Composition of the Extracellular Fluid

The substances dissolved in the extracellular fluid (e. g., salts) are present as electrically charged particles (ions) and the solutions are called electrolytes. Because of their electric charge, ions can migrate in electric fields. For this reason, positively charged ions are also called cations (they migrate to the negative pole, the cathode) and negatively charged ions are called anions (they migrate to the positive pole, the anode). The salt present in the largest amount is common table salt (NaCl), consisting of a positively charged sodium ion (Na+) and a negatively charged chloride ion (Cl−) dissolved in a concentration of about 9 g/l. Other cations and anions are also present, though in distinctly smaller quantities: e. g., potassium (K+), calcium (Ca2+) and magnesium (Mg2 +), as well as
bicarbonate (HCO3 −) and negatively charged proteins. The three compartments of the extracellular space—the interstitial fluid, the blood plasma, and the lymph—differ mainly in the amount of protein dissolved in each. For instance, the walls of the blood and lymph capillaries are permeable only to small ions and smaller organic particles, while large proteins are retained within the lumina of these vessels.