# Pharmacokinetics: Vd, Clearance, Half-life: Calculation Drug Distribution, Elimination, Rate

HelpHippo: Better than learning from Wikipedia. Vd means Volume of distribution, reflecting the extent a drug gets into plasma. The equation is: amount in body, divided by, concentration of drug in plasma. A “low Vd” suggests that there is HIGH drug in plasma, so it can float to where it’s needed in the body.. If you swallow 30mg of drug, and there is 15mg/L of drug in plasma, then your Vd is 2L. A high Vd suggests that there is less drug floating in watery plasma. (maybe because the drug is fat soluble and stuck in fatty tissues) If you swallow 30mg of drug, and there are 5mg/L of drug in plasma, then your Vd is 6L.

How to remember this inverse relationship?: You want low Venereal Disease You also want low Volume of Distribution. because you want it to reflect high concentration of effective drug in plasma. Pause and review Clearance is the rate you eliminate a drug: calculated by “elimination out rate” divided by “concentration of remaining drug” High clearance means there is “high drug” lost in urine, and therefore “less drug” remaining in your body: If you pee out 30 milligrams per hour, and there’s only 5mg/mL remaining in your plasma, then your clearance is 6 mL/hour Low clearance means there is less drug in urine and more drug kept in your body: You pee out 30 milligrams/hr, and there’s still 15 mg/mL dots remaining in your plasma, then your clearance is 2mL/hour Pause and Review Half-life is how long it takes for you to lose half of a drug’s activity.

You start with 40 mg of drug in plasma. 20mg get used up in one day, so the “half life” is one day A longer half-life obviously means it takes longer to lose half a drug’s activity. You start with 40 mg of drug in plasma and it takes TWO days to use up 20 mg of drug activity. The equation is “start” divided by “out”, or Vd/Clearance, with a magic 0.7 correction factor, so the real equation is: half life equals Vd divided by clearance times point seven. Pause and review HelpHippo: Better than learning from Wikipedia..