Question

Which mineral would be found in greatest concentration in the plasma of the marathon runners?

Answer 1

The right answer is E. Sodium  (with 140mmol/L)

Sodium is the most important element in the blood and extracellular liquids of the body.The needs of an adult subject are of the order of 1 to 2 g of sodium per day. The normal diet is much richer than our actual needs, about 4g, so it can largely compensate for the needs of an athlete or a runner.
Losses are majoritarly by feces and sweat. The kidney is able to reabsorb almost all the sodium that is filtered: urinary losses can be tiny.