Composition of Body Fluids
All body fluids - plasma, interstitial and intracellular have one common feature i.e. they are formed mostly of water. You would be amazed to know that despite, our solid appearance we are made mostly of water i.e. 70% water by weight. Of this 50% is intracellular; 15% is interstitial fluid water and the remaining 5% is blood plasma. Apart from water, body fluids contain many inorganic and organic substances. These are mainly inorganic electrolytes and proteins. Figure shows the difference in composition in intracellular and extracellular fluids. These differences are always maintained, despite continuous flow of materials in and out of cells.
Figure: Electrolyte composition of body fluids. Total concentration of each electrolyte is shown. Equal amounts of cations and anions are present in each fluid compartment. Na+ and Cl- are virtually absent from intracellular fluids where amount of proteins is much more.
Among lower invertebrates like cnidarians and flatworms that lack a circulatory system true 'blood' is not found. Only a clear watery fluid with a few phagocytes, some proteins and a mixture of salts similar to seawater is found. The higher invertebrates possess haemolymph which is more complex than the fluid found in lower invertebrates. In vertebrates, blood is a complex liquid tissue composed of plasma, and formed elements or corpuscles (red blood cells, white blood cells and platelets).