Air flow is directly proportional to the pressure difference between the mouth/nose and the alveoli. It is inversely proportional to the resistance of the airways. Between breaths, alveolar pressure equals atmospheric pressure. During inspiration, the diaphragm contracts to increase lung volume, decreasing alveolar pressure and establishing a pressure gradient, which causes air to flow into the lungs. Resistance in the respiratory system is determined by Poiseuille law.
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