u Determinants of simple diffusion For most drugs of small molecules(usually are weak acids or weak bases ) o Lipid-soluble or un-ionized forms o pKa of the drug and pH of the body fluid u The pKa is the ph at which the concentrations of the ionized and un-ionized forms are equalDeterminants of simple diffusion For most drugs of small molecules (usually are weak acids or weak bases): Lipid-soluble or un-ionized forms pKa of the drug and pH of the body fluid The pKa is the pH at which the concentrations of the ionized and un-ionized forms are equal