BIOM1060 Lecture Notes - Lecture 26: Pulmonary Pleurae, Intrapleural Pressure, Thoracic Cavity
BIOM1060 17/04/18
RESPIRATORY MECHANICS AND LUNG VOLUMES
Review
- Recoil of lung: alveolar surface tension (due to fluid of surface lining in alveolar walls)
o Air-water interface produces inward pressure (bipolar water molecules attracted
to each other) - moderated by pulmonary surfactant molecules (reduce water
tension to allow inflation of lungs and prevent collapse of small alveoli into larger)
o Connective tissue fibres within alveolar walls (stretched by inflated lungs)
Pleural sac
- Each lung surrounded by visceral pleura, while diaphragm and ribcage surrounded by
parietal pleura (forms pleural sac - filled with intrapleural fluid)
Air pressure
- Pressure eerted atospheri air aove earth’s surfae
o At sea level: 760mm of mercury (Hg)
- Air moves from high to low pressure
Transmural pressure gradient: differences in pressure
Human breathing
- Negative tidal breathers (create lower than atmospheric pressure in lungs to breathe in)
o Pressure gradient between atmosphere and alveoli
Pressure relationships in thoracic cavity
- Intrapulmonary pressure: pressure in alveoli (rises and falls with phases of breathing)
- Intrapleural pressure: pressure in pleural cavity (fluctuates, but always less than
intrapulmonary pressure - lung has natural tendency to recoil due to elasticity and
surface tension of alveolar fluid - pulls visceral pleura inwards, while thoracic wall pulls
outwards - increased intrapleural space = decreased pressure)
Punctured parietal pleura
- Intrapleural space exposed to atmosphere (intrapleural pressure equalised with
atmospheric pressure
o Lung collapses and rib cage protrudes (springs out)
Quiet breathing
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BIOM1060 17/04/18
- Inspiration: lower diaphragm = lower parietal pleura = increased intrapleural space =
decreased intrapleural pressure = visceral pleura pulled = increased volume of alveoli =
decreased intrapulmonary pressure = air flows into lungs
- Expiration: diaphragm relaxes = lung recoils = decreased volume = increased
intrapulmonary pressure = air flows out
Forced breathing
- Inhalation: assisted by external intercostal muscles in ribs - abduct ribcage (increased
volume = larger pressure gradient = faster inhalation)
- Exhalation: assisted by abdominal muscles - push diaphragm upwards (pressure on
parietal pleura) and internal intercostal muscles flatten ribcage = decrease volume of
alveoli = larger pressure gradient = faster air expiration)
Airflow rate = pressure gradient
airways resistance
Airway resistance
- Due to friction molecules experience along wall of airway
o Larger diameter = lower resistance
Quiet exhalation vs forced exhalation
- Quiet exhalation: diaphragm relaxes, lung recoils, intrapulmonary pressure increases,
intrapleural less than intrapulmonary pressure (pulls airways open), air flows out
- Forced expiration
Dynamic small airway closure
- During forced expiration
o Increased intrapleural pressure and extra recoil (due to forced inspiration) =
increased intrapulmonary pressure = air escapes at much faster rate = pressure
decreases in small airways faster (airway resistance)
- Large airways: cartilage support, but small airways: made up of smooth muscle and
surrounded by intrapleural space (subject to intrapleural pressure)
- Increased intrapleural pressure constant, but pressure in small airways decreases (lower
pressure in alveoli due to decreased volume and airway resistance decreases)
o Eventually reaches equal pressure point, after which intrapleural pressure is
more than pressure in small airways
o Intrapleural space compresses and closes it off
▪ Traps some air (residual air) within lungs (can never fully empty lungs)
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Document Summary
Each lung surrounded by visceral pleura, while diaphragm and ribcage surrounded by parietal pleura (forms pleural sac - filled with intrapleural fluid) Pressure e(cid:454)erted (cid:271)(cid:455) at(cid:373)ospheri(cid:272) air a(cid:271)ove earth"s surfa(cid:272)e: at sea level: 760mm of mercury (hg) Air moves from high to low pressure. Negative tidal breathers (create lower than atmospheric pressure in lungs to breathe in: pressure gradient between atmosphere and alveoli. Intrapulmonary pressure: pressure in alveoli (rises and falls with phases of breathing) Intrapleural space exposed to atmosphere (intrapleural pressure equalised with atmospheric pressure: lung collapses and rib cage protrudes (springs out) Inspiration: lower diaphragm = lower parietal pleura = increased intrapleural space = decreased intrapleural pressure = visceral pleura pulled = increased volume of alveoli = decreased intrapulmonary pressure = air flows into lungs. Expiration: diaphragm relaxes = lung recoils = decreased volume = increased intrapulmonary pressure = air flows out.
