wall disappears so lung recoils and collapses.
11
Q

What is the difference between open and closed pneumothorax?

A
  • 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.
12
Q

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

A
  • 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.
13
Q

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

A

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)
14
Q

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

A
  • 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.
15
Q

What are the 3 consequences of pulmonary surfactant?

A
  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).
16
Q

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

A
  • 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.