During a marathon, a runner drank an excessive amount of water (no electrolytes)

Question

During a marathon, a runner drank an excessive amount of water (no electrolytes). The runner drank so much that her Total ExtraCellular Fluid (ECF) volume increased by 15%. Whereas normally an individual has 14L of ECF, this runner was found to have 16.1L (14L x 1.15 = 16.1L).

1a. If the typical concentration of sodium and potassium in the extracellular fluid is 145mM and 4mM respectively, what are the concentrations of these ions in this runner after her marathon. Assume that the total AMOUNT of extracellular ions (Na and K) has not changed.   Show all work

Explanation / Answer

Answer:

1. (a) Initial volume of extracellular fluid = 14 L

Final volume of extracellular fluid = 16.1 L

There is no change in the total amount of sodium and potassium in the ECF. So, the concentration decreases, i.e, gets diluted after intake of water.

Applying, V1S1 = V2S2

Concentration of sodium = (145 * 14) / 16.1 = 126.086 mM

Concentration of potassium = (4 * 14) / 16.1 = 3.478 mM

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