The document discusses the body's fluid compartments and fluid balance. It describes that the body has extracellular and intracellular fluid compartments, with the extracellular space consisting of plasma and interstitial fluid. Total body water is about 60% of body weight, with 20% being extracellular fluid and 40% intracellular fluid. Fluid balance is regulated to keep the osmolarities of intracellular and extracellular fluids equal. Edema occurs when there is excess fluid accumulation in the interstitial spaces, usually due to increased capillary fluid filtration from factors like increased capillary pressure, decreased plasma proteins, or increased capillary permeability.