Expanded Answers for Study Problems in: Rowland & Tozer's Clinical Pharmacokinetics: Concepts and Applications, 3rd edition.

Chapter 8 - Movement Through Membranes

Study Problems on Page 117

2. a) When distribution of a xenobiotic is perfusion-rate limited, the rate at which a xenobiotic diffuses away from the site of absorption is determined by the blood flow of the tissue. Since perfusion is the slowest step, equilibrium of xenobiotic on both sides of the capillary membrane is rapidly achieved and easily maintained (i.e., the concentration ratio is 1).

b) This is false. Permeability of the membrane is an inherent property of the membrane and is not altered by a simple increase in surface area. However, permeability is sometimes used synonymously with diffusion. If the term is used in this latter sense, then it will increase with an increase in surface area. Remember Fick's Law of Diffusion indicates that the rate of diffusion is proportional to the surface area of the membrane.

e) This is false. The binding of drugs to circulating proteins does not influence the inherent permeability of the membrane. The amount of drug available to cross the membrane will be altered (since only unbound drug can cross the membrane), but the binding to proteins does not influence membrane properties.

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