Osmosis is the net movement of water across a semi-permeable membrane from an area of lower gradient concentration to an area of higher gradient concentration. On the other hand, tonicity is the ability of an extracellular fluid to make water move into or out of a cell through osmosis. A solution's tonicity is related to its osmolarity or the total concentration of the solutes in a solution.
When solutions of varying osmolarities are separated through a membrane that is permeable to water but not to solute, water from the side with lower osmolarity will move to the area with higher osmolarity.
<span>Tonicity is the concentration of a solution compared to another, with regards to the number of solutes in these solutions. Too many solutes in a cell may cause them to be hypertonic hence absorb water too much water and lyse. Therefore, these solutes are excreted to the blood-stream through active transport and facilitated diffusion. These solutes are then excreted by the kidneys. If they are not excreted, the osmotic pressure (due to the inflow of water to the blood stream by osmosis) may cause high blood volumes being pumped by the heart hence high blood pressure</span>
