Explain how the fluids in the compartments differ in composition.
Extracellular fluids generally have similar compositions, including relatively high concentrations of sodium, chloride, and bicarbonate ions, and lesser concentrations of potassium, calcium, magnesium, phosphate, and sulfate ions. The blood plasma fraction contains considerably more protein than either interstitial fluid or lymph.
Intracellular fluid contains relatively high concentrations of potassium, phosphate, and magnesium ions, a somewhat greater concentration of sulfate ions, and lesser concentrations of sodium, chloride, and bicarbonate ions. It also has a greater concentration of protein than plasma.