The total volume of the fluid compartments of the body into which drugs may be distributed is approximately 40 L in a 70-kg adult. These compartments include plasma water (approximately 10 L), interstitial fluid (10 L), and the intracellular fluid (20 L). Total extracellular water is the sum of the plasma and the interstitial water. Factors such as sex, age, edema, pregnancy, and body fat can influence the volume of these various compartments.
The rate at which an equilibrium concentration of a drug is reached in the extracellular fluid of a particular tissue will depend on the tissue's perfusion rate; the greater the blood flow the more rapid the distribution of the drug from the plasma into the interstitial fluid. Thus, a drug will appear in the interstitial fluid of liver, kidney, and brain more rapidly than it will in muscle and skin (Table 3.2). The pharmacokinetic concept of volume of distribution (a derived parameter that relates the amount of drug in the body to the plasma concentration) is discussed more fully in Chapter 5.
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