Cell volume & shape is maintained by

Gibbs-Donnan effect---there are more osmotically active paicles in the cells than the interstitial fluid. This would make them swell and eventually rupture, but prevented by operation of Na+-K+ pump which produces net movement of positive charge out of the cell and keeps the inside a.d outside of the cell in osmotic equilibrium. (Ref: Textbook of physiology AK Jain 5th edition page no.32)