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MECHANISM OF BREATHING: periodic exchange of gas from alveoli with fresh gas of the upper airways

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spontaneous ventilation
mechanical ventilation
anesthetic effects on ventilation

removal and elimination of carbon dioxide from the blood
spontaneous ventilation

general:
alveolar pressure: is atmospheric and is greater than intrathoracic pressure during end expiration/inspiration
pleural pressure : used as a measure of intrathoracic pressure
transpulmonary pressure: equals alevolar pressure minus intrapleural pressure

end expiration
alveolar pressure: equal zero; no flow
intraplueral pressure: equals -5 cm H20
transpulmonary pressure equal +5 cm H20

beginning inspiration
diaphragm contracts therefore chest cavity expands
pressure gradient allows gas to flow from the upper airways into the alveoli
alveolar pressure changes from 0 to -3 cm H20
intraplueral pressure changes from -5 cm H20 to -8 cm H20
transpulmonary pressure

end inspiration
new transpulmonary pressure sustains lung expansion
alveolar pressure returns to zero
intraplueral pressure remains decreased
transpulmonary pressure 5 cm H20

beginning expiration
diaphragm relaxes
elastic recoil of the lung creates a pressure gradient
gas flow from alveoli to the upper airway

alveolar pressure
intrapleural pressure returns to -5 cm H20
transpulmonary pressure no longer supports lung expansion

mechanical ventilation
general: apply intermittent positive pressure to upper airways
inspiration
gas flows into the alveoli with positive pressure until the alveolar and upper airway pressure equal

expiration
positive airway pressure is either removed or decreased
allows for reveral of pressure gradient (of inspiration)
airflow moves from alveoli into upper airway