Two halves of one story
Pharmacology splits into two questions. Pharmacodynamics asks what the drug does to the body — which receptor it grabs, what effect follows. Pharmacokinetics asks the mirror question: what the body does to the drug. This track is entirely about the second half. The body treats every drug like a stranger passing through: it lets the molecule in, moves it around, chemically rebuilds it, and shows it the exit.
The handy acronym is ADME: Absorption, Distribution, Metabolism, Excretion. Read it as a verb sequence happening to a single dose over time. Everything technical in later guides — clearance, half-life, dosing intervals — is just precise bookkeeping on top of these four plain ideas.
Walking the four steps
- Absorption — the drug crosses from where you put it (gut, muscle, skin) into the bloodstream. A swallowed tablet must dissolve, survive the stomach, and seep through the intestinal wall before it counts as 'in'.
- Distribution — blood carries the drug everywhere, but it does not spread evenly. Some collects in fat, some sticks to blood proteins, some reaches the brain and some is locked out. Where it goes decides where it can act.
- Metabolism — mostly in the liver, enzymes chemically rebuild the drug, usually into a more water-friendly form that is easier to flush away. This step can switch a drug off, or occasionally switch it on.
- Excretion — the body ejects the drug and its leftovers, mainly through the kidneys into urine, sometimes through bile into stool. This is the true exit.
Why this story matters at the bedside
A receptor cannot tell whether a molecule arrived from a pill, a patch, or a vein — it only senses how much drug sits in front of it, and for how long. Pharmacokinetics is what produces that concentration-versus-time picture. Get the kinetics wrong and even a perfect drug fails: too little reaches the target, or too much piles up and turns toxic. The single number that captures 'how much of the swallowed dose actually made it into the blood' is bioavailability, and it is where the next guide begins.