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Medicine

How a medicine reaches and leaves the circulation

Archive 01182

Slide 1

A medicine must reach the site where it acts

A medicine must reach a relevant site before it can act. The body’s handling of it helps determine the exposure at that site.

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Pharmacokinetics studies this movement through time. Pharmacodynamics asks what the medicine does to the body, so exposure and effect are connected questions rather than interchangeable terms.

Slide 2

The route determines how it enters circulation

An oral medicine must be released and absorbed before reaching the general circulation. Other routes begin the journey differently.

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Some absorbed drug encounters intestinal and liver metabolism before circulating widely. This first-pass processing helps explain why equal amounts given by different routes need not produce equal exposure.

Slide 3

Blood does not give every tissue the same concentration

Blood carries a medicine through the body, but tissues do not necessarily receive the same concentration. Barriers and binding influence its distribution.

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Binding to blood proteins can change the freely available fraction. A total concentration measured in blood therefore does not directly describe every tissue’s exposure.

Slide 4

Metabolism changes a drug; excretion removes substances

Metabolism changes a medicine’s chemical form. Excretion removes substances from the body, often through urine or bile.

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A metabolite can be inactive or still biologically active. Some medicines are administered in a form that becomes active after processing, so chemical transformation can begin an effect as well as help end it.

Slide 5

Kidneys, liver and interactions change exposure

Kidney function, liver function and interactions can alter a medicine’s time course. The same administered amount need not behave identically in everyone.

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Repeated administration adds new material while earlier material is still being cleared. Understanding that overlap is one reason dosing schedules require clinical evidence rather than guesswork from a single dose.

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