Functions of the gall bladder

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The functions of the gall bladder are to store and concentrate bile, and to deliver it to the duodenum during a meal.

Motility

As it emerges from the gall bladder, the cystic duct combines with the common hepatic duct to form the common bile duct, which joins with the main pancreatic duct to form the ampulla of Vater.

During their passage through the duodenal wall, the common bile duct, the pancreatic duct and the ampulla of Vater are surrounded by an arrangement of smooth muscles, the spincter of Oddi. The closure of this sphincter prevents bile from entering the duodenum. This closure results in formed bile being diverted into the gall bladder. The main stimulus for the relaxation of the sphincter muscle is cholecystokinin (CCK). Thus when levels of CCK increase in the blood during a meal, the gall bladder contracts, the sphincter of Oddi

Left hepatic duct

Right hepatic duct

Common hepatic duct

Cystic duct (to gall bladder)

Common bile duct

Figure 8.1 Attachment of the gall bladder to the liver. Reproduced with permission from Hinchliff et al. (1996).

Table 8.1 Electrolytes and organic molecules in gall bladder bile.

Constituent

Bile (mM)

Electrolytes

Bicarbonate (HCO3-)

10

Chloride (Cl-)

25

Potassium (K+)

12

Sodium (Na+)

130

Calcium (Ca+2)

Organic molecules

Bilirubin

5.1

Cholesterol

16.0

Lecithin

3.9

Bile salts

145.0

relaxes and bile flows into the duodenum. Other factors, such as pharmaceutical agents, sensory input, disease and emotional states, may also influence the filling and contraction of the gall bladder.

Secretion

Bile is an alkaline, greenish-yellow fluid that is secreted continuously by the liver. It is composed of different secretions, one originating in the liver cells (hepatocytes) and the other in the cells which line the bile ducts. The two secretions mix together in the ducts.

Gall bladder bile is an isotonic solution but some of its components are highly concentrated. Its major components are water (which makes up 97% of hepatic bile), bile salts, fatty acids, lipids, inorganic electrolytes and other organic substances (see Table 8.1). When bile flows from the liver through the cystic duct and into the gall bladder for storage, up to 90% of the water is removed.

Bile has a number of functions, which include the emulsification of undigested fats; the activation of both intestinal and pancreatic enzymes; the facilitation of the absorption of fat-soluble vitamins; and providing a route of excretion for bilirubin, cholesterol, thyroid and adrenal hormones. Bile also aids the neutralisation of gastric acid in the small intestine in that its composition is similar to pancreatic juice.

The volume of alkaline secretion, unlike hepatic bile, is not directly determined by the concentration of bile salts in the blood.

Gall bladder contraction

The gall bladder exhibits muscle tone even in the interdigestive period. It also contracts between meals to deliver bile intermittently into the duodenum. The major stimulus for gall bladder contraction after a meal is high blood levels of cholecystokinin (CCK), the duodenal hormone that is released in reponse to fats in the duodenum. Gastrin, which is released in the stomach antrum in response to peptides in the food, also stimulates gall bladder contraction.

Vasoactive inhibitory peptide (VIP), pancreatic polypeptide and stimulation of the sympathetic nerves to the gall bladder, all cause gall bladder relaxation. Bile acids, by means of feedback control, also inhibit gall bladder contraction.

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