EXCRETION OF DRUGS
Drugs, expect the volatile anaesthetics are usually excreted by a route other than that of route of absorption. The important channels of drug excretion
Kidneys: the process which contribute, to the elimination of s drug in the urine are :
- Passive diffusion
- Active tubular secretion
- Passive diffusion across the tubules
I. Ionized drugs which are poorly absorbed are excreted almost entirely by the glomerular filtration. Unionized drugs which are well absorbed are filter at the glomerulus’s, but the can diffuse back from the lumen of the renal tubule into the cell of the renal tubule.
II. Many weak acids and weak base are actively transported across the proximal tubules to the lumen by system that are responsible for the transport of naturally occurring substance such as uric acid. i.e., they are excreted in urine.
III. Passive diffusion is a bidirectional process and drugs may diffuse across the tubules in either direction depending upon the drug conc. And the ph Ex ,salicylates.
IV. In the presence of renal damage, the ability of the kidney to excrete drugs is impaired. Hence great take must therefore be taken while using drugs in patients with renal disease.
V. Protein binding reduces the amount of the drug available for the filtration at the glomerulus’s but protein bound drugs may still be available for the secretion by the proximal renal tubules Ex phenybutazone.
VI. Most of the acidic and the basic drugs are secreted buy the renal tubules. Tubular secretion of weak organic acids such as penicillin can be blocked by probencid and their half –life can be increased. Secretion of weak bases by the renal tubules can also be blocked but the blocking agent are too toxic for any therapeutic use.
The pH of the urine influences the excretions of certain weak acids and bases. Thus weak acids are quickly eliminated in an alkaline medium.Ex-barbiturates and salicylates :while weak base and rapidly in an acidic urine Ex- pethidine and amphetamine .Hence the action of these drug can be prolonged are decreased by adjusting the ph of urine.
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