The transfer of drugs to the place of their action is strongly influenced by the biochemical properties of both the organism and the drugs. Since the drug is transported by blood, it is natural that those parts of the body that are more supplied with blood receive and more drug. Indeed, after the absorption of a drug, the heart, brain, kidneys, liver and other organs that require a lot of blood receive most of it. To parts of the body in which the blood circulation is not so intense (muscles, internal organs and fatty tissue), the drug comes much later. In addition to circulation, the transmission capacity of the membranes and tissues affects the nature of the spread. Fabrics with higher throughput get the drug faster.

Drug properties have a significant effect on distribution. The main example is solubility in fats. The better the drug dissolves in them, the easier it passes through the shells and quickly reaches its place of action.
The solubility of a substance in fats greatly depends on how much it enters the brain. The volume of blood passing through the brain is so large that the brain could become a real depot of drugs (and other chemicals) entering the body. But before any substance enters the brain, it must overcome the barrier. As mentioned in Chapter 3, this barrier serves as a blood filter, and removes toxins from it before they enter the brain. Filtering is based on the fact that the pores of the capillaries of the brain are very small and close to each other, which makes it impossible for foreign substances to pass through them. In addition, the capillaries are surrounded by a thin wall of glial cells that make up the second line of defense. If the drug is well soluble in fats, like benzodiazepine diazepam (Valium), then it easily passes through the capillaries,and through glial cell membranes. But these obstacles are insurmountable for less fat-soluble substances.

Another chemical property of drugs that affects their distribution is the ability to combine with elements of the body. For example, some drugs, such as barbiturates, interact with certain plasma proteins. The more selectively the drug reacts with the elements of the body, the slower it is transferred to the right place. Likewise, the chemical structure of some drugs makes them susceptible to reaction with the tissue of the body. In this case, the drug can be released from this tissue only after a long time. It may appear in the blood, but its release from adipose tissue is so slow that it has a very minor effect on the psyche. A good example of a drug that reacts with fat is marijuana. Due to the selective nature of the compound with the molecules of the body, it does not spread throughout the body,and its effect is weakened. Part of the dose taken can not quickly reach its place of action.

Thus, the processes of absorption and distribution of the drug show that the drug in the body violates the biochemical equilibrium and causes resonance throughout the system. Absorption and distribution are complex bases of bioavailability, that is, the amount of a drug that reaches its site of action. Bioavailability is very important when considering the effects of drugs on the body. To understand the long-term effects of drugs, you need to trace their destruction or elimination from the body.