How Drugs Move through the Body !!

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How Drugs Move through the Body

The effect of a drug on the body depends on a number of processes that the drug undergoes as it moves through the body. All these processes together are known as pharmacokinetics (literally, "motion of the drug"). First in these processes is the administration of the drug after which it must be absorbed into the bloodstream. From the bloodstream, the drug is distributed throughout the body to various tissues and organs. As the drug is metabolized, or broken down and used by the body, it goes through chemical changes that produce metabolites, or altered forms of the drug, most of which have no effect on the body. Finally, the drug and its metabolites are eliminated from the body.

A. Administration

Depending on the drug and its desired effect, there are a variety of administration methods.Most drugs are administered orally—that is, through the mouth. Only drugs that will not be destroyed by the digestive processes of the stomach or intestines can be given orally. Drugs can also be administered by injection into a vein (intravenously), which assures quick distribution through the bloodstream and a rapid effect; under the skin (subcutaneously) into the tissues, which results in localized action at a particular site as with local anesthetics; or into a muscle (intramuscularly), which enables rapid absorption through the many blood vessels found in muscles. An intramuscular injection may also be given as a depot preparation, in which the drug is combined with other substances so that it is slowly released into the blood.

Inhaled drugs are designed to act in the nose or lungs. General anesthetics may be given through inhalation. Some drugs are administered through drug-filled patches that stick to the skin. The drug is slowly released from the patch and enters the body through the skin. Drugs may be administered topically—that is, applied directly to the skin; or rectally—absorbed through an enema (an injection of liquid into the rectum) or a rectal suppository (a pellet of medication that melts when inserted in the rectum).

B. Absorption

Absorption is the transfer of a drug from its site of administration to the bloodstream. Drugs that are inhaled or injected enter the bloodstream more quickly than drugs taken orally. Oral drugs are absorbed by the stomach or small intestine and then passed through the liver before entering the bloodstream.

C. Distribution

Distribution is the transport of a drug from the bloodstream to tissue sites where it will be effective, as well as to sites where the drug may be stored, metabolized, or eliminated from the body.
Once a drug reaches its intended destination, the drug molecules move from blood through cellular barriers to various tissues. These barriers include the walls of blood vessels, the walls of the intestines, the walls of the kidneys, and the special barrier between the brain and the bloodstream that acts as a filtration system to protect the brain from exposure to potentially harmful substances.

The drug molecules move from an area of high drug concentration—the bloodstream—to an area of low drug concentration—the tissues—until a balance between the two areas is reached. This process is known as diffusion. When a drug reaches its highest concentration in the tissues, the body begins to eliminate the drug and its effect on the body begins to diminish. The time it takes for the level of a drug to fall by 50 percent is known as the drug's half-life. Depending on the drug, this measurement can vary from a few minutes to hours or even days. For example, if a drug's highest concentration level in the blood is 1 mg/ml and this level falls to 0.5 mg/ml after five hours, the half-life of the drug is five hours. A drug's half-life is used to determine frequency of dosage and the amount of drug administered.

Distribution of a drug may be delayed by the binding of the drug to proteins in the blood. Because the proteins are too large to pass through blood vessel walls, the drug remains in the blood for a longer period until it is eventually released from the proteins. While this process may increase the amount of time the drug is active in the body, it may decrease the amount of the drug available to the tissues.

D. Metabolism and Elimination

While circulating through the body, a drug undergoes chemical changes as it is broken down in a process called metabolism, or biotransformation. Most of these changes occur in the liver, but they can take place in other tissues as well. Various enzymes oxidize (add oxygen to), reduce (remove oxygen from), or hydrolyze (add water to) the drug. These changes produce new chemicals or metabolites that may continue to be medically active in the body or may have no activity at all. A drug may be broken down into many different metabolites. Eventually, most drugs or their metabolites circulate through the kidney, where they are discharged, or eliminated, into the urine. Drugs can also be excreted in the body's solid waste products, or evaporated through perspiration or the breath.

E. Dose-Response Relationship

The extent of the body's response to a drug depends on the amount administered, called the dose. At a low dose, no response may be apparent. A higher dose, however, may produce the desired effect. An even higher dose may produce an undesirable or harmful response. For example, to relieve a headache most adults require two tablets of aspirin. A half tablet may provide no relief from pain while ten tablets may cause burning pain in the stomach or nausea.

The doses prescribed by physicians are those recommended by each drug's manufacturer to produce the best therapeutic, or medically beneficial, responses in the majority of patients. However, doses may need to be adjusted in certain individuals. For example, a person may be born without the enzyme required to metabolize a particular drug while other individuals may suffer from lung disorders that prevent them from absorbing inhaled drugs. Factors such as alcohol consumption, age, the method of drug administration, and whether or not the individual has taken the drug previously can affect an individual's response to a drug.

F. Receptors

Drugs interact with cell receptors, small parts of proteins that control a multitude of chemical reactions and functions in the body. Receptors have a specific, chemical structure compatible only with certain drugs or endogenous compounds—substances that originate within the body such as hormones and neurotransmitters. This relationship can be compared to that of a lock and key: A drug molecule—the "key"—attaches briefly to its specific receptor—the "lock" that only this molecule can open. The lock-and-key combination of the drug and receptor results in a cascade of chemical events. The extent of the response is determined by the number of receptors activated. Stimulation of only a few receptors may not produce a response while stimulation of a certain number of receptors is needed to produce the desired effect.

IV. Therapeutic Responses and Adverse Reactions

The same receptors can be found in different tissues and organs in the body, but receptors produce different responses depending on their location. As a result, a specific drug can affect the body in more than one way. Desirable effects are called therapeutic or beneficial responses. Undesirable or harmful effects are called adverse reactions. Some adverse reactions, or side effects, can be predicted. The most common side effects are drowsiness, headache, sleeplessness, nausea, and diarrhea. Other reactions, such as those that occur only in specific individuals for unexpected reasons, called idiosyncratic reactions, and those that occur with the triggering of the body's immune system, called allergic reactions, are less predictable.

Drug toxicity, or poisoning, can occur when drugs are given in too large a dose or when individuals take a particular drug over a long period of time—the drug may build up to dangerous levels in the kidneys and liver and damage these organs. For some drugs, such as those used to treat epilepsy, the difference between therapeutic and toxic concentrations is small. Physicians constantly monitor the precise levels of such drugs in an individual's bloodstream to prevent drug poisoning.

Other drugs, such as those used to treat cancer, are known to have toxic effects; however, the benefits outweigh the risks—that is, treatment without them may result in death.

A. Drug Interactions

When taken together, drugs can interact with one another and produce desirable or undesirable results. Some drugs have an additive effect—that is, they increase the effect of other drugs. For example, alcoholic beverages intensify the drowsiness-producing effect of some sedatives. Drugs that displace, or take the place of other drugs present in blood proteins, make the displaced drugs more active in the body, increasing their effect. Other drugs have a reducing effect—that is, they interfere with the action of drugs already present in the body. For example, antacids prevent antibiotics from being absorbed by the stomach. Some drugs combine with other drugs to create a substance that has no medical benefit. In some cases, however, drug interactions can produce desirable results. Doctors have found that using three drugs to fight AIDS is more effective than one drug used alone.

Drugs are most effective when properly prescribed by physicians and taken correctly by patients. Missing doses, taking drugs at the wrong time of the day or with instead of before meals, and stopping drug use too soon can markedly reduce the medical benefits of many drugs.

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