 Volume of distribution, or VD, represents how much of a drug will stay in the blood plasma and how much will move into the surrounding tissue. A drug with a low VD will mostly stay in the blood plasma. A drug with a high VD will mostly move out into tissue with little circulating in the plasma. Again, eventual drug in plasma versus eventual drug that ends up in the tissue. So it's usually given as a number of litres. For paracetamol, it's around 50 litres. So here we've got a 50 litre box filled with water. We're going to crack open this little 500mg capsule of paracetamol and empty it in. If that paracetamol then spread out perfectly evenly, we'd have 500mg across 50 litres or 10mg per litre. If we gave this person here 500mg of paracetamol and then wait an hour or so, the eventual plasma concentration would be about 10mg per litre. That's interesting because we've only got around 3 litres of plasma in us, which would only carry a total of 30mg of paracetamol. So this person has ingested 500mg. 30 of it has ended up in the plasma. The other 470mg has ended up spread out through the tissue. So you've got a nasty headache. We reckon a paracetamol plasma concentration of 20mg per litre will do the trick to fix up that headache. We times the plasma concentration that we want by the volume of distribution we find out what dose we should give. So 20mg per litre times 50 litres, cancel out the common unit and you end up with 1000mg or 2 average tablets of paracetamol. Chuck those in, eventually they'll disperse through your blood and then into your tissue and your headache will hopefully be gone. Warfarin is an example of a drug with a low VD, only 8 litres. Warfarin loves attaching to proteins in the blood plasma and thus doesn't move out of the blood vessels and into the tissue around. So you'd only need 8 litres of water in this box to allow for the same concentration in here as in the plasma if you gave someone say 10mg of warfarin. Let's look at a drug with a really high VD like chloroquine, one of the old anti-malaria medications. It's got a whopping great VD of around 13,000 litres and we'd need 260 of those original containers to have enough fluid to match with a single dose, the concentration through that fluid and the concentration in the plasma. Why is that? Well chloroquine basically looks like a lipid. It has no worries slipping through the fatty membrane of the endothelium, escaping the blood and ending up in the tissue. So here's what the eventual distribution will look like. Pretty much all of the drug has moved out of the blood and into the tissue. Paracetamol as you can see here is somewhere in the middle. So to reiterate, volume of distribution essentially gives you an idea of how much drug will stay in the blood and how much will spread into tissue out of the blood. But the values we've given here are average values for a 70kg person and so they won't reflect the VD for all people. For instance, if a patient has more adipose tissue to soak up the drug, the VD will be higher because there's effectively a bigger volume for the drug to spread into. For that reason, VD is often quoted as a litres per kilogram value. Dijoxone for instance has a VD of 7.5 litres per kilogram. For a 70kg person, that's 70kg times 7.5 to get 525 litres. For a 100kg person, the VD will be 750 litres. So we've discovered that when adjusted for the person we're treating, volume of distribution will help us achieve the exact plasma concentration that we want to achieve the patient outcome that we want. I hope that was useful. Thanks for watching. Hit subscribe and we'll see you next time.