4. Pharmacokinetics

Pharmacokinetics can be split into

• Absorption

• Distribution

• Metabolism

• Elimination

Absorption

the transportation of unmetabolized drugs form the site of administration to the circulatory system

Bioavailability

the extent and rate at which an active drug is absorbed from the medicine and reaches systematic circulation

Distribution

disbursement of an unmetabolized drug as it moves through the body’s blood and tissue

Drugs distribute in different amounts into different parts of the body

depending on their chemical structure and other factors

Metabolism

the chemical alteration of drugs by various bodily systems to create components which are more easily excreted from the body

metabolism mostly occurs in the liver

via enzymes and is an evolutionary (protective) measure

first pass metabolism

where absorbed drug FIRST enters the portal vein which leads to the liver before it can then enter systematic circulation

Elimination

the removal of the administered drug from the body in metabolised/ unmetabolised form mainly via the kidneys, sweating, tears, saliva, resporation, milk,faeces and bile

In pharmacokenetics,

aside from absorption, the other processes occur simultaneously 

absorption: paracetamol

Tablet swallowed
Tablet disintegrates in the stomach,
where the drug (paracetamol)
dissolves in gastric fluids i.e. drug is in
solution

Drug (paracetamol) in solution passes
into the small intestine
Drug absorbs across the wall of the
small intestine

distribution: paracetamol

. Some of the absorbed paracetamol
reaches systemic circulation (i.e. blood
vessels) straight away
2. Via blood vessels, the drug then
‘distributes’ into extracellular fluids (i.e.
leaves blood vessels) where it can interact
with receptors
3. This drug-receptor interaction exerts the
pharmacological effect

metabolisation: paracetamol

Some of the paracetamol is metabolised in
the gut wall after it crosses the gut wall
2. Some of the absorbed paracetamol is
metabolised in the liver before it even
enters systemic circulation (i.e. blood
vessels)
3. Most remaining paracetamol which
reaches systemic circulation (i.e. blood
vessels) is eventually metabolised by the
liver

elimination: paracetamol

. Of the administered dose, all is excreted
via the kidneys as urine
2. In the urine, 95-98% of the original dose
are metabolites, with 2-5% the original
paracetamol molecule