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PharmD clinical pharmacy program Level 3, Semester 2 Biopharmaceutics & Pharmacokinetics (PT608(

 Drug molecules rapidly diffuse through a network of fine capillaries to the tissue

spaces filled with interstitial fluid.

 The interstitial fluid plus the plasma water is termed extracellular water, because

these fluids reside outside the cells.

 Drug molecules may further diffuse from the interstitial fluid across the cell

membrane into the cell cytoplasm.

• Drug distribution is generally rapid, and most small drug molecules permeate

capillary membranes easily.

• The passage of drug molecules across a cell membrane depends on the

physicochemical nature of both the drug and the cell membrane.

Factors affecting distribution:

1. Differing characteristics of body tissues,

2. Disease states that alter physiology,

3. Physico-chemical and lipid solubility of the drug,

4. Regional differences in physiologic pH (e.g., stomach and urine)

5. Extent of protein binding of the drug.

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