hi in this tutorial I'm going to talk about the distribution of drugs within body compartments in the previous tutorial I said that this video would be on the metabolism of drugs but I've decided that it's probably best to talk about the distribution of drugs before we move on to metabolism for the purposes of distribution we can consider that the body is made up of four major compartments which are really collections of fluid and numerous minor compartments these major compartments are the blood or strictly speaking the plasma fat extracellular fluid and intracellular fluid then there are numerous other minor compartments these include the cerebral spinal fluid the peritoneum sinovial fluid in joints and the fetus in pregnancy there are numerous others but for the purposes of this tutorial we'll consider these as one single compartment so if a drug is placed into one of the compartments we can see how it might move and distribute into other compartments often drugs are placed into the blood these are drugs which are given intravenously so when a drug is placed into a compartment it often Associates with binding molecules these binding molecules are often proteins such as albumin this process sequesters the drug within the compartment effectively increasing that compartment storage capacity and there is an equilibrium AED between the bound drug and the Unbound drug the Unbound drug within a compartment is then able to move into a new compartment and thus an equilibrium is maintained between adjacent compartments when a drug moves into a new compartment some of that drug is sequestered by binding molecules within that new compartment this process continues until there is a balance between each compartment and a balance between the amount of fre drug versus bound drug within each compartment the balance between each compartment is determined by a number of factors and this is best represented using an equilibrium constant if you've ever done Much Chemistry then you'll have definitely seen equilibrium constants before but if you haven't then all you need to know is that they are a number which shows whether the balance is in the forward direction or in the back Direction I'm not going to discuss equilibrium constants further here but I am going to talk about the factors that affect whether drugs would prefer to move into one compartment versus another thus I can say that the equilibrium constant is dependent on the permeability of barriers between compartments the pH within the compartment The Binding capacity within the compartment and numerous other factors such as the fat solubility of the drug which is especially important for those moving into the fat compartment now I'm going to talk about the volume of distribution which depending on the level of pharmacology you need to know may or may not be important the volume of distribution denoted v d is a property of the drug which shows how much drug needs to be within the body to get a certain concentration in the plasma it is defined as the total amount of drug in the body divided by the concentration of drug in the plasma I'll give you an example of how it might be used say I want to get a plasma concentration of morphine of 370th of a millgram per liter how much morphine do I have to give a 70 kg patient I know that the volume of distribution of morphine is about 5 L per kilog of body weight and I'm trying to achieve a plasma concentration of 370th of a MGR per liter well by rearranging the above formula I can see that the dose I need to give which in this case is the same as the total amount of drug in the body is the volume of distribution times the desired plasma concentration substituting in the numbers and solving shows that I need 157th of a milligram for every kilogram of body weight therefore for a 70 kg person I need a dose of 15 mg and that's a rundown of the distribution of drugs in the next video I will be talking about the metabolism of drugs and the cytochrome p450 system if you've enjoyed this tutorial please help us produce more by making a at www.h handwritten tutorial.com