What is pharmacology?

What is pharmacology?

Simply, it is the study of medicines and their interaction with the body. Information is needed from the moment the drug enters the body and, indeed, how it got there, how it travels to the site of action, the physiological actions and their consequences, and how it is metabolised and excreted. This is all you need to know about ibuprofen for the adult in one paragraph, simple! However, this chapter herein avoids using this method for two reasons:

you should be making notes that require you to gather information from different sources, which will help you learn in more depth;

it is rather repetitive for me to write and you to read!

How may drugs get to their destination?

Drugs must be absorbed, i.e. they have to get from the outside in. The main routes of administration are:

oral;

sublingual;rectal;

epithelial surface application (skin, nasally, corneal, etc.);

inhalation;

injection (subcutaneous, intramuscular (i.m.), intravenous (i.v.), etc.).

These routes each have additional factors which will contribute to how well the drug is absorbed, for example:

blood flow;

gastrointestinal motility;

size of drug particle;

dissociation constant (kPa/physiochemical factors of the drug

Wherever they are heading, they will need to cross a membrane along their way. They will do this in one of the following ways:

diffusion through the lipid bilayer;

transmembrane carrier protein;

pinocytosis (invagination of the cell membrane creating an intracellular vesicle);

diffusing through a special aqueous channel.

The ionic status of a drug molecule is significant when considering membrane transit. To traverse the lipid bilayer, a molecule should be non-polar and unionised (without charge). As many drugs are weak acids or bases, this presents a complication. The pH of the environment will alter the ratio of ionised to unionised molecules, thus influencing the diffusion of the drug across the membrane. Simply, an environment (pH = acidic or basic) that pushes up the relative number of unionised species will increase its ability to permeate the membrane, thus more drug will be absorbed. If the drug is an acid, A-, (e.g. aspirin) in an environment with a low pH (e.g. the acidic gastric juices of the stomach, pH = 3) it will gain a hydrogen ion, H+,

and thus become unionised, AH, and cross the membrane. To clarify, an acidic drug is better absorbed in an acidic environment and a basic drug is better absorbed in an alkaline environment.

Blood flow may be influenced in your practice as a physiotherapist via heat, relaxation, massage, chest percussion, acupuncture, electrotherapy, etc. If the area to which the drug has been delivered has a good blood supply, for example gut, skin, mucosa, it will be absorbed more rapidly. Also, a drug that enters the systemic circulation but acts locally, for example ibuprofen, may have a higher concentration in the injured area if the blood flow is enhanced. It is always worth considering if you wish to influence this mechanism of drug absorption. An often-quoted scenario is the patient who has a transdermal morphine patch and is advised to apply heat to their shoulder. In doing so, the patient’s uptake of the drug was enhanced and experienced associated side effects.

The sympathetic response is part of the ‘fight or flight’ reaction. Part of the sympathetic effect is to decrease the gastric blood flow in favour of skeletal muscle and reduce peristalsis (this is the fight or flight response which is explained in exquisite clarity in Payne’s Handbook of Relaxation

Techniques: A Practical Guide for the Health Care Professional). By encouraging relaxation you will allow the parasympathetic nervous system to resume control and allow normal absorption of drugs and food from the gut Inhaled drugs (including oxygen), bronchodilators and cystic fibrosis transmembrane regulator therapy require the lungs to be operational enough to utilise the vast surface area and be clear enough so that excessive secretions are not blocking the epithelial transport of the drugs. The surface area and the capillary network allow rapid uptake of the drug, which makes inhalational therapy very useful indeed. Localised drug actions, such as dilating bronchioles, are useful as the lungs also expel the drug rather effectively, thus minimising the systemic effects. This is arguably the biggest influence that a physiotherapist can have on drug delivery, as not only can you mobilise secretions, you can facilitate the musculature to enable deeper breaths and expand more lung.

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