The outermost layer of the body of an organism, be it a cell wall or a multilayered epidermis, is the main barrier which separates a biological system from the outside environment. Even through this layer a number of toxic agents can enter. In addition, there are a number of routes through which a toxic agent can enter a living system.
1. Through the Epidermis:
The skin or epidermis comes in contact with a number of toxic agents. In higher animals fortunately epidermis acts as an efficient lipoid barrier which separates a biological system from its environment. However, there are some chemical agents which can pass through this barrier and cause damages to the system. The capacity of the toxic agent to dissolve in lipids and fats appears to be an important factor in determining their permeability through the skin. For example; carbon tetrachloride can be absorbed through the skin in sufficient concentration to produce liver toxicity.
Stratum corneum, the outermost corny layer of the skin plays an important role in determining the permeability of the epidermis. Abrasions or chemical treatment of this layer raises the permeability of skin significantly. There are found considerable differences in skin permeability in different species. It is this feature which accounts for the differential toxicity of certain insecticides to various groups of animals. DDT, for example, can enter insects more quickly through the chitinous exoskeleton and cause immediate damages while in mammals it is unable to do so. LD50 of DDT in insects and mammals are almost similar when the insecticide is injected but it is considerably less toxic to mammals than insects when sprayed over the skin.
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2. Through Respiratory Track:
Toxic agents absorbed by lungs are usually gases, vapours of various volatile compounds, fine particulate matter, dust and smoke etc. This is a very efficient route of entry of toxic agents as the surface area for absorption is large and blood flow in close proximity of alveolar spaces is very high. Rather quick toxic responses result when poisonous gases, vapours, smoke or dust are inhaled. Carbon monoxide poisoning and silicosis are caused by the entry of toxic substances through this route.
3. Gastro-intestinal Track:
Gastro-intestinal track may be regarded as a tube going through the body. Its contents, although within the body may be treated as exterior to it. Toxic substances present in the intestines do not produce any harm unless they are caustic or irritating to the walls of the gastro-intestinal track. For producing systemic effects the toxic agent has to be absorbed ill the blood stream.
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Stomock contains a strong acid and there are several enzymes as well. A rich microbial population flourishes in the intestinal track. Cumulatively these factors may alter the toxic agent drastically and render it ineffective. For example, snake venom is relatively non-toxic when givers orally as enzymes of the gastro-intestinal track decompose it. Similarly at times harmless substances may be transformed into harmful ones by the low pH, the enzymes present and the microbial population inside the track.
For example, intestinal flora can reduce aromatic nitro-compounds to amines which are goiter-forming or carcinogenic agents. Apart from to transformations, most of the material which is absorbed from the intestines has to pass through hepatic portal system and liver, wherein an efficient enzymatic machinary exists to bring about the biotransformation. The toxic agent, therefore, often gets detoxified and converted into harmless metabolites before it is allowed to pass into blood stream or else it may be excreted via bile into the intestines again.
4. Through Special Routes:
In addition to these common routes of entry, a toxic agent may be administered by means of injections. These may be subcutaneous, intramuscular, intra-peritonial, or intravenous. These specialized routes produce quick responses and may serve to by-pass internal barriers or machinery for their inactivation existent within a biological system. In patients suffering from violent emesis and gastric hypermotility medicines are administered via intramuscular or intravenous injections as oral administration of drugs causes’ immediate vomitting.