Volumes of fluids in the body Background: Drugs are not confined to just the plasma. They can potentially distribute anywhere in the body. Instructions: Read the short section below for examples of different fluids in the body and their approximate volumes. Learning Goal: To learn how to interpret Vd values of drugs and make reasonable predictions on how specific drugs distribute. Information very similar to this was presented back in section 1 of Chapter 6. It is worth repeating here since we have a better understanding of volume of distribution. plasma A 70‐kg patient has 5 L of whole blood. Plasma is the non‐cellular fraction of blood. At 54% of the volume of whole blood, the volume of plasma in a 70‐kg patient is approximately 2.7 L. Because not everyone has a mass of 70 kg, the volume of plasma is very frequently reported as 0.039 L/kg. If a drug has a Vd of only 0.039 L/kg (which would be uncommon), then the drug would most likely be confined to the plasma. interstitial fluid Interstitial fluid is the liquid that sits between the cells of the body. The fluid contains the nutrients and waste of the cells. Drugs reach the interstitial fluid from the capillaries. The walls of the capillaries have pores that allow passage of liquids and anything smaller than the pores. Everything in blood can pass except proteins and cells. All orally‐delivered drugs are small enough to slip into the interstitial fluid and reach the cells in the body. A 70‐kg patient has 10 L of interstitial fluid. That is 0.14 L/kg. Oral drugs have full access to both the plasma and interstitial fluid and should therefore have a Vd of at least 0.18 L/kg (sum of volume of plasma and interstitial fluid). intracellular fluid The total water found within the cells of the body is 25 L for a 70‐kg person. That is about 0.36 L/kg. For a drug to enter cells, it must be able to cross cell membranes. Therefore, a drug that has access to cell water will also have access to the volume of all the cell membranes in the body. The Vd of such a drug is hard to predict, but it could certainly be much higher than 0.54 L/kg ‐ the sum of the plasma, interstitial fluid, and intracellular fluid.