Pulmonary passageway and pulmonary vascular resistance
Respiratory passageway resistance
Upper respiratory passageways – relatively large, very little resistance to airflow (unless obstruction such as from food lodging or cancer)
Lower respiratory passageways – from medium-sized bronchioles on down, can alter diameter based on autonomic stimulation
Parasympathetic – causes bronchoconstriction
Sympathetic – inhibits bronchoconstriction
Epinephrine – used to treat life-threatening bronchoconstriction such as during asthma and anaphylactic shock.
Pulmonary vascular resistance
Although pulmonary vascular resistance is very low at rest, it can decrease further, as in exercise, during which blood flow increases. Neural : Sympathetic nerves to larger vessels – NE released.
Effects somewhat controversial – most likely increases pulmonary vascular resistance and decreases distensibility of larger vessels. Parasympathetic – Ach decreases PVR if tone elevated.
Humoral : Increase PVR: Norepinephrine, alpha adrenergic agonists, serotonin, prostaglandins F2 and E2, angiotensin, endothelin, alveolar hypoxia (hypoxic pulmonary vasoconstriction), alveolar hypercapnia, low pH of mixed venous blood.
Histamine constricts pulmonary veins. Decrease PVR: Acetylcholine, beta adrenergic agonists, bradykinin, prostaglandin E1 and I2 (prostacyclin), nitric oxide.