Download CHAPTER 3: Digestion, Absorption, and Metabolism Objectives

CHAPTER 3: Digestion, Absorption, and
Metabolism
Objectives:
 Describe the processes of digestion,
absorption, and metabolism.
 Name the organs in the digestive system
and describe their functions.
 Name the enzymes or digestive juices
secreted by each organ and gland in the
digestive system.
 Calculate your basal metabolic rate (BMR).
Digestion
 The breakdown of food in the body in
preparation for absorption
 Mechanical digestion: food is broken into
smaller pieces by teeth and moved along
the gastrointestinal tract by peristalsis.
 Chemical digestion: carbohydrates, proteins,
and fats are broken down into nutrients that
tissues can absorb and use.
 Chemical changes occur through hydrolysis.
 Enzymes act on food substances, causing
them to break down into simple compounds.
 An enzyme can act as a catalyst, which
speeds up the chemical reactions without
itself being changed in the process.
Mouth
 Enzyme (salivary amylase) acts on starch.
 Teeth break up food; food mixes with saliva.
 Bolus formed.
 Length of time food is in mouth is brief.
Esophagus
 Food travels through this muscular tube.
 Connects mouth to stomach
 Peristalsis and gravity act to move bolus.
 Cardiac sphincter opens at lower end of
esophagus to allow passage of bolus into
stomach.
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♠ STOP AND SHARE ♠
What symptom results when the cardiac
sphincter does not close properly?
What is the name of the condition?
•
What prevents this condition from
occurring?
- Indigestion or heartburn occurs as a result of
stomach acid flowing back into the
esophagus.
- This is called gastroesophageal reflux.
- The proper closing of the cardiac sphincter
prevents the acidic content of the stomach
from flowing back into the esophagus.
Stomach
 Temporary storage of food
 Mixing of food with gastric juices
 Regulation of a slow, controlled emptying of
food into the intestine
 Secretion of the intrinsic factor for vitamin
B12.
 Destruction of most bacteria that were
inadvertently consumed.
 Hydrochloric acid prepares the gastric area
for enzyme action.
 Pepsin breaks down proteins.
 In children, rennin breaks down milk proteins,
and gastric lipase breaks down the butterfat
molecules of milk.
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•
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♠ STOP AND SHARE ♠
You are preparing a teaching plan for the
nursing staff working on a gastrointestinal
floor.
You plan to review the anatomy and
physiology of the stomach.
Complete the following objectives to prepare
for your class:
• Identify the three parts of the stomach.
• Define chyme.
• Identify the condition that results from lack
of the intrinsic factor in the stomach.
- Fundus: upper portion of the stomach
- Body of the stomach: middle area
- Pylorus: end of stomach near small
intestines
- Chyme: semiliquid mass of food and gastric
juices
- Pernicious anemia: lack of intrinsic factor
Small Intestine
 Hormones released
 Secretin causes pancreas to release
sodium bicarbonate to neutralize acidity
of chyme.
 Cholecystokinin triggers gallbladder to
release bile.
 Bile
 Emulsifies fat after it is secreted into
small intestine
 Produced in liver; stored in gallbladder
 Enzymes are found in the pancreatic juice
that is secreted into small intestine:
 Pancreatic proteases (trypsin,
chymotrypsin, carboxypeptidases): split
proteins
 Pancreatic amylase: converts starches
(polysaccharides) to simple sugars
 Pancreatic lipase: reduces fats to fatty
acids and glycerol
 Prepares foods for absorption
 Produces enzymes
 Lactase, maltase, sucrase convert lactose,
maltose, sucrose to simple sugars.
 Peptidases reduce proteins to amino acids.
Large Intestine
 Colon walls secrete mucus to protect it
against acidic digestive juices in chyme.
 Major tasks of the large intestine:
 Absorb water
 Synthesize some B vitamins and vitamin
K
 Collect food residue
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•
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♠ STOP AND SHARE ♠
You are providing preoperative teaching to a
client who is going to have intestinal surgery.
The client asks you to explain the structure
of the large intestine.
How do you respond?
- The cecum (blind pocket), colon, and rectum
make up the large intestine. Contents travel
through:
- Ascending colon
- Transverse colon
- Descending colon
- Sigmoid colon
- Rectum
- Anal canal
Absorption
 The passage of nutrients into the blood or
lymphatic system.
 Nutrients must be in their simplest form.
