Pharmacology : Medical School Crash Course

Section 1 Chapter 1: Pharmacokinetics Welcome to Section 1 Chapter 1. This Chapter deals with the principles of Pharmacokinetics.

When you think of Pharmacokinetics you think of the flow of a drug through the body. This includes the absorption of the drug, the distribution of the drug, the metabolism of the drug and the elimination of the drug from the body. When you prescribe a drug, you will be directly or indirectly thinking of these principles of Pharmacokinetics.

For a drug to be absorbed, it may need to pass several cell membrane layers on its way to a therapeutic effect. This process involves consideration for the molecular size of the drug, the ionization of the drug, whether the drug is lipid soluble and if the drug binds to blood proteins.

Let’s discuss these cell membranes for a minute. There are four common ways for drugs to cross a membrane.

First, on a basic level, cell membranes are permeable to water. If a drug is not bound to a protein in the blood, then the drug can potentially use the movement of water through the cell membrane to help it cross over into the cell. This is the process of water diffusion. The drug is, in effect, carried across a membrane as water moves across. Some cells, as in the central nervous system, have tighter membranes and a drug being carried by water diffusion is not as likely.

Secondly, the process of moving a drug across a membrane can be aided by a concentration differential. If a drug on the outside of a membrane has a higher concentration, then it can move across the membrane to equalize the concentration. Factors involved in this movement are the concentration gradient, the drug’s solubility in the lipid layer of the membrane and the amount of the drug spread out over the surface area of the membrane. This is the process of using a concentration gradient.

A third way a drug can be transported across a membrane in an active fashion. This activity involves the use of energy from the cell to accomplish the transfer of a drug. Active transport is needed if a drug is to be moved against a concentration gradient. This is the process of active transport.

The fourth and least likely mechanism of transfer across a cell membrane involves something called facilitated diffusion. This facilitation does not involve the use of energy. This facilitated diffusion involves what can be called an assist by another substance that attaches to the drug and helps it cross the membrane. This is the process of facilitated transport.

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