Page 261 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Bile inhibits the hepatic synthesis of bile salts
(cholesterol-7α-hydroxylase; negative feed-
Bile components. Bile contains electrolytes, back; ! B) and (b) stimulates the secretion of
bile salts (bile acids), cholesterol, lecithin bile salts into the biliary canaliculi. The latter
(phosphatidylcholine), bilirubin diglucuro- effect increases the bile flow due to osmotic
nide, steroid hormones, medications etc. water movement, i.e., causes bile salt-depend-
(! A). Bile salts are essential for fat digestion. ent choleresis (! C). Bile salt-independent
Most of the other components of bile leave the choleresis is, caused by secretion of other bile
body via the feces (excretory function of the components into the canaliculi as well as of
–
–
liver ! p. 250). HCO 3 (in exchange for Cl ) and H 2O into the
Bile formation. Hepatocytes secrete ca. 0.7 bile ducts (! C). The latter form is increased by
L/day of bile into biliary canaliculi (! A), the the vagus nerve and secretin.
Nutrition and Digestion canalicular membranes of the hepatocytes closed, hepatic bile (C bile) is diverted to the
fine canals formed by the cell membranes of
Gallbladder. When the sphincter of Oddi be-
tween the common bile duct and duodenum is
adjacent of hepatocytes. The sinusoidal and
gallbladder, where it is concentrated (1 : 10)
contain numerous carriers that absorb bile
and stored (! D). The gallbladder epithelium
components from the blood and secrete them
into the canaliculi, resp.
–
reabsorbs Na , Cl and water (! D1) from the
+
stored bile, thereby greatly raising the con-
Bile salts (BS). The liver synthesizes cholate
bilirubin-di-glucuronide,
salts,
from cholesterol. The intestinal bacteria con-
cholesterol,
phosphatidylcholine, etc.). If bile is used for fat
vert some of them into secondary bile salts
10 and chenodeoxycholate (primary bile salts) centration of specific bile components (bile
and lithocholate. Bile
digestion (or if a peristaltic wave occurs in the
such as deoxycholate
salts are conjugated with taurine or glycine in interdigestive phase, ! p. 240), the gallbladder
the liver and are secreted into the bile in this contracts and its contents are mixed in por-
form (! A). This conjugation is essential for tions with the duodenal chyme (! D2).
micelle formation in the bile and gut.
Cholesterol in the bile is transported inside micelles
Hepatic bile salt carriers. Conjugated bile salts in si- formed by aggregation of cholesterol with lecithin
nusoidal blood are actively taken up by NTCP (Na + and bile salts. A change in the ratio of these three
taurocholate cotransporting polypeptide; secondary substances in favor of cholesterol (! E) leads to the
active transport), and transported against a steep precipitation of cholesterol crystals responsible for
concentration gradient into the canaliculi (primary gallstone development in the highly concentrated
active transport) by the ATP-dependent carrier gallbladder bile (B bile). The red and green dots in E
hBSEP (human bile salt export pump), also referred show the effects of two different ratios.
to as cBAT (canalicular bile acid transporter).
Gallbladder contraction is triggered by CCK
Enterohepatic circulation of BS. Unconjugated (! p. 234), which binds to CCK A receptors, and
bile salts are immediately reabsorbed from the the neuronal plexus of the gallbladder wall,
bile ducts (cholehepatic circulation). Conju- which is innervated by preganglionic para-
gated bile salts enter the duodenum and are sympathetic fibers of the vagus nerve (! D2).
reabsorbed from the terminal ileum by the Na + CGRP (! p. 234) and substance P (! p. 86) re-
symport carrier ISBT (= ileal sodium bile acid leased by sensory fibers appear to stimulate
cotransporter) and circulated back to the liver the gallbladder musculature indirectly by in-
(enterohepatic circulation; ! B) once they have creasing acetylcholine release. The sympa-
been used for fat digestion (! p. 252). The total thetic nervous system inhibits gallbladder con-
bile pool (2–4 g) recirculates about 6–10 times tractions via α 2 adrenoreceptors located on
a day, depending on the fat content of the diet. cholinergic fiber terminals. As cholagogues,
Ca. 20–30 g of bile salts are required for daily fatty acids and products of protein digestion
fat absorption. (! p. 234) as well as egg yolk and MgSO 4 effec-
Choleresis. Enterohepatic circulation raises tively stimulate CCK secretion.
the bile salt concentration in the portal vein to
248
a high level during the digestive phase. This (a)
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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