Q.90 Which of the following cations are found in higher concentration in extracellular fluid as compared
to intracellular fluid in animals?
(A) Na+and Ca++
(B) K+and Ca++
(C) K+and Mg++
(D) Na+and Mg++
Sodium (Na+) and calcium (Ca2+) ions maintain higher concentrations in extracellular fluid compared to intracellular fluid in animals, making option (A) the correct choice.
Correct Answer
(A) Na+ and Ca++ is correct. Na+ concentration reaches about 140 mmol/L extracellularly versus 10 mmol/L intracellularly, while free Ca2+ stays low inside cells (around 0.0001 mmol/L) but higher outside (1-2 mmol/L total).
Option Breakdown
Each option lists cation pairs with their typical distributions:
| Option | Cations | Extracellular vs. Intracellular Concentration |
|---|---|---|
| (A) Na+ and Ca++ | Higher ECF for both | Correct: Na+ ~140 mM ECF vs. 10 mM ICF; Ca2+ ~1.5 mM ECF vs. ~0.1 μM free ICF |
| (B) K+ and Ca++ | K+ higher ICF; Ca2+ higher ECF | Incorrect: K+ ~4 mM ECF vs. 140 mM ICF |
| (C) K+ and Mg++ | Both higher ICF | Incorrect: K+ as above; Mg2+ ~1 mM ECF vs. 13 mM ICF |
| (D) Na+ and Mg++ | Na+ higher ECF; Mg++ higher ICF | Incorrect: Mg2+ as above |
Physiological Role
The Na+/K+ ATPase pump actively maintains these gradients by exporting 3 Na+ for 2 K+ import, creating membrane potential essential for nerve signaling and muscle contraction. Ca2+ gradients support signaling, with intracellular stores buffering free levels tightly.
