Geometric model of the deformed vesicle. The vesicle is abstracted into a geometry consisting of a rotationally symmetric side surface and two round planes at top and bottom. (a) Three-dimensional view. A fixed global right-handed Cartesian coordinate system is set up, with its origin labeled as O, at the center of the vesicle. (b) Front view and top view. In the front view, the outline of the vesicle is constituted by two horizontal straight lines with length 2L, and two semicircles with radius R. The vertical pressure p is along the z axis. In the top view, there appear conceivably two concentric circles with radius L and R + L, respectively.