The liver plays an active role in the process of digestion through the production of bile. Bile is a
mixture of water, bile salts, cholesterol, and the pigment bilirubin. Hepatocytes in the liver
produce bile, which then passes through the bile ducts to be stored in the gallbladder. When food
containing fats reaches the duodenum, the cells of the duodenum release the hormone
cholecystokinin to stimulate the gallbladder to release bile. Bile travels through the bile ducts
and is released into the duodenum where it emulsifies large masses of fat. The emulsification
of fats by bile turns the large clumps of fat into smaller pieces that have more surface area and
are therefore easier for the body to digest.
Bilirubin present in bile is a product of the livers digestion of worn out red blood cells. Kupffer
cells in the liver catch and destroy old, worn out red blood cells and pass their components on to
hepatocytes. Hepatocytes metabolize hemoglobin, the red oxygen-carrying pigment of red blood
cells, into the components heme and globin. Globin protein is further broken down and used as
an energy source for the body. The iron-containing heme group cannot be recycled by the body
and is converted into the pigment bilirubin and added to bile to be excreted from the body.
Bilirubin gives bile its distinctive greenish color. Intestinal bacteria further convert bilirubin into
the brown pigment stercobilin, which gives feces their brown color.
The hepatocytes of the liver are tasked with many of the important metabolic jobs that support
the cells of the body. Because all of the blood leaving the digestive system passes through the
hepatic portal vein, the liver is responsible for metabolizing carbohydrate, lipids, and proteins
into biologically useful materials.
Our digestive system breaks down carbohydrates into the monosaccharide glucose, which cells
use as a primary energy source. Blood entering the liver through the hepatic portal vein is
extremely rich in glucose from digested food. Hepatocytes absorb much of this glucose and store
it as the macromolecule glycogen, a branched polysaccharide that allows the hepatocytes to pack
away large amounts of glucose and quickly release glucose between meals. The absorption and
release of glucose by the hepatocytes helps to maintain homeostasis and protects the rest of the
body from dangerous spikes and drops in the blood glucose level. (See more about glucose in
the body.)
Fatty acids in the blood passing through the liver are absorbed by hepatocytes and metabolized to
produce energy in the form of ATP. Glycerol, another lipid component, is converted into glucose
by hepatocytes through the process of gluconeogenesis. Hepatocytes can also produce lipids like
cholesterol, phospholipids, and lipoproteins that are used by other cells throughout the body.
Much of the cholesterol produced by hepatocytes gets excreted from the body as a component of