Elimination (pharmacology)

These organs or structures use specific routes to expel a drug from the body, these are termed elimination pathways: Drugs are excreted from the kidney by glomerular filtration and by active tubular secretion following the same steps and mechanisms as the products of intermediate metabolism.

In the proximal and distal convoluted tubules, non-ionised acids and weak bases are reabsorbed both actively and passively.

Poisoning treatments use this effect to increase elimination, by alkalizing the urine causing forced diuresis which promotes excretion of a weak acid, rather than it getting reabsorbed.

As the acid is ionised, it cannot pass through the plasma membrane back into the blood stream and instead gets excreted with the urine.

The liver and kidneys of newly born infants are relatively undeveloped and they are highly sensitive to a drug's toxic effects.

For this reason it is important to know if a drug is likely to be eliminated from a woman's body if she is breast feeding in order to avoid this situation.

The «renal clearance» rate will be determined by factors such as the degree of plasma protein binding as the drug will only be filtered out if it is in the unbound free form, the degree of saturation of the transporters (active secretion depends on transporter proteins that can become saturated) or the number of functioning nephrons (hence the importance of factors such as kidney failure).

As «hepatic clearance» is an active process it is therefore determined by factors that alter an organism's metabolism such as the number of functioning hepatocytes, this is the reason that liver failure has such clinical importance.

For elimination via bile please see: Estimation of Biliary Excretion of Foreign Compounds Using Properties of Molecular Structure.