211. SECRETION OF BILE
Some cells in the liver secrete a substance known as bile. Bile contains large
quantities of bile salts, a moderate amount of cholesterol, and a small amount of
bilirubin. Bile is secreted into very small bile canaliculi (vessels), which empty into
terminal bile ducts. The bile ducts become progressively larger and finally reach the
hepatic duct and the common bile duct. From these ducts, bile is either emptied into the
duodenum or directed to the gallbladder. A brief discussion of the three components of
bile is presented below:
a. Bile Salts. Liver cells are constantly forming bile salts. These bile salts are
obtained from cholic acid, a substance similar in structure to cholesterol. Bile salts
serve two main functions in the intestinal tract.
(1) Emulsifying function. Bile salts decrease the surface tension of fatty
particles in food and allow them to be broken up into minute fat globules by the agitation
of the intestinal tract.
(2) Hydrotropic function. Bile salts help in the absorption of fatty acids and
monoglycerides from the intestinal tract through a process known as hydrotropic action.
b. Cholesterol. Excess cholesterol is excreted by the liver in the bile. In some
people, the cholesterol may precipitate to form gallstones in the gallbladder.
c. Bilirubin. Bilirubin is excreted from the liver. When red blood cells have
completed their life spans (approximately 125 days) they rupture and release their
hemoglobin. The body removes the iron from the hemoglobin, which changes and
becomes bilirubin. However, bilirubin is insoluble in the body fluids. Bilirubin, therefore,
is bound to protein in the plasma and is absorbed by the liver in this form.
212. METABOLIC FUNCTIONS
a. Carbohydrate Metabolism. Most of the liver functions in carbohydrate
metabolism are concerned with maintaining a normal blood glucose level.
(1) Storage of glycogen. The liver is capable of taking excess glucose from
the blood and storing it in the liver cells in case blood glucose level should fall. This
process is glycogenesis. This is known as the glucose buffering function of the liver. In
an individual with a malfunctioning liver, the blood glucose level will rise as much as
three times higher after a meal high in carbohydrates.
(2) Glycogenolysis. Glycogenolysis (breakdown of glycogen to glucose)
begins to occur when the blood glucose level falls below the normal level. In cases
where the blood glucose drops significantly below the normal level, the liver is able to
convert amino acids into glucose (this is called gluconeogenesis).