Distribution in Pharmacology is a branch of pharmacokinetics that describes the reversible transfer of one drug from one body to another. Once a drug enters the systemic circulation by absorption or direct administration, it must be distributed in the interstitial and intracellular fluids. Each organ or tissue can receive different doses of the drug, and the drug can stay in a variable amount of time for different organs or tissues. The distribution of a drug between tissues depends on vascular permeability, regional blood flow, tissue perfusion rate and the ability of the drug to bind tissue and plasma proteins to lipid solubility. The pH partition also plays a major role. The drug is easily distributed in highly perfused organs such as the liver, heart and kidneys. It is distributed in small, less perfused tissues such as muscles, fats and peripheral organs.