The Digestion and Absorption of Food
553
tion. Thus, the organic nutrients in the small intestine initi-
ate, via hormonal and neural refl exes, the secretions involved
in their own digestion.
Although most of the pancreatic exocrine secretions are
controlled by stimuli arising from the intestinal phase of diges-
tion, cephalic and gastric stimuli also play a role by way of
the parasympathetic nerves to the pancreas. Thus, the taste
of food or the distension of the stomach by food will lead to
increased pancreatic secretion.
Bile Secretion and Liver Function
As stated earlier, bile is secreted by liver cells into a number
of small ducts, the
bile canaliculi
(
Figure 15–29
), which
converge to form the common hepatic duct (see Figure 15–4).
Bile contains six major ingredients: (1) bile salts; (2) lecithin (a
phospholipid); (3) bicarbonate ions and other salts; (4) choles-
terol; (5) bile pigments and small amounts of other metabolic
end products; and (6) trace metals. Bile salts and lecithin are
synthesized in the liver and, as we have seen, help solubilize
fat in the small intestine. Bicarbonate ions neutralize acid in
the duodenum, and the last three ingredients represent sub-
stances extracted from the blood by the liver and excreted via
the bile.
From the standpoint of gastrointestinal function, the
most important components of bile are the bile salts. During
the digestion of a fatty meal, most of the bile salts entering the
intestinal tract via the bile are absorbed by specifi c sodium-
coupled transporters in the ileum (the last segment of the
small intestine). The absorbed bile salts are returned via the
portal vein to the liver, where they are once again secreted into
the bile. Uptake of bile salts from portal blood into
hepato-
cytes
(liver cells) is driven by secondary active transport cou-
pled to sodium. This recycling pathway from the liver to the
intestine and back to the liver is known as the
enterohepatic
circulation
(
Figure 15–30
). A small amount (5 percent) of
the bile salts escapes this recycling and is lost in the feces, but
the liver synthesizes new bile salts from cholesterol to replace
them. During the digestion of a meal, the entire bile salt con-
tent of the body may be recycled several times via the entero-
hepatic circulation.
In addition to synthesizing bile salts from cholesterol,
the liver also secretes cholesterol extracted from the blood into
the bile. Bile secretion, followed by excretion of cholesterol in
the feces, is one of the mechanisms for maintaining cholesterol
Small intestine
Digestion of fats and protein
Flow of enzymes into small intestine
Pancreas
Enzyme secretion
Small intestine
CCK secretion
Intestinal fatty acids and amino acids
Plasma CCK
Figure 15–29
(a) A small section of the liver showing the location of bile canaliculi
and ducts with respect to blood and liver cells. Bile (green) is formed
by uptake by liver cells (hepatocyte) of bile salts and secretion into
bile canaliculi. (b) Photomicrograph of the liver showing a portal
space with its characteristic small artery and bile duct surrounded by
connective tissue. H&E stain, medium magnifi cation. (c) A central
(centrolobular) vein. H&E stain, medium magnifi cation.
(a) Adapted from Kappas and Alvares; (b, c) Courtesy of M.F. Santos from Junqueira and Cameiro
Basic Histology Image Library (McGraw-Hill).
Figure 15–28
Hormonal regulation of pancreatic enzyme secretion.
(a)
Branch of
hepatic artery
Blood
Central vein
Bile
canaliculus
Liver cells
Bile duct
Branch of
portal vein
Central vein
Hepatocytes
Bile duct
Branch of
portal vein
Branch of
hepatic artery
(b)
(c)
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