Maintenance of the Body
which inhibit gastric secretory activity. Sympathetic activity also
inhibits gastric secretion.
Mechanical breakdown in the stomach is triggered by stomach
distension and coupled to food propulsion and stomach emptying.
Food movement into the duodenum is controlled by the pylorus
and feedback signals from the small intestine. Pacemaker cells in the
smooth muscle sheet set the maximal rate of peristalsis.
Digestive System; Topic: Control of the Digestive System,
pp. 3, 5–6, 8.
The Small Intestine and Associated Structures
Te small intestine is the major digestive and absorptive organ.
It extends from the pyloric sphincter to the ileocecal valve. Its
three subdivisions are the duodenum, jejunum, and ileum. Te
bile duct and pancreatic duct join to form the hepatopancreatic
ampulla and empty their secretions into the duodenum through
the hepatopancreatic sphincter.
Circular folds, villi, and microvilli increase the intestinal surface
area for digestion and absorption.
Te duodenal submucosa contains elaborate mucus-secreting
duodenal glands. Te mucosa of the ileum contains Peyer’s
patches (lymphoid follicles). Te duodenum is covered not with a
serosa but an adventitia.
Intestinal juice is largely water and relatively enzyme-poor. Te
major stimuli for its release are stretch and chemicals.
Te liver is a lobed organ overlying the stomach. Its digestive role is
to produce bile, which it secretes into the common hepatic duct.
Te structural and functional units of the liver are the liver
lobules. Blood ﬂowing to the liver via the hepatic artery and
hepatic portal vein ﬂows into its sinusoids, from which stellate
macrophages remove debris and hepatocytes remove nutrients.
Hepatocytes store glucose as glycogen, use amino acids to make
plasma proteins, and detoxify metabolic wastes and drugs.
Bile is made continuously by the hepatocytes. Bile salts and
secretin stimulate bile production.
Te gallbladder, a muscular sac that lies beneath the right liver
lobe, stores and concentrates bile.
Bile contains electrolytes, a variety of fatty substances, bile
salts, and bile pigments in an aqueous medium. Bile salts are
emulsifying agents; they disperse fats and form water-soluble
micelles, which solubilize the products of fat digestion.
Cholecystokinin released by the small intestine stimulates the
gallbladder to contract and the hepatopancreatic sphincter to
relax, allowing bile (and pancreatic juice) to enter the duodenum.
Te pancreas is retroperitoneal between the spleen and small
intestine. Its exocrine product, pancreatic juice, is carried to the
duodenum via the pancreatic duct.
Pancreatic juice is a HCO
-rich ﬂuid containing enzymes that
digest all categories of foods. Intestinal hormones and the vagus
nerves control secretion of pancreatic juice.
Digestive System; Topic: Secretion, pp. 11–14.
Mechanical breakdown and propulsion in the small intestine mix
chyme with digestive juices and bile and force the residues, largely
by segmentation, through the ileocecal valve. Pacemaker cells set
the rate of segmentation. Ileocecal valve opening is controlled by
the gastroileal reﬂex and gastrin.
The Large Intestine
Te subdivisions of the large intestine are the cecum (and appendix),
colon (ascending, transverse, descending, and sigmoid portions),
rectum, and anal canal. It opens to the body exterior at the anus.
Te 20 deciduous teeth begin to be shed at the age of 6 and are
gradually replaced during childhood and adolescence by the 32
±eeth are classed as incisors, canines, premolars, and molars. Each
tooth has an enamel-covered crown and a cement-covered root. Te
bulk of the tooth is dentin, which surrounds the central pulp cavity.
A periodontal ligament secures the tooth to the bony alveolus.
Food propelled from the mouth passes through the oropharynx
and laryngopharynx. Te mucosa of the pharynx is stratiﬁed
squamous epithelium; skeletal muscles in its wall (constrictor
muscles) move food toward the esophagus.
Te esophagus extends from the laryngopharynx and joins
the stomach at the cardial oriﬁce, which is surrounded by the
Te esophageal mucosa is stratiﬁed squamous epithelium. Its
muscularis is skeletal muscle superiorly and changes to smooth
muscle inferiorly. It has an adventitia rather than a serosa.
Digestive Processes: Mouth to Esophagus
Te mouth and associated accessory organs accomplish food
ingestion and mechanical breakdown (chewing and mixing),
initiate the digestion of starch (salivary amylase), and propel food
into the pharynx (buccal phase of swallowing).
±eeth masticate food. Chewing is initiated voluntarily and then
Te tongue mixes food with saliva, compacts it into a bolus,
and initiates swallowing (the buccal phase). Te pharynx and
esophagus are primarily food conduits that conduct food to the
stomach by peristalsis. Te swallowing center in the medulla and
pons controls this phase reﬂexively. When the peristaltic wave
approaches the gastroesophageal sphincter, the sphincter relaxes
to allow food to enter the stomach.
Digestive System; Topic: Motility, pp. 3–5.
Te J-shaped stomach lies in the upper le² quadrant of the
abdomen. Its major regions are the cardia, fundus, body, and
pyloric part. When empty, its internal surface exhibits rugae.
Te stomach muscularis contains a third (oblique) layer of
smooth muscle that allows it to churn and mix food.
Te stomach mucosa is simple columnar epithelium dotted
with gastric pits that lead into gastric glands. Secretory cells in
the gastric glands include pepsinogen-producing chief cells;
parietal cells, which secrete hydrochloric acid and intrinsic factor;
mucous neck cells, which produce mucus; and enteroendocrine
cells, which secrete hormones.
Te mucosal barrier, which protects the stomach from self-
digestion and HCl, reﬂects the fact that the mucosal cells are
connected by tight junctions, secrete a thick mucus, and are
quickly replaced when damaged.
Protein digestion is initiated in the stomach by activated pepsin
and requires acidic conditions (provided by HCl). Few substances
are absorbed in the stomach.
Both nervous and hormonal factors control gastric secretory
activity. Te three phases of gastric secretion are cephalic, gastric,
and intestinal. Most food-related stimuli acting on the head and
stomach (cephalic and gastric, respectively) stimulate gastric
secretion. Most stimuli acting on the small intestine trigger the
enterogastric reﬂex and release of secretin and CCK, both of