Airway resistance in respiratory physiology is modeled by Poiseuille's law for rigid tubes, R = 8ηL / (πr⁴), in which resistance varies directly with tube length and gas viscosity and inversely with the fourth power of the radius, making radius overwhelmingly the dominant determinant of resistance to airflow. Because the bronchial tree branches into progressively more airways arranged in parallel, total resistance falls with successive airway generations, and radial traction exerted by surrounding lung parenchyma further dilates airways and lowers resistance; conversely any process narrowing the lumen raises resistance and slows expiratory flow, which is why obstructive disease is detected as reduced volume expelled per unit time on spirometry at equivalent effort. A second, mechanically distinct source of resistance is dynamic compression, in which loss of elastic recoil leaves pleural pressure exceeding intraluminal airway pressure at some point along the tube during forced expiration, collapsing the airway. This concept belongs to pulmonary mechanics within respiratory physiology, applying fluid-dynamics principles to the parent discipline of human physiology.
Airway resistance in respiratory physiology is modeled by Poiseuille's law for rigid tubes, R = 8ηL / (πr⁴), in which resistance varies directly with tube length and gas viscosity and inversely with …