1. Introduction, Absorption of Drugs

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Courtesy: Allianze University College Of Medical Science

´╗┐INTRODUCTION

General Pharmacology 

Pharmacology is the science dealing with biochemical and physiologic aspects of drug effects, including absorption, distribution, metabolism, elimination, toxicity doses, and specific mechanisms of drug action. Pharmacology includes three major divisions: theoretical (general), experimental, and clinical. Theoretical pharmacology touches upon common regularities of interactions of drugs with an organism. Experimental pharmacology investigates drugs influence on the organism of animals. Clinical pharmacology examines drugs influence on the organism of patient. Pharmacotherapy studies the use of medicaments for cure of a concrete illness. Some branches of pharmacology are different sciences: phytotherapy, toxicology, vitaminology, endocrinology, and chemotherapy. Pharmacology is closely connected with pharmacy. Pharmacology is based on the advances of physics, chemistry, biology, biochemistry, physiology for the explanation of drugs mechanism of action. Pharmacology is the basis for therapy and other clinical disciplines. The pharmacological effect is the changes of metabolism and function of cells. Mechanism of action is the way by means of which the initial reaction is realized. The initial pharmacological reaction is characterized by biochemical, physiological, physical and chemical changes of metabolism and function of systems and organs. The two main areas of pharmacology include pharmacokinetics and pharmacodynamics. Pharmacokinetics refers to the way the body handles drug absorption, distribution, biotransformation, and excretion. Pharmacodynamics is the study of biochemical and physiological effects of drugs and their mechanisms of action. Pharmacokinetics 

Drug transport. The movement of drug molecules in the body is subject to absorption, distribution, and excretion. Drugs can cross cellular membranes by various mechanisms. The mechanisms of absorption are similar to the mechanisms of membrane transport: passive diffusion, carrier-mediated diffusion, filtration, active transport, or pinocytosis. Being a bimolecular lipid layer, the cell membrane can also act as a barrier to some drugs.

Passive diffusion. Most compounds penetrate into cells by diffusing as the un-ionized moiety through the lipid membrane. Factors affecting the passage of a molecule through a membrane are the molecule’s size and charge, the lipid-water partition coefficient, and the concentration gradient. The two types of passive drug transport are simple diffusion and filtration.

Simple diffusion. Simple diffusion is characteristic of organic acids and alkaline. The greater the concentration gradient, the greater the rate of absorption. 

The larger the absorbing surface, the greater the drug flux. The diffusion constant is directly proportional to the temperature and is inversely related to the molecular size. The greater the lipid-water partition coefficient, the greater the drug flux. In simple diffusion, molecules cross the lipid membrane in an uncharged form. The pH of the 

medium affects the absorption and excretion of a passively diffused drug. Acidum acetylsalicylicum and other weak acids are best absorbed in the stomach because of its acidic environment. Alkalinic drugs are best absorbed in the small intestine, which has a higher pH

Pinocytosis is typical of lipid soluble vitamin drugs. A vacuolar apparatus in some cells is responsible for this process. There exist both fluid-phase pinocytosis for substances such as sucrose and adsorptive-phase pinocytosis for substances such as insulin. 

Bioavailability is the relative rate and extent by means of which a drug reaches the general circulation; this is especially important when a drug is administered orally. Factors that influence bioavailability are: solubility of the drug in the contents of the stomach, dietary patterns, tablet size, quality control in manufacturing and formulation.

Absorption is the rate at which a drug leaves the site of administration and the extent to which this occurs. The absorption of a drug through the mucosal lining of the gastrointestinal tract or through capillary walls depends on the physical and chemical properties of the drug. 

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