Give the concentration of salt solution which is isotonic for RBCs.
A. 0.9% of salt solution
B. 0.4% of salt solution
C. 0.5% of salt solution
D. 0.7% of salt solution

Hint: An isotonic solution is a type of solution which has the same osmolarity as that of the red blood cells. In such a solution, the concentration of solute is the same as the concentration of solute inside the cells. It is marginally below 1%. In such a situation, there is no movement of solvent between the cell and the solution it is kept in.

Complete answer:
The cell membrane of red blood cells is semi-permeable and this can cause influx and efflux of fluids. The movement of the solvent depends on the osmotic gradient. The movement of a solvent across the membrane is from the region of higher concentration to a region of lower concentration.

Since the concentration on both sides of the membrane is the same in the case of an isotonic solution, there is no movement of water.

So, 0.9% of salt solution is isotonic for red blood cells.
Hence, the correct answer is option (A).

Additional information:
> Hypertonic solution - It is a type of solution in which the concentration of solute is more than the concentration of solute on the other side of the membrane. So, if a cell is kept in such a solution, the solvent moves from inside the cell to outside the cell, into the solution.
The cells collapse and this leads to crenation.

> Hypotonic solution - It is a type of solution in which the concentration of solute is less than the concentration of solute on the other side of the membrane. So, if a cell is kept in such a solution, the solvent moves from outside the cell to inside the cell.
The cells get swollen and this leads to haemolysis.

Note: The concentration of hypertonic media is more than that of an isotonic media. For example, 1.8% of sodium chloride. The concentration of a hypotonic media is less than that of an isotonic media. For example, 0.4% of sodium chloride. When a medicine is injected into a person, the solution that carries it is always isotonic.