Chapter 40 Toxicology
In the study of toxicology, a solid appreciation of pharmacokinetics and pharmacodynamics (see Chapters 14 to 19) is very important, as the mechanics of the two areas are very much a part of what makes a chemical harmful to a patient. Poisons or toxicants are chemicals that have harmful or adverse effects on living organisms. A chemical can be poisonous under one set of conditions and not under another. For example, potassium is a vital part of body metabolism, but too much will cause atrial fibrillation.
There are two different types of toxic reaction:
When dealing with chemicals, the following must be kept in mind:
The Occurrence of Poisons
Virtually all chemicals can be considered toxic under certain conditions, e.g. pure water when inhaled is rapidly absorbed across the lung alveoli to cause lysis of red blood cells. Poisoning is either:
Drug Absorption
The tissues that are most susceptible are those directly in contact with the environment, e.g. skin, mucous membranes of the lungs and gut.
Gut
This is a common route of entry. Different parts of the gut allow different rates of absorption:
Respiratory Tract
As much blood passes through the lungs as all the remaining parts of the body together. The lungs are therefore one of the most vascular and effective sites of absorption of the body. This is why it is possible for a patient to have an anaphylactic reaction from inhalation.
Skin
The total skin area of an average human is 2 m2; the skin is a relatively effective barrier to toxins. This is because the toxin has to reach the inner layer – the dermis – before it can diffuse into the bloodstream. The outer layer – the epidermis, through which the toxin must pass – is made up of dead, cornified cells and no active transport takes place. Only lipid-soluble substances will pass through the epidermis. This tends to occur at the sweat glands, sebaceous glands and hair follicles; however, these comprise only 1% of total skin’s surface area.
• Factors Affecting Toxin Absorption through the Skin
Distribution and Transport
The Role of Blood
Enzymes
Chemicals can interfere with the enzymes that are vital as catalysts of metabolic processes in the body (see Chapter 19 ‘Pharmacodynamics: how drugs elicit a physiological effect’, p. 137):
The Importance of Correct Cell Structure
An intact cell membrane is crucial. Sometimes, however, the cell membrane might not be functioning properly: