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McKinley/O’Loughlin
Human Anatomy, 2nd Edition
CHAPTER 26
Answers to “What Did You Learn?”
1.
The GI tract includes the oral cavity, pharynx, esophagus, stomach, small
intestine, and large intestine.
2.
Secretion is the process of producing and releasing fluid products, such as acid,
bile, digestive enzymes, and mucus, in order to facilitate chemical digestion or
passage of material through the GI tract lumen. Absorption involves either the
passive movement or active transport of electrolytes, products of digestion,
vitamins, and water, through the GI tract epithelium and into the lymph vessels
and blood vessels.
3.
The components of saliva are water (moisturizes food and cleanes mouth), ions,
immunoglobulin A (a class of secreted antibodies that inhibit bacterial growth),
lysozyme (an antibacterial enzyme), mucin, and salivary amylase (initiates
chemical digestion of carbohydrate in the mouth).
4.
Permanent teeth include the incisors, canines, premolars, and molars. The
incisors are designed for slicing or cutting into food. Canines have pointed tips
for puncturing and tearing food. Premolars have flat crowns with cusps that are
used to crush and grind ingested materials. Molars have large, broad flat crowns
with distinctive cusps, and are also used for grinding and crushing.
5.
Sequential contraction of the pharyngeal constrictors decreases the diameter of the
pharynx beginning at its superior end and moving toward its inferior end, thus
forcing swallowed material toward the esophagus.
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6.
Human Anatomy, 2nd Edition
Intraperitoneal refers to organs that are completely surrounded by visceral
peritoneum. They include the stomach, part of the duodenum of the small
intestine, the jejunum and the ileum of the small intestine, the cecum, appendix
and transverse and sigmoid colon of the large intestine. Retroperitoneal refers to
organs that typically lie directly against the posterior abdominal wall, so only
their anterolateral portions are covered with peritoneum. Retroperitoneal organs
include most of the duodenum, the pancreas, and the ascending and descending
colon of the large intestine.
7.
The four main tunics and their default patterns are a mucosa (typically lined with
simple columnar epithelium), submucosa (formed from areolar or dense irregular
connective tissue), muscularis (typically formed from two layers of smooth
muscle), and an adventitia or serosa.
8.
The esophageal tunics differ from the default tunic pattern in that the mucosa has
a stratified squamous epithelium and the muscularis has a mixture of skeletal and
smooth muscle.
9.
The three phases of swallowing are the voluntary phase, the pharyngeal phase,
and the esophageal phase. The voluntary phase occurs after the ingestion of food.
Food mixed with saliva forms a bolus that is pushed into the archway leading into
the oropharynx. The appearance of the food bolus at the oropharynx initiates the
pharyngeal phase. The bolus passes quickly and involuntarily through the
pharynx to the esophagus. The pharyngeal phase includes the soft palate
elevation to block the nasopharynx, the reception of the food bolus into the
oropharynx, and the movement of the larynx toward the epiglottis. The
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Human Anatomy, 2nd Edition
esophageal phase (involuntary stage) moves the bolus through the esophagus and
into the stomach.
10.
The four regions of the stomach are: the cardia, fundus, body, and pylorus. The
cardia is a small, narrow, superior entryway into the stomach lumen from the
esophagus. The fundus is the dome-shaped region lateral and superior to the
esophageal connection with the stomach. Its superior surface contacts the
diaphragm. The body is the largest region of the stomach; it is inferior to the
cardiac orifice and the fundus. The pylorus is a narrow, medially directed, funnelshaped region that forms the terminal part of the stomach.
11.
The five types of epithelial cells in the stomach are surface mucous cells (secrete
mucin) , mucous neck cells (secrete alkaline mucin), parietal cells (secrete
hydrochloric acid and intrinsic factor) , chief cells (secrete pepsinogen), and
enteroendocrine cells (secrete gastrin).
12.
