Body Fluid Compartments
- Fluid within the body can be thought of as separated into several functional compartments divided by semi-permeable membranes which allow free movement of water but not of certain classes of solutes. In a healthy individual, fluid is divided between these functional compartments in certain ratios. Below we discuss how these compartments are defined, the nature of their separating barrier, and how much of the fluid volume each compartment contains in a healthy individual.
|Distribution of body fluids|
Roughly 60% of the total body weight is water. Two-thirds of total body water is held within cells as Intracellular Fluid (ICF). The remainder is within the extracellular space as the Extracellular Fluid (ECF). The ECF is itself divided between fluid within the vasculature itself, known as plasma, and that outside the vasculature, known as the interstitial fluid. Each of these compartments is separated by the barriers shown above with unique physio-chemical properties. The plasma membrane is permeable to water but not small solutes or proteins. The vascular wall is permeable to water and small solutes but not proteins.
|Intracellular Fluid Compartment|
- The Intracellular Fluid (ICF) refers to the fluid present inside cells and is considered the sum total of the fluid volume in all of the body's cells. The ICF is separated from the Extracellular Fluid (see below) by the plasma membrane of each, individual cell. Although the plasma membrane is permeable to water, it is highly impermeable to both ionic and small solutes as well as proteins. The ICF contains roughly two-thirds of the total body water or about 40% of total body weight.
|Extracellular Fluid Compartment|
- The Extracellular Fluid (ECF) refers the total volume of fluid outside of cells. As mentioned before, the ECF is separated from the ICF by the plasma membrane of each cell which is impermeable to nearly all classes of solutes. The ECF contains roughly one-third of the total body water or about 20% of total body weight. The ECF is sub-divided into two other fluid compartments known as the Interstitial Fluid and the Plasma. These compartments are separated by the vasculature.
- Interstitial Fluid
- The Interstitial Fluid refers to the total volume of ECF outside of the blood vessels and is separated from the plasma (see below) by the walls of the body's vasculature. As described in microcirculatory physiology, the vasculature allows free movement of water and small solutes but not of proteins. Consequently, the interstitial fluid is composed of roughly the same chemical composition as the plasma except without the proteins.
- The plasma refers to the total volume of ECF within the vascular space. As mentioned, the plasma is separated from the interstitial fluid by the vascular wall which is permeable to water and small solutes but not proteins. It should be pointed out that the plasma volume is not the same as the blood volume, as the volume of erythrocytes contributes nearly 40% of the entire blood's volume. Therefore, it should be remembered that both the ECF found in plasma and the ICF found in erythrocytes, contribute to the total blood volume.