If alveolar o2 decr 30% (pao2 < 70mm hg), adjacent vessels constrict, increasing resistance and reducing blood flow. (blood flow ctrl due to po2 of alveolar gas, not of the blood: effect induced by o2 sensitive k+ channels. Low o2 k+channels close causing depol contraction of sm: ans plays minor role in pulmonary bf distribution, hydrostatic effects, lungs about 30 cm long, top of lung ~ 15 mmhg less than pulmonary arterial pressure. Bottom of lung about 8 mm hg above pulmonary arterial pressure: greater bp at bottom of lung distends vessels reduces resistance. So bf greater at bottom (up to 5x greater): 3 superior-inferior categories, zone 1 upper zone. 29 mm hg: cap hydrostatic p ii, negative interstitial fluid hydrostatic p, total. Interstitial fluid colloid p: forces tending to cause fluid influx into caps, plasma colloid osmotic p, total.