In some animals, including mammals, the extracellular fluid can be divided into two major subcompartments, interstitial fluid and blood plasma. The extracellular fluid also includes the transcellular fluid, which comprises about 2.5 percent of the ECF.
In humans, the normal glucose concentration of extracellular fluid that is regulated by homeostasis is approximately 5 mM/L, and the pH is tightly regulated by buffers around 7.4. The volume of ECF is typically 15 L, of which 12 L is interstitial fluid and 3 L is plasma. Interstitial fluid makes up 16% of human body weight, and blood plasma, 4%.
Extracellular fluid, in conjunction with intracellular fluid, helps control the movement of water and electrolytes throughout the body. In order to maintain osmotic balance, the extracellular compartments of a mammal's body must be able to excrete and absorb water to and from the environment. Inorganic ions must also be exchanged between ECF and the external environment to maintain homeostasis. These exchanges occur across specialized epithelial cells and through a filtration process in the kidneys. Extracellular fluid allows a solute balance between the outside and the inside of the cell, or to produce a gradient (which is commonly used to initiate diffusion, osmosis, facilitated diffusion, active transport, etc.). Blood flow delivers oxygen to arterioles, capillaries, and venules and clears waste products of metabolism. A second flow of fluid important to organ function is filtration of plasma across the microcirculation into the interstitial lymphatic compartment of the ECF. Multiple mechanisms regulate this flow and the distribution of ECF between the plasma volume and interstitial lymphatic fluid compartments.
- Main cations:
- Main anions:
It is poorer in proteins, as compared to intracellular fluid.
- Ionic Composition
Na+ 150 (mM)
K+ 5 (mM)
Cl− 110 (mM)
Extracellular fluid is the body fluid that is not contained in cells. It is often secreted by cells to provide a constant environment for cellular operation.
- Interstitial fluid – the extracellular fluid bathing most tissues, excluding the fluid within the lymph and blood vessels.
- Blood plasma – the yellow or gray-yellow, protein-containing fluid portion of blood in which the blood cells and platelets are normally suspended.