The Gibbs-Donnan equilibrium across capillary walls explains the distribution of electrolytes. Which statement correctly describes this effect in plasma versus interstitial fluid?

• A Cl− concentration is higher in plasma than in interstitial fluid
• B Na+ concentration is slightly higher in plasma than in interstitial fluid due to the net negative charge of impermeant plasma proteins ✓
• C The Donnan effect is negligible because albumin is neutral at physiological pH
• D K+ concentration is higher in interstitial fluid than in plasma

Explanation

Impermeant plasma proteins (mainly albumin) carry a net negative charge at physiological pH and cannot cross the capillary wall. By the Gibbs-Donnan equilibrium, the non-diffusible protein anion in plasma makes diffusible cations (Na+, K+) about 5% higher in plasma than in interstitial fluid, while diffusible anions (Cl−, HCO3−) are about 5% lower in plasma. Therefore Na+ is slightly higher in plasma (option B). Cl− is higher in the interstitium (not plasma), albumin is not neutral at physiological pH, and K+ is also slightly higher in plasma — so the other options are incorrect.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.