Chapter 3 – Digestion, Absorption, and Transport
Learning Objectives
After completing Chapter 3, the student will be able to:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
List the segments of the digestive tract in order from the mouth to the colon.
Explain the mechanical processes of digestion in order of occurrence in the digestive tract.
List the five organs and their secretions, which assist in the breakdown of food.
List the secretions that break down carbohydrates.
List the secretions that break down protein.
List the secretions that break down fat.
Describe the anatomy of the absorptive system.
Explain the role of the circulatory system and the lymphatic system in transport of nutrients.
Identify the hormones involved in digestive and absorptive processes.
Apply the concepts presented in the chapter to explain common digestive tract problems.
I.
Digestion
Digestion is the process of breaking down foods into nutrients to prepare for absorption
while overcoming 7 challenges.
A. Anatomy of the Digestive Tract – The gastrointestinal (GI) tract is the flexible
muscular tube from mouth to anus. The lumen is the inner space of the tract.
1. The mouth is the beginning of the digestive system. Digestion in the mouth involves
mastication (chewing), the stimulation of taste buds, and swallowing. The epiglottis
closes to prevent food from entering the pharynx. After swallowing, the food is
called a bolus.
2. The esophagus is the tube that leads the bolus to the stomach. There is a sphincter at
the upper and lower (also known as the cardiac sphincter) ends of the esophagus.
3. The stomach adds juices and grinds the bolus to a semi liquid mass called chyme.
The pyloric sphincter regulates the flow of partially digested food into the small
intestine.
4. The small intestine receives digestive juices from the gallbladder and the pancreas.
The three segments of the small intestine are the duodenum, the jejunum and the
ileum.
5. The large intestine (colon) begins at the ileocecal valve and ends at the rectum and
anus. The chyme passes by the opening of the appendix.
B. The Muscular Action of Digestion – under autonomic control
1. Peristalsis pushes the digestive contents along.
2. Stomach action involves circular, longitudinal, and diagonal muscles.
3. Segmentation is contractions by circular muscles that contract and squeeze contents
to promote mixing with digestive juices.
4. Sphincter contractions open and close passageways. This prevents reflux and
controls the passage of contents.
C.
The Secretions of Digestion
1. Includes digestive enzymes that act as catalysts in hydrolysis reactions
2. Saliva from the salivary glands moistens foods
3. Gastric juice from the gastric glands includes hydrochloric acid. The goblet cells
of the stomach wall secrete mucus to protect the walls of the stomach from the high
acidity levels that are measured by pH units.
4. Pancreatic juice contains intestinal enzymes (carbohydrase, lipase, protease) and
bicarbonate.
5. Bile is produced by the liver, stored in the gall bladder, and acts as an emulsifier to
suspend fat.
D.
The Final Stage
1. Energy-yielding nutrients are disassembled for absorption.
2. Vitamins, minerals and water can be absorbed.
3. Undigested residues, including some fibers, continue through the digestive tract and
form stool.
4. Recycling of usable materials
II. Absorption
The enormous surface area of the small intestine facilitates nutrient absorption. Nutrients can
be absorbed through simple diffusion, facilitated diffusion, or active transport.
A. Anatomy of the Absorptive System
1. Villi are the fingerlike projections within the folds of the small intestine that move in
a wave-like pattern to trap nutrients.
2. Microvilli are the microscopic hairlike projections on each villi.
3. Crypts are the tubular glands that lie between the intestinal villi.
4. Goblet cells are located between the villi and secrete a protective thick mucus.
B.
A Closer Look at the Intestinal Cells
1. Specialization of the cells to absorb different nutrients
2. “Food combining” which emphasizes separating food for digestive purposes is a
myth.
3. Preparing Nutrients for Transport
a. Water-soluble nutrients and small products of fat digestion are released to the
bloodstream.
b. Fat-soluble vitamins and larger fats form chylomicrons and are released to the
lymphatic system.
III. The Circulatory System
Water-soluble nutrients and small fats are routed to the liver before being transported to the
cells. Fat-soluble nutrients, entering from the lymph, bypass the liver at first and eventually
enter the vascular system.
A.
1.
2.
3.
The vascular system consists of arteries, capillaries and veins.
The hepatic portal vein directs blood from the GI tract to the liver.
The hepatic vein takes blood from the liver to the heart.
The liver protects against toxic substances.
B. The lymphatic system consists of one-way vessels to transport fluid.
1. Lymph is a clear, yellowish fluid without red blood cells or platelets that moves
through the body by muscle contractions.
