Osmolarity

 Osmolarity is the number of particles per litre of solution. It is the measure of soulte concentration. Osmolarity of a solution is based on the number of ions the molecule dissolved in the solution dissociates into and the molar concentration of that solution. So IM NaCl is 2osM as NaCl dissociates inyo 2 ions (Na+ and Cl-) and it is a 1M solution so osmolarity is M x no. of ions.

One way in which osmolarity is regulated is by the pumping out of ions by the sodium/potassium pump.