In the human body plan, there are three major fluid compartments that are functionally interconnected. These are the (1) intracellular fluid compartment, (2) interstitial fluid, and (3) plasma. Fluid, molecules, and ions flow across physical barriers between the fluid compartments. The physical barrier separating the intracellular fluid compartment (i.e., cytoplasm) and the interstitial fluid is the cell plasma membrane. The capillary endothelium is the physical barrier that separates the interstitial fluid from plasma. Nutrient molecules traveling in the blood must first cross the capillary endothelium to enter the interstitial fluid. They then must cross the plasma membrane to enter the cytoplasmic compartment of cells. Waste products produced by cells follow the reverse path from the cytoplasmic compartment to plasma. The left diagram allows for a better demonstration of the relationship between the intracellular fluid, interstitial fluid, and plasma, however, the relative size of each of the compartment is not drawn to scale. The right diagram shows the three major fluid compartments drawn to scale. The intracellular fluid compartment contains most of the water in the body (~67% of total). Interstitial fluid contains ~25% of the total body water. Plasma is the smallest fluid compartment (~8% of total body water). Note that this diagram places focus only on these three major fluid compartments. For a complete diagram of body fluid compartments, see body fluid compartments of a 70-kg man and body fluid compartments of a 55-kg woman.