Q: It seems counter intuitive that in anaphylaxis, the systemic capillaries dilate and the bronchial constrict. Histamine signals dilation (relaxation of the smooth muscles) of blood vessels, and increased permeability of capillaries, causing edema / inflammation.n the other hand, bronchoconstriction is based on muscular spasm and swelling the occurs as a result of fluid accumulation from leaky, dilated vessels in the bronchial capillary beds. Epinephrine has its major effects directly via vascular constriction and bronchial dilation and indirectly by halting vascular leakage.