The normal osmolarity of intracellular and extracellular fluid is 300 mosm, which makes them

The normal osmolality of intracellular and extracellular fluid is 300 mosm, which makes them in osmotic equilibrium. 

This condition is better described as isosmotic. Water moves from high salt concentration to low salt concentration due to osmosis. The osmotic pressure of the extracellular fluid equals the osmotic pressure of the intracellular fluid (300 mosm), therefore no fluid movement across the plasma membrane.

cell regulation is conducted by special proteins.