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Bile, feces: drug & metabolites

Figure 2 Equilibria between blood and various organs, with liver and kidneys as excretion pathways. It is assumed that each binding is governed by a reversible equilibrium. At any one time, the free drug concentrations are assumed to be rapidly exchangeable. The liver (by biotransformation and bile excretion) and kidneys (by urine excretion) continuously disrupt the general equilibrium by removing some of the free drug. At steady-state, the free drug concentration is assumed to be identical in plasma and organs.

concentration (Cu) in all organs, which is considered to be instantaneously exchangeable with the plasma. However, this equilibrium is disrupted since clearing organs excrete a fraction of the free drug. The decrease in Cu is quickly compensated, and a new equilibrium is reached with a decrease in overall drug concentration (Figure 2). At the end of this cascade of equilibria, the entire dose is cleared from the body.

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