The duodenum forms the first region of the small intestine. It is mostly
retroperitoneal and is approximately 10 inches long. It is arched into a C-shape
around the head of the pancreas. The duodenum begins at the pyloric sphincter
and ends at the duodenojejunal flexure where it connects with the jejunum. The
jejunum is the middle region of the small intestine and forms approximately 2/5
the length of the small intestine. The ileum is the last region of the small intestine
and forms approximately 3/5 the length of the small intestine. It extends from its
origin at the end of the jejunum and its distal end terminates at the ileocecal valve.
The jejunum and ileum are intraperitoneal and suspended in the abdomen by a
mesentery. Circular folds are mucosal and submucosal folds in all three regions of
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Human Anatomy, 2nd Edition
the small intestine, but are best developed in the duodenum and jejunum.
Additionally, villi are larger and more numerous in the jejunum.
Peyer's patches
are much more numerous in the ileum than in the jejunum.
13.
The ascending colon, descending colon, rectum, and anal canal are
retroperitoneal. The cecum, vermiform appendix, transverse colon, and sigmoid
colon are intraperitoneal organs.
14.
The movements and reflexes that propel material through the large intestine are
peristaltic movements, haustral churning, and mass movement.
15.
Portal triads are composed of a branch of the hepatic portal vein, hepatic artery,
and the bile duct.
16.
The gallbladder stores and concentrates the bile produced by the liver. The
gallbladder will release this concentrated bile in response to ingesting a fatty
meal.
17.
Pancreatic acini are cells that secrete pancreatic juice, which is an alkaline fluid
that also contains digestive enzymes.
18. A decrease in mucin secretion results in a decrease in thickness and volume of
mucus that coat and protect the luminal lining of GI tract organs. Without this
protection the organ lining will be subject to abrasion by a moving bolus or
chyme and damage by acid or enzymes.
19.
The liver parenchyma, gallbladder, pancreas and biliary apparatus develop from
buds or outgrowths from the endoderm of the duodenum.
Answers to “Content Review”
McKinley/O’Loughlin
1.
Human Anatomy, 2nd Edition
The oral cavity initiates the process of both mechanical and chemical digestion.
The amylase in saliva begins the mechanical digestion of carbohydrates. Saliva
moistens the food, while the teeth break apart the food into smaller components.
The tongue helps mix the saliva and food, and pushes the material against the
palate of the mouth, where it is transformed into a bolus. A bolus is eventually
swallowed.
2.
The general structural plan of the GI tract from the esophagus through the large
intestine is a tube composed of four concentric layers, called tunics. From
innermost to outermost are the tunica mucosa, submucosa, muscularis, and serosa
(or adventitia). The mucosa is composed of an epithelium (typically simple
columnar), a lamina propria, and a muscularis mucosae. The submucosa is
formed from areolar or dense irregular connective tissue, while the muscularis
typically contains two smooth muscle layers (an inner circular and outer
longitudinal). The adventitia is areolar connective tissue, while serosa also has a
layer of peritoneum covering this connective tissue. The stomach’s mucosa is
lined with simple columnar epithelium, but no goblet cells. The mucosa also has
gastric pits which are lined by gastric glands. The muscularis layer has three
layers of smooth muscle instead of the usual two layers.
3.
Gastric juice and pancreatic juice both assist with the chemical digestion of food.
Gastric juice is formed by the gastric glands of the stomach. It is highly acidic
and contains hydrochloric acid, pepsinogen, mucin, gastrin and intrinsic factor
(the latter assists in vitamin B12 absorption). Pancreatic juice, in contast, is
alkaline due to the high percentage of bicarbonate. The bicarbonate acts to
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Human Anatomy, 2nd Edition
neutralize the acidic chyme that enters the duodenum from the stomach. Other
components of pancreatic juice include mucin and digestive enzymes.
4.
Internally, the mucosal and submucosal tunics of the small intestine are thrown
into circular folds. They help increase the surface area of the small intestine,
allowing a greater opportunity for nutrients to be absorbed, and slow down
movement of chyme to ensure that this material remains within the small intestine
for maximal nutrient absorption. Along these circular folds are smaller fingerlike
projections of mucosa only, called villi, that act to increase surface area for
secretion and absorption. Microvilli are apical membrane surface folds to
increase surface area at the absorptive and secretory surface of each cell.
