Give one intravenous bolus and watch the plasma level peak and fall through its rapid, intermediate and slow phases — while the drug fills the blood, vessel-rich and fat/muscle tanks. See how repeated sampling reveals three exponentials.
What you are watching: a single intravenous bolus fills the blood (central) compartment instantly, giving the peak. Plasma level then falls steeply as drug pours into the well-perfused vessel-rich tissues (rapid, α), more gently as it reaches muscle (intermediate, β), and finally along a shallow terminal slope governed by redistribution from fat plus true elimination (slow, γ). Sampling blood at many time-points (the dots) and plotting concentration on a log scale reveals three straight-line phases — the empirical fingerprint that let pharmacologists resolve the curve into C(t)=Ae–αt+Be–βt+Ce–γt and infer a three-compartment structure.