Reactivity of the vessels of collapsed and ventilated lungs to drugs and hypoxia.

• The vessels of the unexpanded foetal lung react more strongly to certain stimuli than those of the expanded foetal or newborn lung; they probably react more strongly than the vessels of adult lungs. Thus acetylcholine and histamine caused profound vasodilatation in unexpanded foetal lungs and comparatively weak dilatation in expanded foetal lungs. Noradrenaline caused a high degree of vasoconstriction in the lungs of immature foetal lambs but only small effects have been observed in the adult lungs of other species." The vascular resistance of unexpanded foetal lungs is frequently high compared with that of ventilated foetal lungs" but this is mainly due to vascular tone.