The document discusses sodium and water physiology. It states that sodium content determines extracellular fluid (ECF) volume, while sodium concentration reflects intracellular fluid (ICF) volume. Hyponatremia leads to swollen cells due to low sodium concentration, while hypernatremia leads to shrunken cells due to high sodium. Thirst and antidiuretic hormone (ADH) work to maintain fluid balance and defend cellular tonicity against small changes in sodium concentration.