Almost any foreign chemical as it gains entry into a biological system is immediately subjected to the protective machinery which either confines its translocation or converts it into non-reactive form or tries to eliminate it from the system.
As soon as the toxic agent enters the blood plasma from extra-cellular interstitial fluid, it is subjected to the following course of action within a biological system:
(a) The toxic agent is exposed to all parts of the body along with blood circulation.
(b) Many endogenous macromolecules, such as proteins, may effectively bind the toxic agent and make it ineffective. This binding is, however, incomplete in the sense that the reactive chemical remains in part in bound state and in part in Free State, the two being in equilibrium with each other.
(c) The toxic agent is acted upon by various enzyme systems present in different parts of the body or organs and altered or biotransformed into various metabolites.
(d) The toxic agent may accumulate in some parts of the body as a consequence of binding, active transport or high solubility in lipids and fats.
(e) The toxic agents and/or their metabolites, if any, may be subjected to excretory mechanism and gradually eliminated from the system.
The term biological half life is often used as a measure of the period of retention of a foreign substance inside a living system. It denotes the duration of time during which the concentration of the toxic agent is reduced to half of its original value. Depending upon the relative proportion of substance in the blood plasma and the concentration of the toxicant required saturating the system, the rate of process of detoxification and elimination shows lot of variations.
The concentration which nearly saturates the system are reacted upon and eliminated at faster rate than lower concentration. However, the ratio between the concentration of substances present in the system and the amount eliminated per unit time is fairly constant.
If the original concentration of 20 gm/1 is reduced to 10 gm/1 in one hour, a concentration of 2 gm/1 shall take the same time, one hour, to get reduced to half its original value. It is for this reason that biological half-life or the period within which half of the chemical agent is eliminated, is used for the purpose which gives a fairly accurate idea of the period needed to eliminate the toxic agent from a biological system.