Chemistry Reference
In-Depth Information
Distal convoluted
tubule
Proximal convoluted
tubule
Glomerulus
Ascending
limb
Descending
limb
Loop of Henle
Figure 2.10
A diagram of a nephron.
minimise the process of reabsorption of drug from the kidney tubule and so
expedite excretion. If a drug is to be reabsorbed by passive diffusion
through the tubule cell membrane, then it must exist predominantly in the
unionised
form. This is entirely in keeping with the process of absorption
into the body following oral administration discussed above. If the pH of
the urine is adjusted to increase the proportion of the drug that is ionised,
then reabsorption will be decreased (since the unionised species crosses the
membrane more easily by passive diffusion). If the drug taken in overdose is
a weak acid (e.g. a barbiturate, phenytoin, or most NSAIDs), then excretion
should be favoured, and reabsorption minimised, by addition of an agent
that will raise the pH of the urine. This technique is called
forced alkaline
diuresis
and should result in more rapid clearance of an acidic drug. An
example of an agent used to raise the pH of urine is sodium bicarbonate,
Na
HCO
3
(the salt of a strong base and a weak acid, which will be basic
by partial hydrolysis), usually administered as an 8.4% w/v infusion.
If the drug taken in overdose is a base, for example, a benzodiazepine
tranquilliser or an antihistamine, excretion should be favoured by acidifica-
tion of the urine. Agents that may be used to achieve this include ammo-
nium chloride, NH
4
Cl
(an acidic salt by partial hydrolysis) and ascorbic
acid (vitamin C). If the pH of the urine is artificially lowered, the technique
is called
forced acid diuresis
.
