It is suggested that failure of infusions because of phlebitis or extravasation usually results from irritation of the endothelium by the infusate, inducing venoconstriction. If such venoconstriction prevents flow, then pressure which builds up in the vein will enlarge the hole in its wall made by the needle or cannula and so allow extravasation of fluid. Alternatively, partial constriction will cause the endothelium to be perfused with undiluted infusate, and this will aggravate the irritation and lead to phlebitis.