- Nearly 60% of an individual's body weight is composed of water. Because a large variety of solutes are mixed in with this water, the composition is known as "Fluid". Balancing of fluid volume between different areas within the body is a key physiological imperatives and the basic principles which govern this balance are discussed here. Because fluid balance is largely determined by fluid osmolarity we review this topic first and then discuss how fluid within the body is functionally compartmentalized. In the final section we summarize the principles by which fluid shifts between these functional compartments.