Data viva — concentration-time curves and the context-sensitive half-time

Stimulus

Two graphs are shown. The first is a log-concentration-versus-time plot after an intravenous bolus, showing a bi-exponential decline with a steep alpha (distribution) phase and a shallow beta (elimination) phase, with the rate constants k12, k21 and k10 indicated. The second plots context-sensitive half-time (vertical, minutes) against infusion duration (horizontal, hours): a flat line at about 3 minutes labelled remifentanil, and rising curves labelled fentanyl, thiopental and alfentanil. Define Vd, CL and t1/2 from the first graph, explain the alpha and beta phases, and explain the shape of the CSHT curves.