 Carbohydrates: simple sugars
 Proteins: amino acids
 Fats: fatty acids and glycerol
 Most absorption occurs in the small
intestine; villi, hairlike projections, increase
surface area for maximum absorption.
 Villi absorb nutrients from the chyme and
transfer them to the bloodstream.
 Water is absorbed in the stomach, small
intestine, and large intestine.
Metabolism
 The transformation of nutrients into energy
within the cell
 Occurs after digestion and absorption;
nutrients are carried by the blood to the cells
of the body
 Aerobic metabolism, or oxidation, combines
nutrients with oxygen within each cell.
 Anaerobic metabolism reduces fats without
the use of oxygen.
 The complete oxidation of carbohydrates,
proteins, and fats is commonly called the
Krebs cycle.
 Anabolism is the process of using energy
from oxidation to create new compounds.
 Catabolism is the breakdown of compounds
during metabolism.
 Controlled primarily by hormones secreted
by the thyroid gland: triiodothyronine (T3)
and thyroxine (T4)
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♠ STOP AND SHARE ♠
What condition is associated with too much
thyroid hormone?
What condition is associated with too little
thyroid hormone?
- Hyperthyroidism: metabolism speeds up and
the body metabolizes its food too quickly;
weight is lost.
- Hypothyroidism: metabolism slows down
and the body metabolizes its food too
slowly; client tends to become sluggish and
accumulate fat.
Energy
 Needed for involuntary and voluntary
activity
 Involuntary activity: maintenance of body
tissue, temperature, and growth
 Voluntary activity: walking, swimming, eating,
reading, and typing
 Three groups of nutrients provide energy:
 Carbohydrates
 Proteins
 Fats
 Carbohydrates should be the primary source
of energy.
 The unit used to measure the energy value
of foods is the calorie.
 A calorie is the amount of heat needed to
raise the temperature of 1 kilogram of water
1 degree Celsius.
 A bomb calorimeter is used to determine the
energy values of foods.
 1 gram of carbohydrate yields 4 calories.
 1 gram of protein yields 4 calories.
 1 gram of fat yields 9 calories.
 1 gram of alcohol yields 7 calories.
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♠ STOP AND SHARE ♠
If you eat a dessert with 19 grams of fat in it,
how many calories from fat does it have?
 171 calories
 Fat contains 9 calories per gram.
 Dessert has 19 grams of fat.
 9 kcal/gram × 19 grams of fat = 171
calories.
Basal Metabolic Rate (BMR)
 The rate at which energy is needed for body
maintenance
 The energy necessary to carry on all
involuntary vital processes while the body is
at rest
 Also known as resting energy expenditure
(REE)
 Factors that affect BMR: lean body mass,
body size, sex, age, heredity, physical
condition, and climate
 BMR is greater in men than women.
 BMR increases during growth and fever.
 BMR decreases with age and during
starvation.
Calculating BMR
 Harris-Benedict equation
 Used by dietitians for persons over age
18
 Uses height, weight, and age
 Female BMR
655 + (9.6 × weight in kg) + (1.8 × height in cm) –
(4.7 × age)
 Male BMR
66 + (13.7 × weight in kg) + (5 × height in cm) – (6.8
× age)
 Another method used to estimate BMR
 Convert body weight from pounds to
kilograms.
 Multiply kilograms by 24 (hours per day).
 Multiply the answer obtained by 0.9 for a
woman and by 1.0 for a man.
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♠ STOP AND SHARE ♠
Calculate the BMR for a woman who weighs
110 pounds.
Calculate the BMR for a man who weighs
170 pounds.
• 110 pound woman: BMR 1,080 calories
• 110 pounds ÷ 2.2 (pounds per kg) =
50 kg
• 50 kg × 24 hours in a day = 1,200
calories
• 1,200 calories × 0.9 = 1,080 calories
• 170 pound man: BMR 1,854 calories.
• 170 pounds ÷ 2.2 (pounds per kg) =
77.27 kg
• 77.27 kg × 24 hours in a day = 1,854
calories
• 1,854 cal × 1.0 = 1,854 calories
Conclusion
 Food is broken down through the processes
of mechanical and chemical digestion into
nutrients that can be absorbed.
 Enzymes act to break down nutrients.
 Absorption occurs mostly in the small
intestines.
 During metabolism, carbohydrates and
proteins are combined with oxygen in a
process called oxidation.
 Energy released during oxidation is
measured in calories.
 A person’s energy requirement can be
measured in part by estimating the basal
metabolic rate (BMR).