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Respiratory Mechanics

Respiratory Mechanics

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Boyle's Law and The Respiratory System:
Boyle's law describes the relationship between the pressure and volume of a gas; dictates the mechanics of respiration.
  • Boyle's Law Equation: (P1)(V1) = (P2)(V2)
    • Pressure and volume of a gas are inversely related (assume temperature is constant).
Key anatomical/mechanical structures
    • Pair of lungs connected by a trachea
    • Lungs sit within a double-walled pleural sac, which separates lungs from thoracic wall.
    • Pleural space, which is the potential space within sac.
    • Sternum and ribs around pleural sac
    • The internal and external intercostal muscles lie between the ribs
    • Diaphragm comprises a sheet of skeletal muscle and forms bottom border of thoracic cavity.
– Contraction/relaxation changes the volume of the thoracic cavity, which alters the intrapulmonary and intrapleural pressures in accordance with Boyle's law.
Air pressures
Dictate movement of air during inspiration and expiration
    • Atmospheric pressure = 760 mm Hg, external to the thoracic cavity.
    • Intrapulmonary pressure (intra-alveolar pressure) = atmospheric pressure at rest, refers to the pressure within the lungs
    • Intrapleural pressure = 756 mmHg, pressure in the pleural space.
Inspiration: actively increases thoracic volume.
Movements during Inspiration:
  • Diaphragmatic Contraction:
    • Diaphragm flattens and moves inferiorly as it contracts.
  • External Intercostal Muscle Contraction:
    • External intercostal muscles contract
    • Forces sternum to move in an anterior and superior direction
Effects:
  • As thoracic volume increase:
– Intrapulmonary pressure decreases to below atmospheric pressure; creates pressure gradient that facilitates inhalation. – Air moves down this pressure gradient: Moves from atmosphere (high P) through trachea into lungs (low P)
  • Gas exchange occurs in lungs ? need to exhale carbon dioxide
Expiration passively decreases thoracic volume.
Movements during Expiration: – Diaphragm relaxes – Ribs back to resting position (move medially) – External intercostal muscles relax – Sternum back to resting position (moves inferiorly)
Effects:
  • As thoracic muscles relax:
– Lungs elastically recoil to their pre-inspiratory volume; Lung volume decreases. – Intrapulmonary pressure increases above atmospheric pressure; creates pressure gradient that facilitates expiration. – Air moves down pressure gradient: Moves from lungs ? out trachea ? into atmosphere, CO2 exhaled.
Compliance
Flow Rate

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