Liver: Tissue Function

Functions of the Liver

The liver has many functions, including protein, carbohydrate, and lipid metabolism; detoxification of many compounds; production of bile for digestion of fats; and secretion of many serum proteins and hormones.

The liver plays an active role in the digestion process. It produces bile, a mixture of bile salts, cholesterol, and bilirubin, which is secreted into the duodenum, part of the small intestine at the ampulla of Vater. When food reaches the duodenum, cells in the duodenum wall release a hormone called cholecystokinin that stimulates the gallbladder to secrete bile. Bile emulsifies fat present in the food, breaking down large molecules for easy digestion. Kupffer cells metabolize red blood cells and release hemoglobin, which is broken down into its heme and globin parts by hepatocytes. The globin part is used for energy purposes, whereas the heme part cannot be recycled and is converted into bilirubin pigment.

Blood leaving the gastrointestinal tract passes through the liver via the portal vein, carrying carbohydrates, proteins, and lipids. These nutrients are metabolized in the liver into biologically useful products. Hepatocytes absorb glucose under the influence of the hormone insulin from the portal vein and store it in the form of a branched polysaccharide, glycogen. This allows the liver to store glucose in large amounts and release it quickly when required, thus helping maintain normal homeostasis and blood glucose levels. When there is a fall in blood glucose levels, the liver metabolizes glycogen and releases the components into the blood under the influence of the [Page 687]hormone glucagon, which is released from the pancreas. Formation of glycogen from glucose in known as glycogenesis; the breakdown of glycogen is glycogenolysis.

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