Chapter 3
Digestion, Absorption, and
Metabolism
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Objectives
• Describe processes of digestion, absorption,
and metabolism
• Name organs in digestive system and describe
their functions
• Name enzymes or digestive juices secreted by
each organ and gland in digestive system
• Calculate basal metabolic rate (BMR)
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Digestion
• Breakdown of food in body in preparation for
absorption
• Mechanical digestion
– Food broken into smaller pieces by teeth and moved along
gastrointestinal tract by peristalsis
• Chemical digestion
– Carbohydrates, proteins, and fats broken down into
nutrients that tissues can absorb and use
(continues)
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Digestion
• Chemical changes occur through hydrolysis
• Enzymes act on food substances
– Causing them to break down into simple compounds
• Enzyme can act as catalyst
– Speeds up chemical reactions without itself being changed
in the process
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Digestion in the Mouth
• Teeth break up food
• Food mixes with saliva
– Forming bolus
• Salivary amylase acts on starch
• Food in mouth for brief length of time
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Digestion in the Esophagus
• Food travels through this muscular tube
– Connects mouth to stomach
• Peristalsis and gravity act to move bolus
• Cardiac sphincter opens to allow passage of
bolus into stomach
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Stop and Share
• Consider the following questions:
– What symptom results when the cardiac sphincter does not
close properly?
– What is the name of this condition?
– What prevents this condition from occurring?
(continues)
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Stop and Share
• Indigestion or heartburn occurs as result of
stomach acid flowing back into esophagus
– Gastroesophageal reflux
• Proper closing of cardiac sphincter prevents
acidic content of stomach from flowing back
into esophagus
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Digestion in the Stomach
• Temporary storage of food
• Kneading and mixing of food with gastric
juices
• Regulation of slow, controlled emptying of
chyme into intestine
• Destruction of most bacteria consumed
(continues)
Copyright © 2011 Delmar, Cengage Learning. ALL RIGHTS RESERVED.
Digestion in the Stomach
• Secretion of intrinsic factor for vitamin B12
• Gastric juices contain hydrochloric acid,
pepsin, and mucus
• In children, two additional enzymes:
– Rennin
– Gastric lipase
Copyright © 2011 Delmar, Cengage Learning. ALL RIGHTS RESERVED.
Stop and Share
• Consider the following scenario:
– 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.
(continues)
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Stop and Share
• Consider the following scenario:
– Complete the following objectives to prepare for your class:
• Identify the three parts of the stomach
• Define chyme
• Identify the condition that results from a lack of the intrinsic factor
in the stomach
(continues)
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Stop and Share
• Fundus
– Upper portion of stomach
• Body
– Middle area of stomach
• Pylorus
– End of stomach near small intestines
(continues)
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Stop and Share
• Chyme
– Semiliquid mass of food and gastric juices
• Pernicious anemia
– Lack of intrinsic factor
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Digestion in the 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 secreted into small intestine
– Produced in liver
– Stored in gallbladder
(continues)
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Digestion in the Small Intestine
• Enzymes found in pancreatic juice secreted
into small intestine
– Pancreatic proteases
• Splits proteins
• E.g., trypsin, chymotrypsin, carboxypeptidases
– Pancreatic amylase
• Converts starches (polysaccharides) to simple sugars
(continues)
Copyright © 2011 Delmar, Cengage Learning. ALL RIGHTS RESERVED.
Digestion in the Small Intestine
• Enzymes found in pancreatic juice secreted
into small intestine
– Pancreatic lipase
• Reduces fats to fatty acids and glycerol
• Small intestine itself produces enzymes
– Lactase, maltase, and sucrase convert lactose, maltose, and
sucrose to simple sugars
– Peptidases reduce proteins to amino acids
Copyright © 2011 Delmar, Cengage Learning. ALL RIGHTS RESERVED.
Stop and Share
• Consider the following scenario:
– A client with a duodenal ulcer asks you where the ulcer is
located. How do you respond?
(continues)
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Stop and Share
• Small intestine divided into three sections:
– Duodenum
• First section
– Jejunum
• Middle section
– Ileum
• Last section
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Digestion in the Large Intestine
• Consists of cecum, colon, and rectum
• Major tasks of cecum:
– Absorbs water and salts from undigested foods
– Kneads content for enhanced absorption
– Collects volatile fatty acids to be absorbed and used as
sources of energy
• Digested food progress through colon to anal
canal
Copyright © 2011 Delmar, Cengage Learning. ALL RIGHTS RESERVED.
