PRESSURE GRADIENTS:

Responsible for the flow of air


Just as blood moves through the cardiovascular system because of the pumping action of the heart, gas flows into and out of the lungs because of pressure gradients created by the DIAPHRAGM and THORACIC CAGE.  Although the abdominal and internal intercostal muscles are used for expiration during exercise or states of increased airway resistance, exhalation is usually a passive process, secondary to the elasticity of the lung-chest wall.

Remeber that the flow of air is proportional to change in pressure over resistance.  Thus, change in pressure drives ventilation and resistance opposes it.  Inspiration occurs when the intrapulmonary pressure decreases to below atmospheric pressure - this can be secondary to an increase in intrapulmonary volume that occurs when your chest wall expands and your diaphragm descends towards the abdomen. Expiration occurs when the intrapulmonary pressure increases to exceed atmospheric pressure (or when intrapulmonary volume decreases). 

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