Transfer across membranes drug metabolism

In the majority of instances the rates at which absorption, distribution, metabolism and excretion of a drug occur are directly proportional to its concentration in the body. In other words, transfer of drug across a cell membrane or formation of a metabolite is high at high concentrations and falls in direct proportion to be low at low concentrations (an exponential relationship). This is because the processes follow the Law of Mass Action, which states that the rate of reaction is directly proportional to the active masses of reacting substances. In other words, at high concentrations, there are more opportunities for crowded molecules to interact with each other or to cross cell membranes them at low, xmcrowded concentrations. Processes for which rate of reaction is proportional to concentration are called first-order. 

Oral bioavailability, a fraction of the oral dose that reaches blood circulation, is also a multimechanism phenomenon. It depends on the drug s solubility, chemical and metabolic stability, and membrane permeability. Most compounds are absorbed through passive diffusion across the membrane, while some are transferred by a carrier or by transporter molecules residing in the membrane. 

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