Question

Which solution is most likely isotonic?

Answer 1

Hypertonic, hypotonic and isotonic are terms used to refer to solutions that are being compared to another solution and are important for understanding osmosis, which is the movement of water through a semipermeable membrane like the cell membrane. Water can move easily through the membrane, but many dissolved substances (solutes) like salt (NaCl) cannot.

If you look at red blood cells in normal (0.9%) saline under a microscope, the cells are isotonic to each other and the solution they are in. Each cell has essentially the same amount of solutes dissolved inside it and the saline is the same concentration as the cells. If you add a 5% salt solution to surround those cells, the cells will shrink because the salt solution is now hypertonic to the cells. There is far more salt dissolved in the outside solution in comparison to solutes in the cells. Osmotic pressure will cause water to leave the cells to try to bring the solutions inside and outside the cells into balance or equilibrium. If you replace the salt solution with plain water, the cells are now in a hypotonic solution and water will flow into the cells since the solutes dissolved inside the blood cells are now at a higher concentration than the solution they are in. The blood cells are not constrained by cell walls or surrounded by a dense packing of other cells, so they can burst before equilibrium is reached. That is the reason that intravenous fluids always contain something like salt or dextrose. Using plain water to replace lost blood would make the problem worse by destroying some of the remaining blood cells.

Hypertonic saline may be used to bring swelling down in some cases of traumatic brain injury since the salt concentration will cause water to leave the swollen brain cells and relieve the pressure inside the skull. Hypotonic saline can be used to treat severe dehydration by safely replacing lost water.

Answer 2

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