Question

write an essay on absorption, distribution, metabolism and excretion of drugs

Answer 1

Pharmacology is the study of drugs and how these drugs bring about changes in the body. This involves studying the absorption, distribution, metabolism and excretion of the drugs through various systems.

ABSORPTION-First and foremost, we must learn about the various routes of administration of drugs as they play and important role in the absorption. The faster a drug reaches the circulatory system i.e blood, the faster will the drug act. Hence, IV drugs which are directly injected into the blood stream act faster while swallowable capsules and pills take much longer as the disintegrate in the stomach and then eventually get absorbed by the blood.

The various routes of administration of drugs are can be broadly classified as Local and Systemic.

The site of action of the tropical medicine is localised to that specific area. Examples are- Tropical Anaesthetic gel.

Systemic have a non local effect. Acts centrally and can further be classified as,

• Oral. Examples are Capsules, pills, syrups
• Parenteral which could be injectable(example- Antibiotics, insulin injections for Diabetes control), Inhalational (example- Salbutamol inhalers) and Transdermal (example- Contraceptive skin patches, Nicotine patches)

Absorption takes place when the drug finally reaches the blood stream from the site of administration. For this to happen, the molecules of the drug should be able to cross the cell membranes. The various ways in which absorption takes place through the cell membrane are

1. Passive diffusion- Movement from high concentration to low concentration. The size of the drug molecule, pH and the lipid solubility affects the rate of absorption
2. Active Diffusion- the movement of drug molecule takes place against the concentration gradient which also means that energy has to be used to get these drugs from one place to another. The sites of absorption are limited to only a few specific parts of the intestine
3. Facilitative Passive diffusion- The drug combines with another molecules reversibly merely for the purpose of transportation across the membrane, passes along the concentration gradient and then detaches from the molecule
4. Pinocytosis- a very small portion of drugs get absorbed via this means of transportation

DISTRIBUTION- Once the drugs reaches the circulatory system, it should then travel in the blood to reach the carious acting sites where the distribution again depends upon the tissue binding, pH of that particular region, size of the molecule and the form in which the drug is circulating i.e bound or un-bound form. The cell membrane permeability and the perfusion of blood also affect the distribution in various sites.

If the flow of blood to a particular region is high, the distribution also would be high in such areas as well as the tissue mass and division between the tissue and the blood vessel. Drug distribution after it reaches the tissues manly depends on the perfusion in the interstitial fluids.

drugs that need to act on the central nervous system have to cross the blood brain barrier for its distribution which mainly consists of the capillaries in the brain and the cerebrospinal fluid. Very few drugs which are lip soluble can easily cross the blood brain barrier. The space between the cells of the brain are much more less, i.e the cells are in close proximation as compared to other parts of the body making it difficult for the water soluble molecules to cross.

METABOLISM- This process inactivates the drug make it in affective by chemically modifying it. The prime site of action for this process is the Liver and the enzymes required are present in high concentration in the liver.

Drugs can be metabolised by various ways such as oxidation, hydrolysis, reduction, conjugation, hydration etc. They mainly can the original structure and properties of the drug to make them inactive. One of the two things happen in drug metabolism, either the drug is chemically modified to form a new functional group or cleavage or the drug is altered very similar to the original drug with reduced therapeutic or toxicity level. the enzymes responsible for the metabolism of drugs in the liver is the group of cytochrome P-450 enzymes. The drug can either have the ability to be broken down by the enzyme or it can break the enzymes itself. If the former takes place, the affect of the increases and in the case of latter, the affect of the drug decreases.

EXCRETION- just like most waste products of the body need to be extrcreted so do the drugs. The excretion of drugs after its metabolism and chemical alteration, either takes place in the kidney as most other waste products or via the liver in the form of bile.

In the kidney, The drug metabolite is excreted in the form of urine. The functioning of the kidney, blood supply to kidney greatly affects this form of excretion.

Some form of excretion of the drug also takes place in the liver, where post the metabolisim, the product re enters the GI tract in the form of bile, where it either gets re-absorbed or excreted in the form of feces.

Sweat, saliva etc also contain a small amount of the excreted drug metabolite.