![]() ![]() For a complete diagram of body fluid compartments, see body fluid compartments of a 70-kg man and body fluid compartments of a 55-kg woman. Note that this diagram places focus only on these three major fluid compartments. Plasma is the smallest fluid compartment (~8% of total body water). Interstitial fluid contains ~25% of the total body water. The intracellular fluid compartment contains most of the water in the body (~67% of total). The right diagram shows the three major fluid compartments drawn to scale. The left diagram allows for a better demonstration of the relationship between the intracellular fluid, interstitial fluid, and plasma, however, the relative size of each of the compartment is not drawn to scale. Waste products produced by cells follow the reverse path from the cytoplasmic compartment to plasma. They then must cross the plasma membrane to enter the cytoplasmic compartment of cells. Nutrient molecules traveling in the blood must first cross the capillary endothelium to enter the interstitial fluid. The capillary endothelium is the physical barrier that separates the interstitial fluid from plasma. The physical barrier separating the intracellular fluid compartment (i.e., cytoplasm) and the interstitial fluid is the cell plasma membrane. Fluid, molecules, and ions flow across physical barriers between the fluid compartments. These are the (1) intracellular fluid compartment, (2) interstitial fluid, and (3) plasma. Therefore, it should be remembered that both the ECF found in plasma and the ICF found in erythrocytes, contribute to the total blood volume.In the human body plan, there are three major fluid compartments that are functionally interconnected. 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. 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. The plasma refers to the total volume of ECF within the vascular space.Consequently, the interstitial fluid is composed of roughly the same chemical composition as the plasma except without the proteins. As described in microcirculatory physiology, the vasculature allows free movement of water and small solutes but not of proteins. c: half of the total fluid is within our cells and half is outside our cells. b: two-thirds of the total fluid is within our cells and one-third is outside our cells. 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. A comparison of the two major body fluid compartments reveals that a: two-thirds of the total fluid is outside our cells and one-third is within our cells.These compartments are separated by the vasculature. The ECF is sub-divided into two other fluid compartments known as the Interstitial Fluid and the Plasma. The ECF contains roughly one-third of the total body water or about 20% of total body weight. 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 Extracellular Fluid (ECF) refers the total volume of fluid outside of cells.The ICF contains roughly two-thirds of the total body water or about 40% of total body weight. Although the plasma membrane is permeable to water, it is highly impermeable to both ionic and small solutes as well as proteins. The ICF is separated from the Extracellular Fluid (see below) by the plasma membrane of each, individual cell. ![]()
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