The liver is engaged in numerous metabolic conversions involving substrates delivered by blood from the digestive tract, pancreas, and spleen. Some of these products are involved in the production of bile, an exocrine secretion of the liver. Bile contains conjugated and degraded waste products that are returned to the intestine for disposal, as well as substances that bind to metabolites in the intestine to aid in absorption (Table 18.1). Bile is carried from the parenchyma of the liver by bile ducts that fuse to form the hepatic duct. The cystic duct then carries the bile into the gallbladder, where it is concentrated. Bile is returned, via the cystic duct, to the common bile duct, which delivers bile from the liver and gallbladder to the duodenum (see Fig. 18.15).
The endocrine-like functions of the liver are represented by its ability to modify the structure and function of many hormones.
The liver modifes the action of hormones released by other organs. The liver’s endocrine-like actions involve  vitamin D, which is converted by the liver to 25hydroxycholecalciferol, the predominant form of circulating vitamin D (page 629).
 thyroxine, a hormone secreted by the thyroid gland as tetraiodothyronine (T4), which is converted in the liver to the biologically active form, triiodothyronine (T3), by deiodination.
 growth hormone (GH), a hormone secreted by the pituitary gland. The action of GH is modified by liver-produced growth hormone–releasing hormone (GHRH) and inhibited by somatostatin, which is secreted by enteroendocrine cells of the gastrointestinal tract.
 insulin and glucagon, both pancreatic hormones. These hormones are degraded in many organs, but the liver and kidney are the most important sites of their degradation.
Blood Supply to the Liver
To appreciate the myriad functions of the liver introduced previously, one must first understand its unique blood supply and how blood is distributed to the hepatocytes. The liver has a dual blood supply consisting of a venous (portal) supply via the hepatic portal vein and an arterial supply via the hepatic artery. Both vessels enter the liver at a hilum or porta hepatis, the same site at which the common bile duct, carrying the bile secreted by the liver, and the lymphatic vessels leave the liver. Therefore, bile flows in a direction opposite to that of the blood.
The liver receives the blood that initially supplied the intestines, pancreas, and spleen.
The liver is unique among organs because it receives its major blood supply (about 75%) from the hepatic portal vein, which carries venous blood that is largely depleted of oxygen.