Digestion, Absorption, and Transport
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 semiliquid 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
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.
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 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.
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2.
3.
4.
5.
6.
Gastrin is secreted by the stomach.
Pyloric sphincter
Secretin is secreted by the duodenum.
Pancreatic secretions change based on the content of the diet.
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.
VI: 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 larnyx 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. Vomiting
1. Body’s adaptive mechanism
2. Dehydration is a concern.
3. 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. Constipation
1. Defecation habits are different among people.
2. Many causes are possible.
3. Hemorrhoids may be a problem.
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4. 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. Ulcers
1. Peptic ulcers can be gastric or duodenal.
2. Strategies
a. Take prescribed medicine.
b. Avoid caffeine- and alcohol-containing foods.
c. Minimize aspirin and ibuprofen use.
d. No smoking.
1. Describe the obstacles associated with digesting food and the solutions offered by the
human body.
.
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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 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?
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Food Habits, Digestion, and Absorption
Digestion transforms the foods we eat into nutrients and absorption moves nutrients from the GI
tract into the blood. Optimal digestion and absorption depends on the good health of the
digestive tract, which is affected by such lifestyle factors as sleep, physical activity, state of
mind, and the meals you eat.
1. Identify which of these foods and food habits promote or impede healthy digestion and
absorption.
Foods and Food Habits
a. Take small bites of food.
Promote

Impede

b. Chew thoroughly before swallowing.


c. Exercise immediately after eating to prevent weight gain.


d. Eat a low-fiber diet.


e. Drink plenty of fluids.


f. Eat a few large meals instead of several smaller ones.


g. Eat quickly and then lie down to rest.


h. Create a meal using citrus fruits and meat.


i. Tackle family problems at the dinner table.


2. Do you experience GI distress regularly?
3. What changes can you make in your eating habits to promote GI health?
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Transport of Nutrients into the Blood
Nutrient
Transport into Blood
Water-soluble nutrients
Carbohydrates
Monosaccharides
Lipids
Glycerol
Short-chain fatty acids
Medium-chain fatty acids
Proteins
Amino Acids
Vitamins
Vitamins B and C
Minerals
Directly into blood
Directly into blood
Directly into blood
Directly into blood
Directly into blood
Directly into blood
Directly into blood
Fat-soluble nutrients
Lipids
Long-chain fatty acids
Monoglycerides
Triglycerides
Cholesterol
Phospholipids
Vitamins
Vitamins A, D, E and K
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Made into triglycerides
Made into triglycerides
To lymph, then blood
To lymph, then blood
To lymph, then blood
To lymph, then blood