Biliary secretion

The liver secretes bile which is necessary for the digestion and absorption of lipids. All hepatic cells continually form a small amount of bile which is secreted into bile canaliculi. It is stored and concentrated in the gall bladder in man. Approximately 600 ml of hepatic bile is produced per day, but within 4 hours, up to 90% of the water present in the hepatic bile can be removed by the gall bladder. Concentration takes place by removal of sodium ions, and chloride and water then follow passively.

Bile is a variable and complex mixture of water, organic and inorganic solutes. The major organic solutes are bile acids, phospholipids (particularly lecithin), cholesterol and bilirubin. Sodium and potassium ions are found in proportions similar to that found in plasma whilst the concentrations of Cl- and HCO3- are often lower and the bile acids make up the remainder of the ion balance. The bile acids are derivatives of cholesterol in which hydroxyl and carboxylic acid groups are attached to the steroid nucleus, converting it into a powerful natural surfactant. The major pigment of bile is bilirubin. Its formation is of considerable biological significance as it is the most important means by which haem, produced by the breakdown of haemoglobin, is eliminated. Up to 20% of the bilirubin present in bile is produced from other resources such as myoglobin and cytochromes.

Bile salts have two important actions:

(i) emulsification of the fat content of food, producing small droplets of fat in aqueous suspension.

(ii) assisting in the absorption of fatty acids, monoglycerides, cholesterol and other lipids from the intestinal tract by forming submicron clusters of fat and surfactant called mixed micelles.

Brief periodic bursts of bile flow occur under fasting conditions, coincident with the passage of phase 3 of the migrating motor complex (MMC) through the duodenum. When a meal is ingested, the gall bladder contracts and the bile salts are secreted into the duodenum where they can emulsify dietary fat. Bile acids are poorly absorbed in the proximal small intestine, unlike the majority of nutrients, but are absorbed by an active process in the terminal ileum. After absorption, bile acids have a high hepatic clearance and are re-secreted in the bile. This process is known as enterohepatic recirculation.

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