One of the most important concepts in toxicology is the dose or the overall amount of a toxicant an organism is exposed. Any chemical can be toxic if the dose is appropriately high enough since the dose makes the chemical toxic. Therefore it is important to consider not only whether a substance is toxic, but also the amount of the substance that is necessary to cause harm. There are many factors that affect an organisms' response to a toxic chemical. These include the dose, uptake into the organisms' body, distribution within the body, biotransformation, and elimination, (Hodgson, 2010).
The primary routes of exposure for toxicants are through dermal (skin), respiratory (inhalation), gastrointestinal tract (ingestion), and injection. Biotransformation, storage, and elimination are influenced by the path in which a toxin takes through the body as well as the time involved in the process, (Hodgson, 2010). Toxins that are absorbed through the gastrointestinal tract are transported to the liver, then the heart, then the lungs and on to other organs. Toxins absorbed through the lungs or skin enters the blood where they are transported to the heart and then are distributed to various other organs before reaching the liver. .
Distribution occurs when a toxicant is absorbed and enters the blood or lymph for transport to other areas of the body. Distribution occurs by means of the cardiovascular circulatory system and the lymphatic systems. There are many factors that affect distribution that include chemical and physical properties of a toxicant, level of concentration of toxicant in the tissues and blood, the volume of blood flow through the tissues, target tissue of toxicant, and barriers to the toxicant, (Thrush, 2008). .
Regardless of their route of entry, toxicants must first pass through a cell membrane or cell wall separating the external environment from the internal biological environment.