Amount and Composition of Body Fluids
Approximately 60% of a typical adult’s weight consists of fluid (water and electrolytes). Factors that influence the amount of body fluid are age, gender, and body fat. In general, younger people have a higher percentage of body fluid than older people, and men have proportionately more body fluid than women.
Obese people have less fluid than thin people because fat cells contain little water. Body fluid is located in two fluid compartments: the intracellular space (fluid in the cells) and the extracellular space (fluid outside the cells). Approximately two thirds of body fluid is in the intracellular fluid (ICF) compartment and is located primarily in the skeletal muscle mass. The extracellular fluid (ECF) compartment is further divided into the intravascular, interstitial, and transcellular fluid spaces. The intravascular space (the fluid within the blood vessels) contains plasma. Approximately 3 L of the average 6 L of blood volume is made up of plasma. The remaining 3 L is made up of erythrocytes, leukocytes, and thrombocytes. The interstitial space contains the fluid that surrounds the cell and totals about 11 to 12 L in an adult. Lymph is an example of interstitial fluid. The transcellular space is