Chemistry Reference
In-Depth Information
the presence of
microvilli
, small tube-like structures, like tiny hairs, which
extend into the gut lumen. The small intestine is also supplied with a rich
blood supply, which means that food or drug molecules, once they cross the
gut membrane, are carried away in the bloodstream, initially to the liver,
and from there are distributed around the body.
Figure 2.6
A diagram of the small intestine, showing the convoluted surface made up
of villi, which are lined with microvilli.
What all this means in practice is that drugs are absorbed quite effec-
tively from the small intestine even if they exist in a predominantly ionised
form. The absorption process obeys the
law of mass action
, which was
introduced in Chapter 1. This law is fundamentally an equilibrium process,
and, as with any equilibrium, rapid removal of the 'products' or
compounds on the right-hand side of the equilibrium arrow will shift the
equilibrium in that direction. This is exactly what happens in drug absorp-
tion across the gut membrane: a small amount of unionised drug is
absorbed by passive diffusion and whisked away by the rich blood supply of
