PHARMACOLOGY

 Tubular reabsorption ::

The drugs may be filtered by glomerular filtration but may not appear in urine because of tubular reabsorption The extent of tubular reabsorption determines the rate of excretion of drugs and this depends on ionization and pH of urine 

    Unionized drugs can be reabsorbed well so that they return to the tubules and hence unionized drugs cannot be excreted whereas ionized drugs are poorly reabsorbed so they cannot pass back from the tubular and hence are excreted out In urine weak acids like barbiturates and salicylates are quickly ionized if the pH of urine is alkaline hence they cannot be reabsorbed and excreted readily weak bases e.g. pethidine amphetamine etc are quickly ionized if pH of urine is acidic thus they cannot be reabsorbed and excreted readily The elimination of weak acids and weak bases can be accelerated by marking urine alkaline or acidic respectively 

Tubular secretion ::

Certain drugs are excreted from tubules as a sequence of secretion which is an active transport mechanism Drugs like penicillin salicylic acid chlorthiazide probenecid p-amino -hippuric acid glucuronic acid conjugates etc are excreted by this mechanism 

        Substances that are transported by the same mechanism may compete with each other if the substance is transported slowly (secreted slowly ) it will be inhibiting the transport of other substances that are secreted faster e.g.probenecid inhibits the tubular secretion of penicillin