Pesticide Excretion - Pesticide Biotransformation and Disposition - page 181

Figure 8.3 Mammalian liver structure. Reproduced from Wallace and Meyer, (2010) , Figure 13.1.
Reprinted with permission of John Wiley & Sons .
BILIARY EXCRETION
Excretion by the liver, through the biliary system, has been known for a consider-
able time but, because of the difficulty in obtaining uncontaminated bile, has been
less intensively investigated than renal excretion. A brief review of hepatic excretion
may be found in LeBlanc (2010) , and a representation of liver architecture is shown in
Figure 8.3 . Bile is secreted by the liver cells into the bile canaliculi. It then flows into
the terminal branches of the bile duct, the hepatic duct, and the gallbladder. The con-
tents of the gallbladder are discharged into the gut under the influence of hormones
whose release is triggered by food ingestion. In species that lack a gallbladder, such as
the rat, bile flows continuously into the duodenum. Secretion of xenobiotics or their
metabolites into the bile is largely a function of molecular mass and may occur by pas-
sive diffusion or by active transport.
Enterohepatic circulation is an important aspect of biliary excretion ( Figure 8.4 ).
Nonpolar xenobiotics are normally oxidized and then conjugated. If the molecu-
lar mass of the conjugate is appropriate for biliary excretion, it enters the gut, where
hydrolysis by intestinal microflora or gut conditions may occur. The compound, then
being again in a less polar form, can be reabsorbed by the intestine and returned to the