Pharmacodynamics Pharmacodynamics describes the many ways drugs affect the body. After being swallowed, injected, or absorbed through the skin, most drugs enter the bloodstream, circulate throughout the body, and interact with a number of target sites. For example, atropine, a drug given to relax muscles in the gastrointestinal tract, may also relax muscles of the eye and the respiratory tract as well as decrease sweat and mucous gland secretion. Other drugs are highly selective and affect mainly a single organ or system. For example, digitalis, a drug given to people with heart failure, acts primarily on the heart to increase its pumping efficiency.
Sleep aids target certain nerve cells of the brain.
Nonsteroidal anti-inflammatory drugs such as aspirin and ibuprofen are relatively selective because they act wherever inflammation is present. The answer lies in how they interact with cells or substances such as enzymes. Most cells have many surface receptors, which allow the activity of the cell to be influenced by chemicals such as drugs or hormones located outside the cell. A drug's selectivity can often be explained by how selectively it attaches to receptors. Nature probably didn't create receptors so that drugs would someday be able to attach to them.