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Chapter 3
Digestion and Absorption
Key Terms
1.
gustatory cells: Surface cells in the throat and on the taste buds in the mouth that transmit
taste information.
2.
absorption: The movement of substances into or across tissues; in particular, the passage
of nutrients and other substances into the walls of the gastrointestinal tract and then into
the bloodstream.
3.
chemosenses: The chemical sensing system in the body, including taste and smell.
4.
amylase: A salivary enzyme that catalyzes the hydrolysis of amylose, a starch.
5.
villi: Small, fingerlike projections that blanket the folds in the lining of the small
intestine.
6.
chyme: A mass of partially digested food and digestive juices moving from the stomach
into the small intestine.
7.
cholecystokinin (CCK): A hormone produced by cells in the small intestine that stimulate
the release of digestive enzymes from the pancreas and bile from the gallbladder.
8.
esophagus: The food pipe that extends from the pharynx to the stomach.
9.
catalyze: To speed up a chemical reaction.
10.
diarrhea: Watery stools due to reduced absorption of water.
11.
concentration gradient: A difference between the solute concentrations of two substances.
12.
flatus: Lower intestinal gas that is expelled through the rectum.
13.
lumen: Cavity or hollow channel in any organ or structure of the body.
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14.
gastrin: A hormone released from the walls of the stomach and duodenum that stimulates
gastric secretions and motility.
15.
segmentation: A series of muscular contractions that occur in the small intestine, thereby
dividing and mixing chyme.
16.
mucosa: The lining of the gastrointestinal tract, also called the intestinal wall. It is
composed of epithelial cells and glands.
17.
ulcer: A craterlike lesion that occurs in the lining of the stomach or duodenum.
18.
bolus: A chewed, moistened lump of food that is ready to be swallowed.
19.
passive diffusion: The movement of substances into or out of cells without the
expenditure of energy or the involvement of transport proteins in the cell membrane. Also
called simple diffusion.
20.
intrinsic factor: A protein released from cells in the stomach wall that binds to and aids in
absorption of vitamin B12.
21.
hydrochloric acid: A very strong acid of chloride and hydrogen atoms made by stomach
glands and secreted into the stomach. Also called gastric acid.
22.
sphincters: Circular bands of muscle fibers that surround the entrance or exit of a hollow
body structure (e.g., the stomach) and act as valves to control the flow of material.
23.
acrolein: A pungent decomposition product of fats, generated from dehydrating the
glycerol component of fats; responsible for the coughing attacks caused by the fumes
released by burning fat. This toxic water-soluble liquid vaporizes easily and is highly
flammable.
24.
pepsinogen: The inactive form of the enzyme pepsin.
25.
hydrolysis: A reaction that breaks apart a compound through the addition of water.
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26.
ileum: The terminal segment of the small intestine (about 150 cm [5 ft] long), which
opens into the large intestine.
27.
digestion: The process of transforming the foods we eat into units for absorption.
28.
emulsifiers: Agents that blend fatty and watery liquids by promoting the breakup of fat
into small particles and stabilizing their suspension in a watery solution.
29.
irritable bowel syndrome: A disruptive state of intestinal motility with no known cause.
30.
bile: An alkaline, yellow-green fluid that is produced in the liver and stored in the
gallbladder. It emulsifies dietary fats, aiding fat digestion and absorption.
31.
circular muscle: Layers of smooth muscle that surround organs, including the stomach
and the small intestine.
32.
gallbladder: A pear-shaped sac that stores and concentrates bile from the liver.
33.
enzymes: Proteins in the body that speed up the rate of chemical reactions but are not
altered in the process.
34.
mucus: A slippery substance secreted in the GI tract (and other body linings) that protects
cells from irritants.
35.
microvilli: Minute, hairlike projections that extend from the surface of absorptive cells
facing the intestinal lumen.
36.
longitudinal muscle: Muscle fibers aligned lengthwise.
37.
pepsin: A protein-digesting enzyme produced by the stomach.
38.
dyspepsia: A condition also known as upset stomach or indigestion; refers to difficulty
with digestion.
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Fill-in-the-Blank
1.
Also called the preabsorptive phase responses, the cephalic phase responses are those
responses of the parasympathetic nervous system to the sight, smell,
thought, and
sound of food.
2.
The common chemical sense is a chemosensory mechanism that contributes to
our
senses of smell and taste. It comprises thousands of nerve endings, especially
on the
moist surfaces of the eyes, nose, mouth, and throat.
