The body fluids are distributed in three compartments. One is within the cells, the intracellular fluid and the other two, outside the cells and forming the extra cellularfluid and includes interstitial fluid and blood plasma. Between these last two compartments the exchange of diffusible substances is easy and continuous, so they have a similar content of water and salts. Blood is made up of a liquid, the blood plasma, in which ells (erythrocytes, leukocytes, and platelets) and minute particles are suspended. When blood coagulates, it first becomes solid clot and then a fluid oozes out from clot, which is called the serum.
The internal environment
Muticellular organisms are surrounded by an external environment- air or water- but their cells live in a fluid environment which Claude Bernard in 1878 named the “milieu interior” (internal environment). This milieu is formed by the extra cellular fluids, i.e.
- The interstitial or tissue fluids, which bathe the cells and circulate slowly
- The lymph contained in the lymphatic vessels, which circulates between tissues and blood
- The blood plasma, which circulates rapidly through all organs of body.
The cerebrospinal fluid of central nervous system, the aqueous humor of eyes, and the fluid in the pleural and peritoneal cavities, joints, and the synovial sheaths are particular forms of interstitial fluid.
The blood is the part of milieu interior that circulates rapidly within a closed system of vessels. An outstanding feature of blood is its uniform chemical composition and physical properties, thus assuring constant conditions for the functioning of the cells. The blood is being continuously renewed by incoming and outgoing cells and solutes. The functions of organism are regulated so as to maintain the stability of the internal environment which is a physiological fact of great importance pointed out Claude Bernard.
The fluid compartments in healthy, normal men and women differ, because on weight basis, the female body contains more fat. The body water is effectively compartmentalized into two major divisions:
- Intracellular fluid (ICF): means the fluid present inside cells. It comprises about 2/3 of the body water. If your body has 70% water, ICF is about 50% of your weight. The ICF is primarily a solution of potassium and organic anions, proteins etc.
- Extra cellular fluid (ECF): means the fluid present outside the cells. Remaining 1/3 of body water is ECF. ECF accounts for 20% of body weight. ECF is primarily a NaCl and NaHCO3 solution. ECF is further subdivided into three compartments:
- Interstitial fluid (ISF): it surrounds the cells but does not circulate. It comprises about ¾ of the ECF (15% of body weight).
- Plasma: circulates as the extra cellular component of blood. It makes up about ¼ of the ECF (5% of body weight).
- Transcellular fluid: is a set of fluids that are outside of the normal compartments. Digestive juices, cerebrospinal fluid, mucus etc comes under this category.
All the body fluid compartments are in osmotic equilibrium (except for transient change). The ions and small solutes that constitute the ECF are in equilibrium with similar concentrations in each sub compartment. The ECF volume is proportional to the total sodium content.
Measurement of fluid compartments: In a living human it is possible to estimate various fluid compartments with fair accuracy. Very simple dilution principle is applied for these estimations. A known amount (A) of specific indicator substance is injected in blood and allowed to disperse evenly into specific compartment to be measured. A sample of blood is obtained after some time to analyze the extent to which the indicator is diluted. Volume of fluid compartment is calculated after dividing quantity injected (A) by concentration obtained in blood sample (B). Different indicators are used to estimate different fluid compartments simultaneously.