A small portion of the extracellular fluid volume is separated by membranes into subcompartments.
The extracellular fluid compartment includes the inter-stitial fluid, the plasma within blood vessels, and the fluid in the lymphatic vessels. It constitutes approximately one-third of the total body water. The extracellular fluid compartment includes all the fluid outside the cells. Approximately two-thirds of all the water in the body is in the intracellular fluid compartment. Both the composition of the fluid in all these compartments and the regulation of fluid movement across all these cell membranes are similar. The cell membranes of the individual cells enclose the intracellular compartment, which actually consists of trillions of small compartments. The intracellular fluid compartment includes the fluid inside all the cells of the body. Water and ions move between these compartments, but their movement is regulated. Water and the ions dissolved in it are distributed in two major compartments: the intracellular fluid compartment and the extracellular fluid compartment (table 18.2). A smaller percentage of the body weight of an adult female consists of water because females generally have a greater percentage of body fat than do males. Because the water content of adipose tis-sue is relatively low, the fraction of the body’s weight composed of water decreases as the amount of adipose tissue increases. Approximately 50% of the total body weight of an adult female is water. Approximately 60% of the total body weight of an adult male consists of water.