Which of the following best describes the changes in cell volume that will occur when red blood cells (previously equilibrated in a 280-milliosmolar solution of NaCl) are placed in a solution of 140-millimolar NaCl containing 20-millimolar urea, a relatively large but permeant molecule?
A) Shrink, then swell and lyse
B) Shrink, then return to original volume
C) Swell and lyse
D) Swell, then return to original volume
E) No change in cell volume
The Correct Answer and Explanation is :
Correct Answer: A) Shrink, then swell and lyse
Explanation:
Red blood cells (RBCs) are highly sensitive to osmotic changes in their environment. The tonicity of the solution determines how water moves across the RBC membrane.
- Initial Shrinkage Due to Hypertonicity:
- The RBCs are initially equilibrated in a 280-milliosmolar (mOsm) NaCl solution, which is isotonic to them.
- The new solution contains 140 mM NaCl and 20 mM urea.
- Since NaCl dissociates into Na⁺ and Cl⁻, the total osmolarity from NaCl is 140 × 2 = 280 mOsm, making it isotonic in terms of total osmolarity.
- However, urea is a permeant molecule, meaning it can cross the RBC membrane.
- Initially, urea is absent inside the cell, so it does not contribute to the tonicity at the moment of exposure.
- The external environment is effectively hypertonic in terms of non-permeant solutes (NaCl), causing water to leave the RBC, leading to cell shrinkage.
- Delayed Swelling Due to Urea Influx:
- Over time, urea equilibrates across the membrane, entering the RBC and increasing intracellular osmolarity.
- Once urea reaches equilibrium, the intracellular osmolarity increases, reducing the effective hypertonicity of the extracellular fluid.
- Water re-enters the RBC, causing cell swelling.
- Lysis Due to Overhydration:
- Since urea freely diffuses into the RBC but does not contribute to tonicity outside, water influx continues beyond the initial volume, leading to cell swelling and eventual lysis.
Thus, the RBCs will initially shrink due to water loss, then swell due to urea influx, and ultimately lyse due to excessive water entry.