Page 953 - Fundamentals of anatomy physiology

940 Unit 5 Environmental Exchange
Emulsification creates tiny emulsion droplets with a su-
perficial coating of bile salts. The formation of tiny droplets
increases the surface area available for enzymatic attack. In
addition, the layer of bile salts facilitates interaction between
the lipids and lipid-digesting enzymes from the pancreas.
After lipid digestion has been completed, bile salts pro-
mote the absorption of lipids by the intestinal epithelium.
More than 90 percent of the bile salts are themselves reab-
sorbed, primarily in the ileum, as lipid digestion is completed.
The reabsorbed bile salts enter the hepatic portal circulation.
The liver then collects and recycles them. The cycling of bile
salts from the liver to the small intestine and back is called the
enterohepatic circulation of bile.
&T i p s T r i c k s
Oil and water ordinarily don’t mix. The emulsification process
helps mix oils and water-soluble enzymes so that fats can be
broken down.
The Gallbladder
24 The gallbladder is a hollow, pear-shaped organ that stores and
concentrates bile prior to its excretion into the small intestine.
This muscular sac is located in a fossa, or recess, in the posterior
surface of the liver’s right lobe (Figure 24–21a). The gallbladder
is divided into three regions: (1) the fundus, (2) the body, and
(3) the neck. The cystic duct extends from the gallbladder to the
point where it unites with the common hepatic duct to form
the common bile duct. At the duodenum, the common bile
duct meets the pancreatic duct before emptying into a chamber
called the duodenal ampulla (am-PUL-luh) (Figure 24–21b),
which receives buffers and enzymes from the pancreas and bile
from the liver and gallbladder. The duodenal ampulla opens
into the duodenum at the duodenal papilla, a small mound.
The muscular hepatopancreatic sphincter (sphincter of
Oddi) encircles the lumen of the common bile duct and, gener-
ally, the pancreatic duct and duodenal ampulla as well.
Physiology of the Gallbladder
A major function of the gallbladder is bile storage, but it is re-
leased into the duodenum only under the stimulation of the
intestinal hormone CCK (Figure 24–21d). Without CCK, the he-
patopancreatic sphincter remains closed, so bile exiting the liver
in the common hepatic duct cannot flow through the common
bile duct and into the duodenum. Instead, it enters the cystic
duct and is stored within the expandable gallbladder. When-
ever chyme enters the duodenum, CCK is released, relaxing the
hepatopancreatic sphincter and stimulating contractions of the
gallbladder that push bile into the small intestine. The amount
of CCK secreted increases markedly when the chyme contains
large amounts of lipids.
The gallbladder also functions in bile modification. When
full, the gallbladder contains 40–70 mL of bile. The composi-
tion of bile gradually changes as it remains in the gallbladder:
Much of the water is absorbed, and the bile salts and other
components of bile become increasingly concentrated.
If bile becomes too concentrated, crystals of insoluble min-
erals and salts begin to form. These deposits are called gall-
stones. Small gallstones are not a problem so long as they can be
flushed down the bile duct and excreted. In cholecystitis (ko. -le. -
sis-TI.-tis; chole, bile + kystis, bladder + -itis, inflammation), the
gallstones are so large that they can damage the wall of the gall-
bladder or block the cystic duct or common bile duct. In that
case, the gallbladder may need to be surgically removed. This
does not seriously impair digestion, because bile production
continues at normal levels. However, the bile is more dilute,
and its entry into the small intestine is not as closely tied to the
arrival of food in the duodenum.
The Coordination of Secretion and Absorption
A combination of neural and hormonal mechanisms coor-
dinates the activities of the digestive glands. These regulatory
mechanisms are centered on the duodenum, where acids must
be neutralized and appropriate enzymes added.
Neural mechanisms involving the CNS prepare the diges-
tive tract for activity (through parasympathetic innervation) or
inhibit its activity (through sympathetic innervation). Neural
mechanisms also coordinate the movement of materials along
the length of the digestive tract (through the enterogastric, gas-
troenteric, and gastroileal reflexes).
In addition, motor neurons synapsing in the digestive tract
release a variety of neurotransmitters. Many of these chemicals
are also released in the CNS. In general, their functions are
poorly understood. Examples of neurotransmitters that may be
important include substance P, enkephalins, and endorphins.
We will now summarize the information presented thus
far on the regulation of intestinal and glandular function. We
will also consider some additional details about the regulatory
mechanisms involved.
Intestinal Hormones
The intestinal tract secretes a variety of peptide hormones with
similar chemical structures. Many of these hormones have multi-
ple effects in several regions of the digestive tract, and in the acces-
sory glandular organs as well. The origins and primary effects of
these important digestive hormones are shown in Figure 24–22.
Other intestinal hormones are produced in relatively small
quantities. Examples include motilin, which stimulates intesti-
nal contractions; villikinin, which promotes the movement of
villi and the associated lymph flow; and somatostatin, which in-
hibits gastric secretion. Functional interactions among gastrin,
GIP, secretin, CCK, and VIP are diagrammed in Figure 24–23.