Pharmacodynamics and Pharmacokinetics: Distribution, Main Elements and Affinity

This means that part of the drug remains active and inactive.

Often this depends on the pH of the solution (i.e., its acidity or Alka-salinity ) at which the drug dissolves. For example, a weak acid should not dissolve much if dissolved in an acidic environment. This means that the drug can pass through the membranes more actively than if it had dissolved in a neutral solution.

The patient must take medicine as prescribed by the prescriber to obtain the best therapeutic value. Therefore a nurse can explain to the patient the reason and the mechanisms for you to take the medications concretely.


Once the drugs have been administered and absorbed, they have to be distributed to your site of action. The application site is known for some medications, and such drugs are available for local or topical application. All other medications must be distributed throughout the body.

There are four main elements for this:

  1. Distribution in body fluids. These are mainly plasmatic, interstitial fluid, and Intracellular fluid. Molecular targets for drugs are those found in these areas.
  2. Absorption in the body tissues/organs. Specific tissue-specific tissues take some medications – for example, iodine and the thyroid gland.
  3. Plasma protein binding. Proteins such as albumin allow the molecules of the drugs to unite. This varies widely among the drugs. Medicines destined for plasma proteins are pharmacologically inert; only the free medications are active. Some drugs do not bind (for example, caffeine), and some are highly linked (for example, warfarin, which is 99 percent bound to plasma proteins).
  4. Step through the barriers. The two prominent examples are the placenta and the blood-brain barrier. Medications must be highly soluble in lipids to pass through these barriers, or they may not be able to reach their site of action.

Some medications enter the circulation and pass through the liver when absorbed into the stomach. This is known as the first-pass effect.

The action of the drug can also be affected by drug affinity.

The greater the affinity, the better the action of the drug.

The action of agonist and antagonist drugs:

The agonists activate the receptors to produce a response. Antagonists bind with receptors but do not activate or elicit a response. They can block the activation of the receptors. Partial agonists have an answer. However, this is less than expected for a complete agonist drug.


Reverse agonists are drugs that can reduce the regular activity of the cell. Competitive antagonists are drugs that prevent the activation of the cell by its average agent. Non-competitive antagonists are drugs that can block the recipient but not permanently.