How Drugs Move — Walkelia Fabián

Pharmacology · Case 09 · Learning Sample

How Drugs Move

Every time a patient swallows a tablet, four things happen.

Absorption enters blood

Distribution reaches tissues

Metabolism liver breaks it down

Elimination kidneys clear it

The drug dissolves in the gut, crosses the intestinal wall, and enters the bloodstream.

From the blood, the drug spreads into tissues throughout the body. A drug that stays in the blood stays concentrated. One that distributes widely becomes diluted.

Volume of
distribution.

Half-life.

Enzymes in the liver break the drug down. The kidneys filter what remains. How fast this happens determines the half-life.

Plot the concentration against time. It rises to a peak, then falls — Cmax, Tmax.

Too high → toxic. Too low → no effect.
The window is the space in between.

Pharmacokinetics is how you predict
where the curve lands.

Move the parameters. Watch the curve respond.

Parameters · Dosing

Dose D 400 mg

How much drug enters the body at once

Bioavailability F 80%

Fraction that reaches systemic circulation

Volume of distribution Vd 30 L

How widely the drug distributes into tissues

Half-life t½ 4 hr

Time for concentration to fall by half

Absorption rate k_a 2.0 /hr

Speed of absorption from the GI tract

Dosing interval τ 8 hr

How often the dose is repeated

Therapeutic Window

Min effective level 6 µg/mL

Max safe level 24 µg/mL

Concentration–Time Curve

—

C_max

peak concentration

—

T_max

time to peak

—

t½ effective

elimination

—

AUC (rel.)

total exposure

Tutor annotation

Adjust the parameters to start the simulation.

Reading the curve

C_max is the peak — the highest concentration the body reaches after one dose.

T_max is when the curve peaks — absorption rate vs. elimination rate.

Yellow band is the therapeutic window — drug must stay inside it to work without toxicity.

AUC is the total drug exposure — scales with dose and bioavailability.

This simulation uses a one-compartment oral PK model: C(t) = (F·D·k_a) / (Vd·(k_a−k_e)) · (e^−k_et − e^−k_at). All parameters, drug names, and clinical scenarios are fictional and for educational purposes only. This is not clinical guidance.

Walkelia Fabián