The Digestive System
absorbed by active transport processes, except for fat digestion
products, fat-soluble vitamins, and most water-soluble vitamins
(which are absorbed by diﬀusion).
Fat breakdown products are solubilized by bile salts (in micelles),
resynthesized to triglycerides in the intestinal mucosal cells, and
combined with other lipids and protein as chylomicrons that
enter the lacteals. Other absorbed substances enter the villus
blood capillaries and are transported to the liver via the hepatic
Developmental Aspects of the Digestive System
Te glandular accessory organs (salivary glands, liver, pancreas,
and gallbladder) form from outpocketings of the foregut
endoderm. Te mucosa of the alimentary canal develops from
the endoderm, which folds to form a tube. Te remaining three
tunics of the alimentary canal wall are formed by mesoderm.
Important congenital abnormalities of the digestive tract include
cle± palate/lip, tracheoesophageal ﬁstula, and cystic ﬁbrosis. All
interfere with normal nutrition.
Various inﬂammations plague the digestive system throughout life.
Appendicitis is common in adolescents, gastroenteritis and food
poisoning can occur at any time (given the proper irritating factors),
and ulcers and gallbladder problems increase in middle age.
Te eﬃciency of all digestive system processes declines in the
elderly, and periodontal disease is common. Diverticulosis, fecal
incontinence, and GI tract cancers such as stomach and colon
cancer appear with increasing frequency in an aging population.
Te longitudinal muscle in the muscularis is reduced to three
bands (teniae coli), which pucker its wall, producing haustra.
Te mucosa of most of the large intestine is simple columnar
epithelium containing abundant goblet cells.
Te major functions of the large intestine are absorption of water,
some electrolytes, and vitamins made by enteric bacteria, and
defecation (evacuation of food residues from the body).
Te defecation reﬂex is triggered when feces enter the rectum. It
involves parasympathetic reﬂexes leading to contraction of the
rectal walls and is aided by Valsalva’s maneuver.
Digestive System; Topic: Anatomy Review, pp. 3–5.
Physiology of Digestion
Digestion is accomplished by hydrolysis, catalyzed by enzymes.
Most digestion is done in the small intestine by intestinal (brush
border) enzymes and, more importantly, by pancreatic enzymes.
Alkaline pancreatic juice neutralizes the acidic chyme and
provides the proper environment for operation of the enzymes.
Both pancreatic juice (the main source of lipases) and bile are
necessary for fat breakdown.
Virtually all of the foodstuﬀs and most of the water and
electrolytes are absorbed in the small intestine. Most nutrients are
(Some questions have more than one correct answer. Select the best
answer or answers from the choices given.)
Te peritoneal cavity
is the same thing as the abdominopelvic
is ﬁlled with air,
like the pleural and pericardial
cavities is a potential space containing serous ﬂuid,
the pancreas and all of the duodenum.
Obstruction of the hepatopancreatic sphincter impairs digestion
by reducing the availability of
bile and HCl,
pancreatic juice and intestinal juice,
pancreatic juice and bile.
Te action of an enzyme is inﬂuenced by
its speciﬁc substrate,
the presence of needed
cofactors or coenzymes,
all of these.
Carbohydrates are acted on by
peptidases, trypsin, and
amylase, maltase, and sucrase,
peptidases, lipases, and galactase.
Te parasympathetic nervous system inﬂuences digestion by
relaxing smooth muscle,
stimulating peristalsis and
none of these.
Te digestive juice product containing enzymes capable of
digesting all four major foodstuﬀ categories is
Te vitamin associated with calcium absorption is
Someone has eaten a meal of buttered toast, cream, and
eggs. Which of the following would you expect to happen?
Compared to the period shortly a±er the meal, gastric
motility and secretion of HCl decrease when the food reaches the
gastric motility increases even as the person is
chewing the food (before swallowing);
fat will be emulsiﬁed in
the duodenum by the action of bile;
all of these.
Te site of production of cholecystokinin is
the small intestine,
the large intestine.
Which of the following is not characteristic of the colon?
divided into ascending, transverse, and descending portions;
it contains abundant bacteria, some of which synthesize
it is the main absorptive site;
much of the water and salts remaining in the wastes.
is attached to the pancreas,
stores and concentrates bile,
Te sphincter between the stomach and duodenum is
the gastroesophageal sphincter,
the ileocecal sphincter.
In items 13–17, trace the path of a single protein molecule that has
Te protein molecule will be digested by enzymes made by
the mouth, stomach, and colon,
the stomach, liver, and
the small intestine, mouth, and liver,
pancreas, stomach, and small intestine.
Te protein molecule must be digested before it can be
transported to and utilized by the cells because
only useful directly,
protein has a low pH,
proteins in the
circulating blood produce an adverse osmotic pressure,
protein is too large to be readily absorbed.
Te products of protein digestion enter the bloodstream largely
through cells lining
the small intestine,
the large intestine,
the bile duct.