A multi-lumen catheter comprising a longitudinally extending elongated body portion, with at least two septa extending longitudinally through the entire body portion, and defining a central lumen and two side lumens. The central lumen is oblong shape in cross section and positioned between the side lumens. The septa forming the central lumen are configured to flex outwards when the central lumen is under pressure, whereby a cross sectional area of the central lumen resultantly increases to increase flow capacity through the central lumen. In some embodiments, the central lumen has a defining inner surface such that a radius of curvature of the central lumen inner surface is, in all locations, equal to or greater than a radius of curvature of an outer surface of a guide wire used for insertion of the multi-lumen catheter. The septa are each configured to fail under pressure before failure of the outer wall.