5.
The teniae coli in the wall of the large intestine helps form sacs called haustra.
Movement of digested materials through these sacs is called haustral churning. It
occurs after a relaxed haustrum fills with digested/fecal material to a point where
its distension stimulates reflex contractions in the muscularis causing a churning
and movement of the material to more distal hasutra.
6.
At the periphery of hepatic lobules, there are several portal triads, composed of a
least a branch of the hepatic portal vein, hepatic artery, and the bile duct. The
hepatic portal vein carries blood that has already passed through the capillary beds
of the GI tract, spleen, and pancreas. This blood is rich in nutrients and other
absorbed substances but relatively poor in oxygen. The hepatic artery, a branch
of the celiac trunk, carries well-oxygenated blood and supplies the remaining
blood to the liver. Blood from branches of these two vessels mixes in passing to
and through the liver lobules and drained into the central veins. Bile produced by
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Human Anatomy, 2nd Edition
hepatocytes is secreted into bile canaliculi: it flows through these tiny channels to
the portal triad where it enters the bile ducts and eventually exits the liver. Bile
emulsifies fat arriving in the small intestine. Without this emulsification, the fat
in our consumed food could not be chemically digested.
7.
Attached to the inferior surface of the liver, a sac-like organ, called the
gallbladder, stores and concentrates bile produced by the liver. Bile is a yellowgreen fluid produced by hepatocytes; its primary digestive function is to aid in the
emulsification of fat.
8.
Ingested material enters the GI tract via the oral cavity. The material is
mechanically digested by the teeth and tongue, while chemical digestion of
carbohydrates occurs from the amylase in saliva. The ingested material is now
called a bolus. When the bolus is swallowed, it leaves the oral cavity and enters
the oropharynx, laryngopharynx, and esophagus. The bolus is then propelled into
the stomach, where both mechanical and chemical ingestion of the material occur.
The paste-like material that leaves the stomach is called chyme. Chyme leaves
the stomach and enters the small intestine, where mechanical and chemical
digestion will be completed. The chyme first enters the duodenum, where it will
mix with bile and pancreatic juices secreted via the major duodenal papilla. Then
the material travels through the jejunum and ileum, and then enters the large
intestine, where the material is referred to as feces, or fecal matter. Material first
enters the cecum of the large intestine, and then travels into the ascending colon,
transverse colon, descending colon, sigmoid colon, rectum and anal canal. Feces
leaves the body through the anus.
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Human Anatomy, 2nd Edition
9. The mucin-producing cells throughout the GI tract collectively produce a
covering layer of viscous mucus. This mucus lubricates the lining and prevents
desiccation of the lining cells. Additionally the mucus protects the epithelial
lining from abrasion, or damage as a result of exposure to acid or enzymes in the
contents of the tract.
10. Most of the small intestine and the proximal part of the large intestine are formed
from a section of the embryologic gut tube called the midgut. During the fifth
week of development, the midgut elongates and forms a primary intestinal loop.
The upper portion of the loop is called the cranial loop, while the lower part is
called the caudal loop. The cranial loop will form most of the small intestine,
while the caudal loop will form the proximal part of the large intestine.
Beginning the sixth week, the intestinal loop grows and herniates into the
umbilicus (due to space constraints in the abdominal cavity). As it herniates, it
undergoes a 90 degree counterclockwise rotation, as viewed from the front of the
embryo. The loop remains herniated until week 10, when the abdominal cavity
has grown spacious enough to house all of the intestines. During weeks 10-11,
the intestinal loop retracts into the body and in so doing, it rotates another 180
degrees counterclockwise. Now the cranial limb is on the left side of the body
and the caudal limb is on the right side of the body. These limbs will attain their
postnatal position and develop into most of the small and large intestine.