The Lab / Core medicine / Lesson 17
Swallow a tablet and its level in your blood climbs, peaks and fades. Take it again and again and the level settles on a plateau. Build that curve one piece at a time, then watch what liver enzymes, weaker kidneys and a vein route do to it.
Educational, not for clinical use
Build the idea
Pharmacokinetics is what the body does to a drug over time. A swallowed drug passes from the gut (stomach and intestines) into the blood, spreads around the body, and is broken down by the liver or sent out in urine by the kidneys.
Build the idea
What matters is the drug’s blood level: how much drug there is in each drop of blood. Test a drop now and then, mark each result against time, and the marks trace a curve. Every picture in this lesson is that curve.
Build the idea
A swallowed tablet dissolves in the gut, and the drug crosses the gut wall into the blood: absorption. While drug comes in faster than the body removes it, the level climbs. It peaks when the two balance, then falls.
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Blood from the gut reaches the liver first. Its enzymes, proteins that speed up chemical changes, break some drug down (metabolism) on this first pass. The share that reaches the blood unchanged is the bioavailability. Dotted curve: none lost.
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From the blood, drug spreads into the body’s tissues: distribution. A drug that spreads widely leaves less in the blood, so the same amount peaks lower. Less in the blood also means slower removal, so the dotted curve falls slowly and ends higher.
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Drug in the blood keeps passing the liver, which breaks it down by metabolism, and the kidneys, which pass it into urine: excretion. Together they are elimination. How quickly they clean drug out of the blood is its clearance.
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The body removes a fixed share of what is there, not a fixed amount, so the fall slows as the level drops. The half-life is the time for the level to fall by half. Each half-life halves it again: half, a quarter, an eighth.
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Each bar is the drug left after one more half-life. The bars shrink fast: after a few half-lives so little remains that the drug counts as gone. A longer half-life stretches the whole fall out over more time.
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Many drugs are taken at regular times. Each new amount arrives before the last has gone, so it adds to what is left: build-up. The level does not climb forever, because more drug in the blood means faster removal.
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In time, each gap removes as much as each new amount adds. The level then rises and falls around a flat line: steady state. Getting there takes a few half-lives, so a short half-life gets there sooner; slow absorption adds a little.
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A drug helps only when its level is high enough, and harms when it is too high. Between the two lies its working range. The aim is a steady state that stays inside it, neither dipping below nor peaking above.
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Absorbed, trimmed by the first pass, spread out, then removed: the curve sums it all up. Three measures tell the story: how much reaches the blood, the half-life, and the steady-state level. Next, see what changes them.
What changes it
Enzyme inducers, such as the herb St John’s wort, switch up the liver enzymes that break other drugs down. Less survives the first pass and the rest is cleared faster: a shorter half-life and a lower steady state that can sink below the working range.
What changes it
Enzyme inhibitors, such as grapefruit or the antibiotic erythromycin, slow the liver enzymes that break other drugs down. More survives the first pass and the rest lingers: a longer half-life, a later and higher steady state that can climb above the working range.
What changes it
When kidney disease leaves the kidneys passing less drug into urine, that is reduced kidney clearance. A drug the kidneys remove gets a longer half-life: it builds for longer, so steady state comes later and higher, and can rise above the working range.
What changes it
Intravenous means into a vein. The drug skips absorption and the liver’s first pass, so all of it reaches the blood at once: each level jumps straight up and peaks higher. Removal is unchanged, so the half-life stays the same.
Start with Next, or use the arrow keys. Curves are illustrative shapes, not measurements.
Loading reference values…
Normal teaching baseline, by mouth.
The blue line on each bar, and the large number beside it, show where an illustrative teaching model sits: not a patient measurement and not a sourced value. Shaded bands and black typical-value marks are the reference values quoted under each row. Selecting a drug, condition or route shows the expected direction of each value from the quoted text and applies that state to the model.
How the half-life rows are read: the chapter’s numbers are counted in a drug’s own half-lives in a healthy body (“assuming a physiologically healthy individual”). The model keeps counting time in that usual half-life, so when a drug or disease changes the half-life, the markers show how much more or less is gone by then, and how much sooner or later steady state arrives.
How the kidneys filter the blood is taught in Lesson 2: The Nephron and Diuretics.
An illustrative teaching model, not a real drug. Times are in model hours and levels in model units; nothing here is an amount to give anyone. Change how the drug is given and how the body handles it. The dashed curve is the normal teaching drug; the solid curve is your settings.
One compartment, first-order everything. After one amount given by mouth, the level is F × ka ÷ (Vd × (ka − k)) × (e−kt − e−ka·t); straight into a vein it is e−kt ÷ Vd. Amounts given at regular intervals simply add up. F is the share surviving the first pass, ka the absorption rate, Vd how widely the drug spreads and k the elimination rate.
Clearance is the liver’s part plus the kidneys’ part, CL = CLh + CLr. The half-life is proportional to Vd ÷ CL, the form of Equation 3 in the StatPearls half-life chapter (quoted with its constant in the Clinical values tab), scaled so the normal teaching drug has a half-life of six model hours. The plateau’s average level is F ÷ (k × Vd × interval).
Steady state is counted as reached once the level stays within one twentieth of its settled level at every point between amounts, from then on. The model works this out from the repeated curve itself, absorption included: a drug that is absorbed slowly takes longer to settle than its half-life alone suggests. The one-twentieth rule is this model’s choice, not a sourced value.
Set both clearances to zero and nothing removes the drug: there is no half-life, a single amount stays in the blood for good, and amounts given at regular intervals keep adding up with no steady state.
The working range band is an illustrative zone drawn around the normal teaching drug’s plateau, not a real drug’s range. The lesson states change the model in the cited direction by illustrative amounts: an inducer raises liver clearance and first-pass loss, an inhibitor lowers both, reduced kidney clearance lowers the kidney part, and the vein route gives the whole amount at once. Left out: protein binding, a second tissue compartment, saturable (zero-order) removal, active breakdown products and every real difference between people. Nothing here is a prediction for anyone and nothing here is a treatment plan.