2. The thoracic duct is the lymph’s route to the heart.
3. The subclavian vein provides a return of lymph to the vascular system.
4. Lacteals are the lymphatic vessels of the intestine that absorb nutrients and pass them
to the lymphatic system.
IV.
The Health and Regulation of the GI Tract
The principle of homeostasis is important in the functioning of the digestive and absorptive
systems. The body’s hormonal and nervous control systems keep conditions normal.
A.
Gastrointestinal Bacteria
1. A healthy GI tract has many different non-disease-causing bacteria known as flora or
microflora.
2. Probiotics are bacteria found in the GI tract that can be beneficial to health. An
example is the bacteria found in yogurt.
3. Prebiotics are foods that are used as food by intestinal bacteria.
B. Gastrointestinal Hormones and Nerve Pathways
1. Hormones act as messengers, and those involved in the GI tract are known as
enterogastrones.
2. Gastrin is secreted by the stomach.
3. Pyloric sphincter
4. Secretin is secreted by the duodenum.
5. Pancreatic secretions change based on the content of the diet.
6. The pancreas is protected against enzymes by creating enzyme precursors called
proenzymes or zymogen.
7. Cholecystokinin targets the gall bladder.
8. Motility slows for foods that take longer to be digested.
C. The System at Its Best
1.
A healthy digestive tract is essential.
2.
Balance, moderation, variety and adequacy of meals are important.
V. Highlight: Common Digestive Problems
Many digestive problems reflect hurried lifestyles. Remedies include handling stress,
relaxing, getting enough sleep, and eating in a leisurely manner, in addition to diet strategies.
A. Choking
1. Food becomes lodged in the trachea.
2. The larynx cannot make sounds.
3. The Heimlich maneuver may need to be used.
4. Strategies
a. Small bites
b. Chew thoroughly.
c. Don’t talk or laugh with food in the mouth.
d. Don’t eat when breathing hard.
B.
1.
2.
3.
Vomiting
Body’s adaptive mechanism
Dehydration is a concern.
May be self-induced as in eating disorders
C.
Diarrhea
1. Frequent, loose, watery stools
2. Irritable bowel syndrome or colitis is one of the common GI disorders.
3. Strategies
a. Rest
b. Drink fluids
c. Medical help is needed if it persists.
D.
1.
2.
3.
4.
Constipation
Defecation habits are different among people.
Many causes are possible.
Hemorrhoids may be a problem.
Diverticulosis is a condition in which the intestinal walls weaken and bulge. The
bulging pockets are called diverticula. Diverticulitis is a worsened condition and
requires intervention.
5. Use of laxatives, enemas and mineral oil may not be necessary with lifestyle
changes.
6. Colonic irrigation is the internal washing of the large intestine and can be
hazardous.
7. Strategies
a. High-fiber diet
b. Increased fluids
c. Exercise regularly.
d. Respond quickly to the urge to defecate.
E. Belching and Gas
1.
Strategies
a. Eat slowly.
b. Chew thoroughly.
c. Relax while eating.
d. Watch bothersome foods.
2.
Hiccups are triggered by eating or drinking too fast.
F.
Heartburn and “Acid Indigestion”
1. Gastroesophageal reflux is the backward flow of stomach contents into the
esophagus.
2. Antacids and acid controllers may help indigestion.
3. Strategies
a. Small meals
b. Liquids between meals
c. Sit up while eating.
d. Wait 1 hour after eating before lying down.
e. Wait 2 hours after eating before exercising.
f. Refrain from tight-fitting clothing.
g. Avoid bothersome foods.
h. Refrain from tobacco use.
i. Lose weight if overweight.
G.
1.
2.
a.
b.
c.
d.
Ulcers
Peptic ulcers can be gastric or duodenal.
Strategies
Take prescribed medicine.
Avoid caffeine- and alcohol-containing foods.
Minimize aspirin and ibuprofen use.
No smoking.
Q & As for Discussion
1. Describe the obstacles associated with digesting food and the solutions offered by the
human body.
2. Describe the path food follows as it travels through the digestive system. Summarize the
muscular actions that take place along the way.
3. Name five organs that secrete digestive juices. How do the juices and enzymes facilitate
digestion?
4. Describe the problems involved with absorbing nutrients and the solutions offered by
the small intestine.
5. How is blood routed through the digestive system? Which nutrients enter the
bloodstream directly? Which are first absorbed into the lymph?
6. Describe how the body coordinates and regulates the processes of digestion and
absorption.
7. How does the composition of the diet influence the functioning of the GI tract?
8. What steps can you take to help your GI tract function at its best?