554
Chapter 15
homeostasis in the blood (Chapter 16) and is also the process
by which some cholesterol-lowering drugs work. Dietary fi ber
also sequesters bile and thereby lowers plasma cholesterol.
Cholesterol is insoluble in water, and its solubility in bile is
achieved by its incorporation into micelles (whereas in blood,
cholesterol is incorporated into lipoproteins). Gallstones, con-
sisting of precipitated cholesterol, will be discussed at the end
of this chapter.
Bile pigments
are substances formed from the heme
portion of hemoglobin when old or damaged erythrocytes
are digested in the spleen and liver. The predominant bile pig-
ment is
bilirubin,
which is extracted from the blood by liver
cells and actively secreted into the bile. Bilirubin is yellow and
contributes to the color of bile. During their passage through
the intestinal tract, some of the bile pigments are absorbed
into the blood and are eventually excreted in the urine, giving
urine its yellow color. After entering the intestinal tract, some
bilirubin is modifi ed by bacterial enzymes to form the brown
pigments that give feces their characteristic color.
The components of bile are secreted by two different cell
types. The bile salts, cholesterol, lecithin, and bile pigments
are secreted by hepatocytes, whereas most of the bicarbonate-
rich salt solution is secreted by the epithelial cells lining the
bile ducts. Secretion of the salt solution by the bile ducts, just
like the secretion by the pancreas, is stimulated by secretin in
response to the presence of acid in the duodenum.
Bile salt secretion is controlled by the concentration of
bile salts in the blood, unlike the pancreas, whose secretions
are controlled by intestinal hormones. The greater the plasma
concentration of bile salts, the greater their secretion into
the bile canaliculi. Absorption of bile salts from the intes-
tine during the digestion of a meal leads to their increased
plasma concentration and thus to an increased rate of bile salt
secretion by the liver. Although bile secretion is greatest dur-
ing and just after a meal, the liver is always secreting some
bile. Surrounding the common bile duct at the point where it
enters the duodenum is a ring of smooth muscle known as the
sphincter of Oddi.
When this sphincter is closed, the dilute
bile secreted by the liver is shunted into the gallbladder. Here,
the organic components of bile become concentrated as NaCl
and water are absorbed into the blood.
Shortly after the beginning of a fatty meal, the sphincter
of Oddi relaxes and the gallbladder contracts, discharging con-
centrated bile into the duodenum. The signal for gallbladder
contraction and sphincter relaxation is the intestinal hormone
CCK—appropriately so, because as we have seen, the presence
of fat in the duodenum is a major stimulus for this hormone’s
release. It is from this ability to cause contraction of the gall-
bladder that cholecystokinin received its name:
chole,
bile;
cysto,
bladder;
kinin,
to move.
Figure 15–31
summarizes the factors
controlling the entry of bile into the small intestine.
Small Intestine
Secretion
Approximately 1500 ml of fl uid is secreted by the walls of the
small intestine from the blood into the lumen each day. One
of the causes of water movement (secretion) into the lumen is
Liver
Gallbladder
Duodenum
Bile salts
Bile salts
Common
bile duct
Synthesis 5%
Hepatic
portal vein
Ileum
5% lost
in feces
Small intestine
Figure 15–30
Enterohepatic circulation of bile salts. Bile salts are secreted into
bile (green) and enter the duodenum through the common bile
duct. Bile salts are reabsorbed from the intestinal lumen into hepatic
portal blood (red arrows). The liver (hepatocytes) reclaims bile salts
from hepatic portal blood.
Figure 15–30
physiological
inquiry
In addition to the hepatic portal vein, can you name another
portal vein system and explain the meaning of the term “portal”?
Answer can be found at end of chapter.
Gallbladder
Contraction
Sphincter of Oddi
Relaxation
Plasma CCK
Duodenum
Fatty acids
CCK secretion
Bile flow
into duodenum
Bile flow into
common bile duct
Figure 15–31
Regulation of bile entry into the small intestine.
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