Overview Airflow, Pressure, and Resistance Q = ΔP / R airflow is directly proportional to pressure difference between mouth/nose (atmospheric) and alveoli airflow is inversely proportional to airway resistance Airway Resistance R = 8ηl / πr4 η = viscosity of inspired air l = length of airway r = radius of airway total resistance depends on the aggregate cross-sectional area of a zone highest in the bronchi between the 4th and 8th generations of branching Changes in Airway Resistance changes in airway diameter provide major mechanism for altering airway resistance and airflow Autonomic Nervous System sympathetic stimulation bronchial smooth muscle is innervated by sympathetic adrenergic nerve fibers sympathetic stimulation → (+) β2 receptors → relaxation → ↑ airway diameter, ↓ resistance to airflow β2 agonists (e.g., epinephrine, isoproterenol, albuterol) relax bronchial smooth muscle and dilate airway in treatment of asthma parasympathetic stimulation bronchial smooth muscle is innervated by parasympathetic cholinergic nerve fibers parasympathetic stimulation → (+) muscarinic receptors → constriction → ↓ airway diameter, ↑ resistance to airflow Pulmonary Vascular Resistance (PVR) PVR = [P(pulmonary artery) - P(left atrium)] / cardiac output P(pulmonary artery) is pressure in pulmonary artery P(left atrium) is pulmonary capillary wedge pressure
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