CDS (I) 2023 General Knowledge · Question 3 of 120

Correct — D, No change in the diameters of the RBCs. Normal saline is a 0.9 per cent solution of sodium chloride, a strength picked precisely because its osmotic effect matches that of blood plasma. Isotonic is the word for that match: water potential inside the cell equals water potential outside it, so water molecules cross the membrane in both directions at the same rate and there is no net movement either way. With no net movement the cell neither takes on water nor loses it, and it keeps the biconcave disc shape and the diameter it started with. The stem hands you the reasoning in its own parenthesis, and the answer is to trust it.

• (a)The RBCs will swell — Swelling needs a hypotonic surrounding — a solution more dilute than the cell contents, such as saline weaker than 0.9 per cent. Water would then move inward down its potential gradient. An isotonic fluid creates no such gradient.
• (b)The RBCs will swell and burst — Bursting, or haemolysis, is what distilled water does to a red cell: water floods in until the membrane gives way. It is the extreme of the hypotonic case, and it is why saline rather than plain water is used for an intravenous drip.
• (c)The RBCs will shrink — Shrinking, with the surface puckering into spikes, is crenation, and it happens in a hypertonic solution such as concentrated salt water, which pulls water out of the cell. Again the opposite of isotonic.

Osmosis is the net movement of water across a selectively permeable membrane, from where water potential is higher to where it is lower. A cell's fate in a solution therefore depends on how the solution's concentration compares with the cell's own. A hypotonic solution is more dilute than the cell, so water enters and the cell swells and may burst; a hypertonic solution is more concentrated, so water leaves and the cell shrivels; an isotonic solution matches, so nothing changes on balance. Normal saline, 0.9 per cent sodium chloride, is the isotonic solution used with human blood.

This item rewards reading the bracket. Having told you the saline is isotonic, the paper is asking a definition question dressed as an experiment, and three of the four options describe what would happen in some other solution. It is worth carrying the practical reason with the definition: fluids given into a vein are made isotonic exactly so that the patient's red cells are neither burst nor shrivelled by the drip. Water alone into a vein would haemolyse them.

• Normal saline is 0.9 per cent weight-by-volume sodium chloride, isotonic with blood plasma.
• A human red blood cell is a biconcave disc roughly 7 to 8 micrometres across.
• In distilled water a red cell takes in water until it bursts — haemolysis.
• In a concentrated salt solution it loses water and its surface puckers — crenation.
• The biconcave shape gives the cell extra surface area for gas exchange and lets it bend through narrow capillaries.

• Reading past the word isotonic in the stem and answering from a half-remembered experiment.
• Confusing haemolysis with crenation — the first is the hypotonic outcome, the second the hypertonic one.
• Assuming a plant cell in pure water would burst as a red cell does; the cell wall prevents it.

As a what-happens-to-the-cell item across the three tonicities, or as a definition item on osmosis and the direction of net water movement.

• practice — not a real PYQ

  Human red blood cells placed in distilled water will most likely

  • (a)shrink and become spiky
  • (b)swell and burst
  • (c)remain unchanged
  • (d)divide rapidly

  Answer(b) swell and burst — distilled water is hypotonic to the cell, so water moves in until the membrane ruptures, a process called haemolysis.

• practice — not a real PYQ

  The concentration of sodium chloride in the normal saline used in intravenous drips is about

  • (a)0.09 per cent
  • (b)0.9 per cent
  • (c)9 per cent
  • (d)19 per cent

  Answer(b) 0.9 per cent — that strength is isotonic with blood plasma, so the patient's red cells neither swell nor shrink.