3.
The central nervous system (CNS) consists of the brain and the spinal cord. It transmits
signals that control muscular actions and glandular secretions along the entire
gastrointestinal (GI) tract.
4.
Chronic pain in the upper abdomen not due to any obvious physical cause is called
functional dyspepsia.
5.
Diverticulosis occurs when small pouches push outward through weak spots in the colon.
6.
The movement of substances into or out of cells against a concentration gradient is called
active transport. This requires energy and involves carrier proteins in the cell
membrane.
7.
Lingual lipase is a fat-splitting enzyme secreted by cells at the base of the tongue.
8.
The part of the central nervous system that regulates the automatic responses of the body
is the autonomic nervous system.
9.
The enteric nervous system is a network of nerves located in the GI wall.
10.
The largest glandular organ in the body, the liver, produces and secretes bile, detoxifies
harmful substances, and helps metabolize carbohydrates, lipids, proteins, and
micronutrients.
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11.
Elimination is a term that refers to the removal of undigested food from the body.
12.
The esophageal sphincter is the opening between the esophagus and the stomach that
relaxes and opens to allow the bolus to travel into the stomach, and then closes behind it.
It also acts as a barrier to prevent the reflux of gastric contents.
13.
The middle section of the small intestine (about 120 cm [4ft] long), lying between the
duodenum and ileum, is the jejunum.
14.
Infrequent and difficult bowel movements, followed by a sensation of incomplete
evacuation, is called constipation.
15.
The vascular system is a network of veins and arteries through which the blood carries
nutrients. It is also called the blood circulatory system.
16.
Facilitated diffusion is a process by which carrier (transport) proteins in the cell
membrane transport substances into or out of cells down a concentration gradient.
17.
Recycling of certain compounds between the small intestine and the liver is called
enterohepatic circulation.
18.
The gastrointestinal (GI) tract is the connected series of organs and structures used for
digestion of food and absorption of nutrients.
19.
The sphincter at the junction of the small and large intestines is the ileocecal valve.
20.
Tissue damage to the esophagus due to the reflux of gastric contents is known as
gastroesophageal reflux disease (GERD).
21.
The lacteal is a small lymphatic vessel in the interior of each intestinal villus that picks
up fat-soluble compounds from intestinal cells.
22.
Gastric lipase is an enzyme in the stomach that primarily breaks down butterfat.
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23.
The pancreas secretes enzymes that affect the digestion and absorption of nutrients and
releases hormones, such as insulin, that regulate metabolism as well as the way nutrients
are used in the body.
24. The hormone gastric inhibitory peptide is released from the walls of the duodenum and
slows the release of the stomach contents into the small intestine and stimulates release of
insulin from the pancreas.
25.
The lymphatic system is a system of small vessels, ducts, valves, and organized tissue
(e.g., lymph nodes) through which lymph moves from its origin in the tissues toward the
heart.
26.
Peristalsis is the wavelike, rhythmic muscular contractions of the GI tract that propel its
contents down the tract.
27.
The pyloric sphincter is a circular muscle that forms the opening between the duodenum
and the stomach. It regulates the passage of food into the small intestine.
28.
The enlarged, muscular, saclike portion of the digestive tract between the esophagus and
the small intestine is the stomach. It has a capacity of about 1 quart.
29.
The tube (approximately 3 meters [10 ft] long) where the digestion of protein, fat, and
carbohydrate is completed and where the majority of nutrients are absorbed is the small
intestine. It is divided into three parts: the duodenum, the jejunum, and the ileum.
30.
Salivary glands are glands in the mouth that release saliva.
31.
pH is a measurement of the hydrogen ion concentration, or acidity, of a solution.
32.
The blind pouch at the beginning of the large intestine into which the ileum opens from
one side and which is continuous with the colon is called the cecum.
33.
The portion of the large intestine extending from the cecum to the rectum is the colon.
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34.
The large intestine is the tube (about 150 cm [5 ft] long) extending from the ileum of the
small intestine to the anus. It includes the appendix, cecum, colon, rectum, and anal
canal.
35.
Digestive secretions are substances released at different places in the GI tract to speed
the breakdown of ingested carbohydrates, fats, and proteins.
36.
Burning pain behind the breastbone area caused by acidic stomach contents backing up
into the esophagus is called heartburn.
37.
