Airway narrowing within the parent bronchus at branching sites. (A) Cross sectional images of a relaxed parent bronchus at a branching site and immediately proximal and distal to the branching site. The parent bronchus opens out to the daughter airway at the centre of the branching site. Airway narrowing to adventitial (B, N = 4) and luminal carbachol (C, N = 4) was compared at the three locations identified in (A). There was no difference in narrowing of the parent bronchus between branching or non branching sites irrespective of whether carbachol was administered to the adventitial or luminal surface. (Two-way ANOVA).