What's the Difference Between Dehydration, Hypernatremia, and Hyponatremia?

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Sweat is usually more dilute than blood plasma. During prolonged exercise, unreplaced sweating can therefore reduce plasma volume and raise plasma sodium concentration, potentially causing hypernatremia. Increased plasma osmolality stimulates thirst and antidiuretic hormone (ADH) release, promoting water conservation by the kidneys.

By contrast, drinking water faster than it can be excreted can dilute plasma sodium. In exercise-associated hyponatremia, water enters cells by osmosis; swelling of brain cells can cause headache, confusion, seizures, or, in severe cases, death. Hypernatremia draws water out of cells and may also impair neurological and physical function.

A common misconception is that every dehydrated athlete is hypernatremic. Dehydration may occur with normal, high, or low sodium concentration because the outcome depends on the relative losses and replacement of water and electrolytes.

IB Exam Technique

For A.2.1, distinguish a change in total body water from a change in sodium concentration. In an explanation question, identify the relative water and sodium changes, connect them to osmosis and plasma volume, and predict the direction of water movement across cell membranes.