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We use normal sline to wash red cells because both are isotonic.  But if we add more sline this solution then why its remain isotonic. I mean we increase sline volume then its means more sline outer rbc. Then why it would not become hypotonic  solution?

Tonicity depends on the number of particles in solution and the nature of the solute. Concentration of nonpenetrating solutes in a solution, not the total osmolarity, determines its tonicity. Penetrating solutes do not contribute to the tonicity of a solution.

Hi,

By any chance, you meant saline? Which is just a specific ratio between NaCl salt and water.
isotonic is a comparison between 2 concentrations, not volume. Concentration is a ratio between the solute and the solution. By adding more saline or sline, you're pretty much adding both solute and solvent with the same ratio at the same time. Therefore, the solution would still be at the same concentration before, and is still isotonic to the rbc.

Let me know if this helps

Ok.  Sir.  Now i understand that,  rbc in a unit have solute and normal saline in a unit have a same concentration. This concentration have pressure.  When we add more saline. Its same concentration.  Then its pressure  remains same.  So its isotonic. Right.?
But i have one more question,  saline have only nacl.  But rbc have more solutes like,  calcium,  potassium etc.  Then how its isotonic  with saline?

Hi,

In this case it's, the concentration that matter, since only water can move across rbc membrane.

The osmolarity pressure is Π = iMRT, where Π = pressure, M is concentration, i = van't Hoff's factor (M and i together is the total concentration) R and T  = gas constant and temperature, respectively, and they should be constant. Saline and rbc although have different solute but they have the same total concentration, and R and T are constant. Therefore, the osmotic pressure is the same for the saline solution and rbc.

Thank you so much sir

Tonicity, this has to do with the concentration of a solution surrounding a cell. This is concerning osmosis. Tonicity on the grounds that they will dependably be in equivalent focuses on both sides of the layer. There are three groupings of tonicity that one arrangement can have in respect to another. The three are hypertonic, hypotonic, and isotonic. In an isotonic arrangement, the extracellular liquid has the same osmolarity as the cell there will be no net development of water into or out of the cell.
Hypertonicity, whenever a solution surrounding a cell is highly concentrated this causes for water to flow outside of the cell.
Hypotonicity, whenever a solution surrounding a cell is lowly concentrated this causes for water to flow into the cell.
Isotonicity, this is equilibrium. When the solution surrounding the cell isn't too concentrated, the amount of water within the cell is balanced.