If the red blood cells (RBCs) of human blood are isolated and are diluted in normal saline (an isotonic solution to blood), what will happen to the RBCs?
- A.The RBCs will swell
- B.The RBCs will swell and burst
- C.The RBCs will shrink
- D.No change in the diameters of the RBCs
Correct answer
Explanation
The correct answer is D, No change in the diameters of the RBCs. In an isotonic solution the concentration outside the cell equals that inside, so there is no net movement of water.
Water moves across the cell membrane by osmosis, from the side with lower solute concentration to the side with higher concentration. Normal saline is 0.9% sodium chloride, which has the same concentration as blood plasma. Water enters and leaves the red blood cells at equal rates, so they keep their normal biconcave shape and size. This is why normal saline is used for drips and for washing blood cells in laboratories.
A is wrong: RBCs swell only in a hypotonic solution, such as distilled water, where water rushes in.
B is wrong: swelling and bursting, called haemolysis, also happens in a strongly hypotonic solution, not an isotonic one.
C is wrong: RBCs shrink, called crenation, only in a hypertonic solution such as strong salt water.
Exam tip: hypotonic = swell and burst (haemolysis), isotonic = no change, hypertonic = shrink (crenation).
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