The mode of exposure to the toxic substance determines the route of entry of the substance into the body. The mode of exposure is either by a physical contact like handling the substance or through inhaling the toxicant or through intake of the toxic substance. In case of physical contact with the substance the route of entry is skin and in case of inhalation the route of entry is lungs and gastrointestinal tract in the scenario of direct intake. Inspite of these multiple routes of entry the toxicant ultimately enters the blood stream. Besides all these routes the toxicant gains direct entry into the blood stream by injecting the substance either intravenously, intraperitoneal, subcutaneously or intramuscularly.
The skin which acts as the barrier between the body and the environment is a quite challenging route for the toxicants to enter the body. The epidermal layer of cells, the hair follicles, oil gland and sweat gland present on the skin are the possible route of entry for a toxicant to reach the blood stream. The entry through epidermal layer is difficult for the toxicants as it has to cross underlying layers such as the germinal layer and corium to reach the blood stream. The rate of diffusion of the toxicant is directly proportional to the permeable nature of the skin. Except for few toxics like acids and alkali makes the skin highly permeable by damaging the skin layer. The permeability of the skin is different for different species.
The toxic agents present in the air enter the body through respiratory tract upon inhalation. The toxic gases like carbon monoxide, sulphur dioxide and nitrogen dioxide and the volatile substances like benzene, the suspended particulate matter in the air enters the lungs through respiration. The absorption of these toxics in lungs is influenced by the larger area of the lungs and increased blood flow to the lungs. The quantity of the toxic gas in the air and the partial pressure of the same are the two factors governing the entry of the toxic gases through inhalation. The solubility nature of the toxic element decides the rate of diffusion of the substance from the lungs to the blood stream.
The next route of entry to be discussed is the gastrointestinal tract where the toxins make its entry via food chain. Also direct intake of toxic substance intentionally reaches the gastrointestinal tract. Thus the toxins reaching the GI tract is mostly harmless till the process of absorption of the toxin by the GI tract takes place. As the GI tract has several transport channels for the various nutrients, minerals and aminoacid present in the food, some of the toxins uses these transport system to reach the blood stream. The micro flora, acid and enzymes present in the GI tract are considered as important factors determining the fate of the toxin entering the GI tract. There is a possibility that the action of any of these factors on the toxic substance can reduce the toxicity of the substance. The rate of toxic element entering the blood stream from the GI tract depends upon the dissolution nature of the element.
Once entering the blood stream through different routes the toxicants either reaches their target organs or gets collected at various sites. The various such collection or storage units are the plasma proteins (albumin, globulin, and transferrin), the fat deposit of the body, the bone, the liver and the kidney. The toxicity of the element is organ specific and hence it need not be exhibited at the storage points.
Finally the distributed compounds are eliminated from the body by the proper functioning of the two vital organs, the liver and the kidney. The liver metabolizes the toxic element and releases into the bile for excretion and the kidney filters the blood and eliminates the toxins.
The amount of toxic substance, the length of exposure to the toxic substance, the age and health of the exposed individual are factors to be observed and analyzed to understand the effects of the toxin on the individual.