BMS 123 · Common Core (Tronc commun)Section 1.1 · Pharmacokinetics and drug absorption
Section 1 of 27
What pharmacokinetics studies
Define pharmacokinetics, list the four ADME processes in order, and explain why the concentration at the site of action decides onset, intensity and duration.
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Why this matters
Almost every drug is given outside the blood yet is expected to act on the whole body, so something must carry it there and something must remove it afterwards. Pharmacokinetics is the subject that explains that journey. Without it you cannot understand why the same tablet acts faster on an empty stomach, why an oral dose is often larger than an injected one, or why a failing kidney turns a safe medicine into a dangerous one. Every later unit of this course builds on the four words introduced here.
What you need to know first
No calculation is needed here. You only need the everyday picture of swallowing a tablet and expecting it to work, plus the idea that blood carries substances around the whole body.
Words to know (5)
Pharmacokinetics
The study of what the body does to a drug: absorption, distribution, metabolism and excretion. Following a tablet from the mouth to the urine is a pharmacokinetic study.
ADME
The four letters that name the four pharmacokinetic processes in order: absorption, distribution, metabolism and excretion. “The drug has good ADME” means it is absorbed, distributed, changed and removed in a predictable way.
Systemic circulation
The blood circulation of the whole body, which receives a drug after absorption and carries it to every tissue. A drug injected into a vein enters the systemic circulation at once.
Site of action
The place in the body where the drug must be present at the right concentration to produce its effect. For morphine the site of action is in the central nervous system, not in the stomach.
Clinical pharmacokinetics
The use of pharmacokinetic principles to choose and adjust drug treatment safely in a particular patient. Reducing the dose interval for a patient with kidney failure is clinical pharmacokinetics.
One idea at a timeIdea 1 / 5
In one sentence Pharmacokinetics is the study of what the body does to a drug after it has been given.
Pharmacokinetics is what the body does to the drug
Although most drugs are given outside the blood — swallowed into the gut or injected under the skin — the majority are intended to act on the whole body. To do that they must first reach the blood, which is the central fluid that receives them. Pharmacokinetics is the study of drug absorption, distribution, metabolism and excretion, remembered as ADME. It follows the drug from the moment it enters the body to the moment it leaves, and asks how much of it is in the body, where it is, and for how long.
For example
A paracetamol tablet swallowed for a headache is not in the blood yet. Pharmacokinetics follows it as it dissolves, crosses the gut wall, is carried by the blood, is changed in the liver and is finally removed by the kidneys.
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In one sentence, what does pharmacokinetics study?
Show the answer
It studies what the body does to the drug: absorption, distribution, metabolism and excretion, remembered as ADME.
Read detailed explanations
Why we measure plasma instead of the site of action
The site of action of a drug is usually inside an organ, so its concentration cannot be measured in a living patient. Pharmacokinetics therefore relies on a substitute: the concentration of the drug in plasma. The substitute is useful because, for most drugs, plasma and tissue concentrations rise and fall together once distribution has equilibrated. Two conditions make the substitute honest. First, only the free fraction of the drug — the part not bound to plasma proteins — can leave the blood and reach the target, so two patients with the same total plasma level may have different free levels if their protein binding differs. Second, equilibration takes time: immediately after an intravenous injection the plasma level is high while the tissue level is still low, so the plasma curve does not yet describe the target. This is why therapeutic drug monitoring reports a plasma level together with the time at which the sample was taken, and why the level must be interpreted against the patient’s protein status, liver function and kidney function rather than read as an isolated number.
Where the chain from dose to response can break
The path from a dose to a clinical response contains several links, and each one can fail for a different reason. The first link is pharmaceutical: a tablet that does not disintegrate or dissolve never becomes available for absorption. The second is absorption itself, which falls when contact time is short, blood flow is poor or the drug is unstable in the gut. The third is transport through the liver and gut wall before the drug reaches the general circulation, the first-pass effect, which removes part of an oral dose. The fourth is distribution: a drug may be carried away from the target, bound to plasma proteins or stored in fat. The fifth is elimination: fast metabolism or fast renal excretion shortens the effect, while slow elimination causes accumulation and toxicity on repeated dosing. Finally the target itself may answer poorly. Because these links are in series, two patients given an identical dose can end with very different concentrations and very different responses. Reading a treatment failure as one of these named links, rather than as a single vague problem, is the practical value of pharmacokinetics.
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What ADME means
One dose passes through four stages — absorption, distribution, metabolism and excretion — and together they set the concentration at the site of action.
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01 / 5Absorption
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Absorption
The passage of a drug from its site of administration to the systemic circulation through biological membranes. It decides how quickly the effect begins and how much of the dose is available to the rest of the body.
Schematic sequence, not to scale. Real drugs may be metabolised before distribution is complete, and some are excreted unchanged without any metabolism at all.
Key points to remember
✓Pharmacokinetics is the study of what the body does to a drug, remembered as ADME.
✓The order is absorption, distribution, metabolism and excretion, and each step limits the next.
✓The concentration at the site of action decides the onset, the intensity and the duration of the effect.
✓Clinical pharmacokinetics applies these principles to safe and effective treatment in real patients.
✓Pharmacokinetics fixes the concentration; pharmacodynamics turns that concentration into an effect.
Go deeper
Worked example
Following a paracetamol tablet from mouth to effect. A patient swallows a paracetamol tablet for fever. Half an hour later the fever begins to fall, and several hours later the effect has gone. You are asked to place each event at the right pharmacokinetic step.
Absorption: the tablet disintegrates, the paracetamol dissolves and crosses the gut wall into the blood, which is the systemic circulation.
Distribution: the blood carries the drug to the tissues, including the central nervous system where the temperature-regulating centre lies.
Metabolism: the liver changes much of the paracetamol into inactive conjugates that are easier to eliminate.
Excretion: the kidneys remove the drug and its metabolites in the urine, so the concentration falls and the effect ends.
The tablet, the blood level and the fall in fever belong to one single chain. The height of the concentration curve decides how strong the effect is, and the speed of elimination decides how long it lasts.
Common misconception
Beginners often think pharmacokinetics means “how the drug works”. It does not. Pharmacokinetics is what the body does to the drug; how the drug works on its target is pharmacodynamics.
Check your reasoning
A colleague says: “The patient took the full dose, so the drug must have arrived.” What can happen to an oral dose between the mouth and the site of action, and why does the tablet size not settle the question?
Reveal an answer
The tablet must disintegrate and dissolve, then cross the gut wall; part may be destroyed in the gut or liver before reaching the blood (first-pass metabolism); distribution, metabolism and excretion then lower the concentration over time. A swallowed dose is therefore never the same as a dose delivered to the target, which is why the concentration at the site of action, not the dose taken, decides the effect.
Where this comes from
BMS 123 General Pharmacology II lecture notes — Part 1, Principles of Pharmacokinetics · Introduction to pharmacokinetics; key learning points
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About this course and its sources
Bilingual lessons organised from the supplied BMS 123 General Pharmacology II lecture notes (Principles of Pharmacokinetics and Principles of Pharmacodynamics). Diagrams are original educational schematics; pharmacology, nursing and French-language review are pending.