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 (ie, 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 .

It is very important that the patient take the medicine as prescribed by the prescriber in order 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 in a concrete way.


Once the drugs have been administered and absorbed, they have to be distributed to your site of action. For some medications the application site is known 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 Intracel – lular 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 that allows the molecules of the drugs to unite. This varies widely among the drugs. Drugs destined for plasma proteins are pharmacologically inert; only the free medications are active. Some drugs do not bind (for example, caffeine) and some that are highly linked (for example, warfarin that is 99 percent bound to plasma proteins).
  4. Step through the barriers.  The two main 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 when absorbed into the stomach enter the circulation and pass through the liver. 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 is the action of the drug.

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 actually block the activation of the receptors. Partial agonists produce a response. However, this is less than what would be expected for a complete agonist drug.

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