3. Distribution of drugs.

   Distribution in pharmacology is a branch of pharmacokinetics which describes the reversible transfer of a drug from one location to another within the body.

Once a drug enters into systemic circulation by absorption or direct administration, it must be distributed into interstitial and intracellular fluids.

 Each organ or tissue can receive different doses of the drug and the drug can remain in the different organs or tissues for a varying amount of time.

The distribution of a drug between tissues is dependent on vascular permeability, regional blood flow, cardiac output and perfusion rate of the tissue and the ability of the drug to bind tissue and plasma proteins and its lipid solubility. 

pH partition plays a major role as well. The drug is easily distributed in highly perfused organs such as the liver, heart and kidney. 

It is distributed in small quantities through less perfused tissues like muscle, fat and peripheral organs.

Factors that affect distribution

Physical volume of an organism

Any drugs within an organism will act as a solute and the organism’s tissues will act as solvents.

The chemical characteristics of a drug will determine its distribution within an organism.

For example, a liposoluble drug will tend to accumulate in body fat and water-soluble drugs will tend to accumulate in extracellular fluids.

The volume of distribution (VD) of a drug is a property that quantifies the extent of its distribution.

VD can be defined as the theoretical volume that a drug would have to occupy (if it were uniformly distributed), to provide the same concentration as it currently is in blood plasma.

It can be determined from the following formula: Vd=Ab/Cp. Where Ab is total amount of drug in body and Cp is drug’s plasma concentration.

Removal rate   

A drug’s removal rate will be determined by the proportion of the drug that is removed from circulation by each organ once the drug has been delivered to the organ by the circulating blood supply.

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