• Bring in oxygen: fuels cell metabolic activity.
• Release carbon dioxide: rids body of metabolic waste.
• Regulate blood pH: via regulation of blood oxygen and carbon dioxide concentrations.

Three major components:

1. Muscular and skeletal components of the chest wall (“the pump”).
2. Alveoli = interfaces for gas exchange (“gas exchanger”).
3. Areas of the brain that stimulate respiratory function (“the controller”).

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.