Extracellular fluid
In cell biology, extracellular fluid (ECF) denotes all body fluid outside the cells of any multicellular organism. Total body water in healthy adults is about 50–60% (range 45 to 75%) of total body weight;[1] women and the obese typically have a lower percentage than lean men.[2] Extracellular fluid makes up about one-third of body fluid, the remaining two-thirds is intracellular fluid within cells.[3] The main component of the extracellular fluid is the interstitial fluid that surrounds cells.
Extracellular fluid is the internal environment of all multicellular animals, and in those animals with a blood circulatory system, a proportion of this fluid is blood plasma.[4] Plasma and interstitial fluid are the two components that make up at least 97% of the ECF. Lymph makes up a small percentage of the interstitial fluid.[5] The remaining small portion of the ECF includes the transcellular fluid (about 2.5%). The ECF can also be seen as having two components – plasma and lymph as a delivery system, and interstitial fluid for water and solute exchange with the cells.[6]
The extracellular fluid, in particular the interstitial fluid, constitutes the body's internal environment that bathes all of the cells in the body. The ECF composition is therefore crucial for their normal functions, and is maintained by a number of homeostatic mechanisms involving negative feedback. Homeostasis regulates, among others, the pH, sodium, potassium, and calcium concentrations in the ECF. The volume of body fluid, blood glucose, oxygen, and carbon dioxide levels are also tightly homeostatically maintained.
The volume of extracellular fluid in a young adult male of 70 kg (154 lbs) is 20% of body weight – about fourteen liters. Eleven liters are interstitial fluid and the remaining three liters are plasma.[7]
Oxygenation[edit]
One of the main roles of extracellular fluid is to facilitate the exchange of molecular oxygen from blood to tissue cells and for carbon dioxide, CO2, produced in cell mitochondria, back to the blood. Since carbon dioxide is about 20 times more soluble in water than oxygen, it can relatively easily diffuse in the aqueous fluid between cells and blood.[16]
However, hydrophobic molecular oxygen has very poor water solubility and prefers hydrophobic lipid crystalline structures.[17][18] As a result of this, plasma lipoproteins can carry significantly more O2 than in the surrounding aqueous medium.[19][20]
If hemoglobin in erythrocytes is the main transporter of oxygen in the blood, plasma lipoproteins may be its only carrier in the ECF.
The oxygen-carrying capacity of lipoproteins, reduces in ageing and inflammation. This results in changes of ECF functions, reduction of tissue O2 supply and contributes to development of tissue hypoxia. These changes in lipoproteins are caused by oxidative or inflammatory damage.[21]