Definition of substance exchange
Material exchange refers to the material exchange between intracellular fluid and extracellular fluid through the internal environment. Cells are immersed in the interstitial fluid. Only the cell membrane is interposed between the intracellular fluid and the interstitial fluid. Water and everything that can penetrate the cell membrane can be exchanged between the intracellular fluid and the interstitial fluid. Between the interstitial fluid and the plasma, only the capillary wall is separated, and water and everything that can penetrate the capillary wall can be exchanged between the two.