intracellular fluid

intracellular fluid, substance within living cells that is made up primarily of water and molecules such as dissolved ions and is a major component of the cytoplasm and cytosol. A variety of chemical reactions within cells take place in the cytoplasm and therefore the intracellular fluid. The intracellular fluid bathes cellular organelles, and, in adult humans, it accounts for about 40 percent of total body weight.

Intracellular fluid is distinguished from extracellular fluid, which is found between cells in multicellular organisms and serves to conduct nutrients, waste products, and other substances throughout the tissues of the body. Relative to extracellular fluid, intracellular fluid is slightly more acidic: the pH of intracellular fluid is between roughly 6.8 and 7.4, depending on the tissue and cell type, whereas extracellular fluid pH is between about 7.3 and 7.5. Intracellular and extracellular fluid are separated by the cell membrane, which is composed of a semipermeable lipid bilayer.

Fat-soluble substances, such as oxygen and carbon dioxide, penetrate lipid bilayer membranes easily. Water can slowly diffuse across such membranes, whereas charged molecules, such as ions, typically require designated transport proteins to gain entry into cells. This results in a so-called ion gradient across the membrane, whereby the concentration of certain ions is higher in the extracellular compartment than in the intracellular one and vice versa. In general, in the human body, intracellular fluid contains only a small amount of sodium and chloride ions and very few calcium ions but contains relatively large amounts of potassium and phosphate ions. Magnesium and sulfate ions are present in moderate amounts. Intracellular fluid also contains large amounts of protein.

The ion balance between intracellular and extracellular fluid is critical for cell and tissue function. For example, the maintenance of sodium and potassium ion gradients across the cell membrane in the kidney is essential for the regulation of electrolyte levels, the filtration of waste products, and the reabsorption of amino acids and glucose in the blood. Furthermore, the movement of fluid and ions between the intracellular and the extracellular compartments is important in hydration and the maintenance of cell volume. Drastic changes in the fluid and ion balance can lead to cell swelling or cell dehydration.