Step 1: Understanding the Concept:
An isotonic solution is a solution that has exactly the same osmotic pressure and solute concentration as the internal fluid of a cell. When a cell is placed in an isotonic environment, the rate of water entering the cell matches the rate of water exiting the cell. Because the net osmotic flow is zero, the cell maintains its normal volume and shape perfectly without swelling or shrinking.
Step 2: Detailed Explanation:
Medical and physiological applications require an intimate knowledge of blood tonicity. Extensive experimental measurements show that human blood plasma has an osmotic pressure that matches a specific mass-by-volume concentration of common table salt:
- A 0.9% (w/v) aqueous Sodium Chloride (NaCl) solution (commonly known as normal saline or physiological saline) exerts an identical osmotic pressure to the fluid inside human red blood cells.
- Any concentration lower than 0.9% (such as 0.1% or 0.5%) behaves as a hypotonic solution relative to blood.
- Any concentration higher than 0.9% (such as 2%) behaves as a hypertonic solution relative to blood.
Because a 0.9% NaCl solution has the same osmotic concentration as the interior of blood cells, it is completely isotonic with human blood. This corresponds to option (C).
Step 3: Final Answer:
The solution that is isotonic with human blood is the 0.9% NaCl solution.