Physiologic Anatomy of Biliary Secretion

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Bile is secreted in two stages by the liver: (1) The initial portion is secreted by the principal functional cells of the liver, the hepatocytes; this initial secretion contains large amounts of bile acids, cholesterol, and other organic constituents. It is secreted into minute bile canaliculi that originate between the hepatic cells.

Regulation of pancreatic secretion

(2) Next, the bile flows in the canaliculi toward the interlobular septa, where the canaliculi empty into terminal bile ducts and then into progressively larger ducts, finally reaching the hepatic duct and common bile duct. From these the bile either empties directly into the duodenum or is diverted for minutes up to several hours through the cystic duct into the gallbladder, shown in Figure 64-11.

In its course through the bile ducts, a second portion of liver secretion is added to the initial bile. This additional secretion is a watery solution of sodium and bicarbonate ions secreted by secretory epithelial cells that line the ductules and ducts. This second secretion sometimes increases the total quantity of bile by as much as an additional 100 per cent. The second secretion is stimulated especially by secretin, which causes release of additional quantities of bicarbonate ions to supplement the bicarbonate ions in pancreatic secretion (for neutralizing acid that empties into the duodenum from the stomach).

Storing and Concentrating Bile in the Gallbladder. Bile is secreted continually by the liver cells, but most of it is normally stored in the gallbladder until needed in the duodenum. The maximum volume that the gallbladder can hold is only 30 to 60 milliliters. Nevertheless, as much as 12 hours of bile secretion (usually about 450 milliliters) can be stored in the gallbladder because water, sodium, chloride, and most other small electrolytes are continually absorbed through the gallbladder mucosa, concentrating the remaining bile constituents that contain the bile salts, cholesterol, lecithin, and bilirubin.

Most of this gallbladder absorption is caused by active transport of sodium through the gallbladder epithelium, and this is followed by secondary absorption of chloride ions, water, and most other diffusible

Secretin via blood stream stimulates liver ductal secretion

Bile acids via blood stimulate parenchymal secretion

Vagal stimulation causes weak contraction of gallbladder

Bile acids via blood stimulate parenchymal secretion

Vagal stimulation causes weak contraction of gallbladder

Bile stored and concentrated up to 15 times in gallbladder

Liver And Biliary System

Stomach

Pancreas

Sphincter of Oddi

Duodenum

Cholecystokinin via blood stream causes:

1. Gallbladder contraction

2. Relaxation of sphincter of Oddi

Stomach

Bile stored and concentrated up to 15 times in gallbladder

Pancreas

Sphincter of Oddi

Duodenum

Cholecystokinin via blood stream causes:

1. Gallbladder contraction

2. Relaxation of sphincter of Oddi

Figure 64-11

Liver secretion and gallbladder emptying constituents. Bile is normally concentrated in this way about 5-fold, but it can be concentrated up to a maximum of 20-fold.

Composition of Bile. Table 64-2 gives the composition of bile when it is first secreted by the liver and then after it has been concentrated in the gallbladder. This table shows that by far the most abundant substances secreted in the bile are bile salts, which account for about one half of the total solutes also in the bile. Also secreted or excreted in large concentrations are biliru-bin, cholesterol, lecithin, and the usual electrolytes of plasma.

In the concentrating process in the gallbladder, water and large portions of the electrolytes (except calcium ions) are reabsorbed by the gallbladder mucosa; essentially all other constituents, especially the bile salts and the lipid substances cholesterol and lecithin, are not reabsorbed and, therefore, become highly concentrated in the gallbladder bile.

Emptying of the Gallbladder—Stimulatory Role of Cholecys-tokinin. When food begins to be digested in the upper gastrointestinal tract, the gallbladder begins to empty, especially when fatty foods reach the duodenum about 30 minutes after a meal. The mechanism of gallbladder emptying is rhythmical contractions of the wall of the gallbladder, but effective emptying also requires simultaneous relaxation of the sphincter of Oddi, which guards the exit of the common bile duct into the duodenum.

By far the most potent stimulus for causing the gallbladder contractions is the hormone cholecystokinin.

Table 64-2

Composition of Bile

Table 64-2

Composition of Bile

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  • tim
    What the bile secretes?
    1 year ago

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