202
chapter
12
Gastrointestinal Digestion and Absorption
the beginning of the pancreatic duct system, and centroaci-
nar cells, which secrete a bicarbonate solution. The acinar
cells are joined to one another by tight junctions, desmo-
somes, and gap junctions. The tight junctions probably
prevent leakage of pancreatic secretions into the extra-
cellular spaces. Acinar cells are pyramidal in shape with
the nucleus located away from the lumen and secretory
granules located toward the lumen. Centroacinar and aci-
nar cells have microvilli on their free borders.
Enzymes are synthesized in the acinar cells by ribo-
somes situated on the rough endoplasmic reticulum and
sequestered in membrane-bound granules (Chapter 25)
stored in the apex of the cell. Upon stimulation by the
vagus nerve or by cholecystokinin, the contents of the
granules are released into the lumen.
The centroacinar and duct cells secrete bicarbonate
ions under the control of gastrointestinal hormones. The
bicarbonate ions are formed by the action of carbonic
anhydrase:
carbonic anhydrase
C 0
2
+ H20 <
>
H
2
C 0
3
H+ + HCO”
The exact mechanism of HCOj" secretion into ductules is
not known.
Composition of Pancreatic Juice
Pancreatic juice is alkaline because of its high content of
HCO^. It neutralizes the acidic chyme from the stomach,
aided by the bile and small intestinal juices. In the jejunum,
chyme is maintained at a neutral pH as required for the
optimal functioning of the pancreatic digestive enzymes.
A normal pancreas secretes about 1.5-2 L of juice per day.
Pancreatic juice contains the proenzymes trypsinogen,
chymotrypsinogen, procarboxypeptidases, and proelas-
tase. All are activated by trypsin in the intestinal lumen.
Enteropeptidase located in the brush border of the jejunal
mucosa converts trypsinogen to trypsin. A trypsin inhibitor
in pancreatic juice protects against indiscriminate autodi-
gestion from intraductal activation of trypsinogen. Other
enzymes of pancreatic juice and their substrates are listed
below.
Enzyme
Substrate
Prophospholipase,
activated to phospholipase
Cholesteryl ester hydrolase
Lipase and procolipase,
converted to colipase
Amylase
Ribonucléase
Deoxyribonuclease
Phospholipids
Cholesteryl esters
Triacylglycerols
Starch and glycogen
RNA
DNA
Large Intestine
The large intestine extends from the ileocecal valve to
the anus. It is wider than the small intestine except for
the descending colon, which when empty may have the
same diameter as the small intestine. Major functions of
the colon are absorption of water, Na+, and other elec-
trolytes, as well as temporary storage of excreta followed
by their elimination. The colon harbors large numbers of
mostly anaerobic bacteria that can cause disease if they in-
vade tissues. These bacteria metabolize carbohydrates to
lactate, short-chain fatty acids (acetate, propionate, and bu-
tyrate), and gases (C02, CH4, and H2). Ammonia, a toxic
waste product, is produced from urea and other nitroge-
nous compounds. Other toxic substances are also produced
in the colon. Ammonia and amines (aromatic or aliphatic)
are absorbed and transported to the liver via the portal
blood, where the former is converted to urea (Chapter 17)
and the latter is detoxified. The liver thus protects the rest
of the body from toxic substances produced in the colon.
Colonic bacteria can also be a source of certain vitamins
(e.g., vitamin K, Chapter 36).
12.2 Gastrointestinal Hormones
Gastrointestinal hormones are chemical messengers that
regulate intestinal functions (e.g., motility and secretion),
which are also regulated by the autonomic nervous sys-
tem. The endocrine cells of the GI tract are interspersed
with mucosal cells and do not form discrete glands. The
GI tract has been described as the largest endocrine or-
gan in the body. In addition, the GI tract is noted for its
special significance in the development of endocrinology.
The first hormone discovered and characterized as a chem-
ical messenger by Bayliss and Starling in 1902 was the GI
hormone secretin.
The GI hormones are a heterogeneous group of peptides
that are released into the bloodstream in response to spe-
cific stimuli, bind to specific receptors on target cells to
produce biological responses, and are subject to feedback
regulation. Some may affect neighboring cells by being
transported through intercellular spaces (paracrine secre-
tion), and others serve as neurotransmitters in peptidergic
neurons (Chapter 30). Peptidergic neurons are present in
the GI tract and in the peripheral and central nervous sys-
tems. These peptides are synthesized in the GI tract and in
the brain, although in the GI tract some may originate from
vagal nerve endings. This dual localization of “brain-GI
tract” peptides is an example of biological conservation.
Of the many regulatory gastrointestinal peptides, four
are well understood and are considered to be authentic
