A--Drug A is an undissolved drug. It is not in solution and it cannot be
absorbed.
B--B is a molecule of a drug in solution. It is unionized and can be
absorbed.
C--C is a molecule of a drug attached to a "carrier molecule" at the cell
wall.
D--D is an ion of a drug. It cannot be absorbed since the fatty layer
repels it.
E--E is a molecule of a drug. It is in circulation and will be carried away.
F--F is a molecule of the drug that is being released into circulation by a
carrier molecule.
Figure 3-3. Illustration of drug transport concepts.
NOTE: Passive transport or diffusion has absorbed Molecule B. Once it has been
absorbed, the circulating blood will carry it away. Consequently, the concentration of
the drug will always remain higher in the gut and diffusion will continue. As the
unionized particles are absorbed, the ionized particles will attach to each other to form
more unionized drug. This occurs because the drug has equilibrium established
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