The duodenum is the portion of the small intestine closest to the stomach. It is 25 to 30
cm (10 to 12 in.) long and wider than the remainder of the small intestine.
38.
Fluid that travels through the lymphatic system, made up of large fat particles and fluid
drained from the areas between cells, is called lymph.
39.
The process of separating and removing waste products of metabolism is excretion.
Fill-in-the-Blank Summaries
Putting It All Together: Digestion and Absorption
The digestive process begins in the mouth. Chewing breaks food into smaller pieces, increasing
the surface area available to enzymes. Saliva contains the enzyme amylase, which breaks
down starch into small sugar molecules. In the mouth, saliva and mucus blend with the
food to form a bolus. This ball of chewed food then slides through the esophagus to the
stomach. Once in the stomach, the esophageal sphincter keeps the food from sliding back
into the esophagus. Hydrochloric acid creates an acidic environment in the stomach that
kills many pathogenic bacteria and aids in the digestion of protein. When the chyme is
ready to leave the stomach, about 30–40 percent of carbohydrate, 10–20 percent of protein,
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and less than 10 percent of fat have been digested. Very little absorption occurs in the
stomach. In the small intestine, most digestion takes place in the duodenum. Fats do not
usually mix with water, but this process is facilitated by bile once it is released from the
gallbladder into the duodenum. Nutrients absorbed through the intestinal lining pass into
the interior of the villi. Each villus contains blood vessels and a lacteal. Most minerals,
with the exception of potassium, sodium, and chloride, are absorbed in the small intestine.
Undigested material and some liquid move on to the large intestine, where water and
electrolytes are absorbed, leaving waste material to be excreted as feces.
Nutrition and GI Disorders
A diet low in fiber and water and high in fat is the most common cause of constipation. As
people age, the colon develops small pouches that bulge outward through weak spots. This
condition is known as diverticulosis. Eating a diet high in fiber can help prevent this
condition. GERD occurs when the lower esophageal sphincter relaxes inappropriately.
The esophagus has no protective mucous lining, so acid can quickly damage it. Doctors
recommend avoiding foods that can weaken this sphincter, such as peppermint. The cause
of IBS remains a mystery. Some foods that are often offenders are beans and chocolate.
Without treatment, IBS will not progress to a more serious illness. It is likely that a number
of physical and psychosocial factors combine to trigger this disorder. After lung cancer,
colon cancer is the most common form of cancer in the United States. Research to date
shows that diets high in vegetables and regular physical activity are the most significant
factors in reducing risk of this type of cancer. Flatus composition depends largely on
carbohydrate intake and the activity of the colon’s bacterial population. The major cause
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of ulcers is Helicobacter pylori, which creates ulcers by weakening the protective mucus
coating around the lining of the stomach.
Short Answer
1.
2.
The GI tract has a large variety of functions. Explain the six primary functions.
1.
Ingestion—the receipt and softening of food
2.
Transport of digested food
3.
Secretion of digestive enzymes, acid, mucus, and bile
4.
Absorption of end products of digestion
5.
Movement of undigested material
6.
Elimination—the transport, storage, and excretion of waste products
In addition to enzymes, other chemicals support the digestive process. List these four
chemicals.
Acid in the stomach; neutralizing base in the small intestine; bile that prepares fat for
digestion; mucus secreted along the GI tract
3.
How does eating a high fiber diet help to prevent constipation?
Some fibers dissolve easily in water and take on a soft, gel-like texture in the intestine.
Other fibers pass along unchanged through the intestines. The bulk and soft texture of
fiber help prevent hard, dry stools that are difficult to pass.
4.
The digestion of protein, fat, and nearly all carbohydrates is completed and most nutrients
are absorbed in which part of the digestive tract?
Small intestine
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5.
Which body organ is considered the “chemical factory,” performing over 500 chemical
functions that include the production of blood proteins, cholesterol, and sugars?
Liver
6.
Which nutrients are absorbed into the lymph system in the small intestine and leave the
GI tract bypassing the liver?
Fat-soluble nutrients and most end products of fat digestion
7.
What substance present in a healthy stomach helps prevent foodborne illnesses?
Hydrochloric acid
8.
Identify four ways you can prevent and manage most GI disorders.
Add fiber-rich foods and water to your diet; exercise; manage stress; maintain a healthy
weight
Labeling
Identify the structural organization of the GI tract wall.
Serosa; Longitudinal muscle; Circular muscle; Mucosa; Submucosa; Lumen
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