Stop and Share
• Consider the following scenario:
– 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?
(continues)
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Stop and Share
• Cecum (blind pocket), colon, and rectum make
up large intestine
(continues)
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Stop and Share
• Contents travel through:
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Ascending colon
Transverse colon
Descending colon
Sigmoid colon
Rectum
Anal canal
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Absorption
• Passage of nutrients into blood or lymphatic
system
(continues)
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Absorption
• Nutrients must be in simplest form
– Carbohydrates
• Simple sugars
– Proteins
• Amino acids
– Fats
• Fatty acids and glycerol
Copyright © 2011 Delmar, Cengage Learning. ALL RIGHTS RESERVED.
Absorption in the Small Intestine
• Where most absorption occurs
• Villi
– Hairlike projections that increase surface area for
maximum absorption
– Absorb nutrients from chyme and transfer them to
bloodstream
• Water absorbed in stomach, small intestine,
and large intestine
Copyright © 2011 Delmar, Cengage Learning. ALL RIGHTS RESERVED.
Absorption in the Large Intestine
• Mucus protects colon from digestive juices
• Major tasks of large intestine:
– Absorbs water
– Synthesizes vitamin B and vitamin K
– Collects food residue
• Undigested food excreted as feces
Copyright © 2011 Delmar, Cengage Learning. ALL RIGHTS RESERVED.
Metabolism
• Transformation of nutrients into energy within
cell
• Occurs after digestion and absorption
– Nutrients carried by blood to cells of body
• Aerobic metabolism combines nutrients with
oxygen within each cell
– Also known as oxidation
(continues)
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Metabolism
• Anaerobic metabolism reduces fats without
use of oxygen
• Krebs cycle
– Complete oxidation of carbohydrates, proteins, and fats
• Anabolism
– Process of using energy from oxidation to create new
compounds
(continues)
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Metabolism
• Catabolism
– Breakdown of compounds during metabolism
• Controlled primarily by hormones secreted by
thyroid gland
– Triiodothyronine (T3)
– Thyroxine (T4)
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Stop and Share
• Consider the following questions:
– What condition is associated with too much thyroid
hormone?
– What condition is associated with too little thyroid
hormone?
(continues)
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Stop and Share
• Hyperthyroidism
– Metabolism speeds up and body metabolizes food too
quickly
– Weight loss
• Hypothyroidism
– Metabolism slows down and body metabolizes food too
slowly
– Feeling of sluggishness and accumulation of fat
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Energy
• Needed for involuntary and voluntary activity
• Involuntary activity
– E.g., maintenance of body tissue, temperature, and growth
• Voluntary activity
– E.g., walking, swimming, eating, reading, typing
(continues)
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Energy
• Three groups of nutrients provide energy:
1. Carbohydrates
2. Proteins
3. Fats
• Carbohydrates should be primary source of
energy
(continues)
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Energy
• Kilocalorie
– Unit used to measure energy value of foods
– Also known as calorie
• Calorie
– Amount of heat needed to raise temperature of 1 kilogram
(kg) of water 1 degree Celsius
• Bomb calorimeter determines energy values of
foods
(continues)
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Energy
•
•
•
•
1 gram (g) of carbohydrate yields 4 calories
1 g of protein yields 4 calories
1 g of fat yields 9 calories
1 g of alcohol yields 7 calories
Copyright © 2011 Delmar, Cengage Learning. ALL RIGHTS RESERVED.
Stop and Share
• Consider the following question:
– If you eat a dessert with 19 g of fat in it, how many calories
from fat does it have?
(continues)
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Stop and Share
• Fat contains 9 calories per gram
• Dessert has 19 g of fat
• 9 kilocalories per gram × 19 g of fat = 171
calories
Copyright © 2011 Delmar, Cengage Learning. ALL RIGHTS RESERVED.
BMR
• Rate at which energy is needed for body
maintenance
• Energy necessary to carry on all involuntary
vital processes while body at rest
• Also known as resting energy expenditure
(REE)
(continues)
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BMR
• Affecting factors:
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Lean body mass
Body size
Sex
Age
Heredity
Physical condition
Climate
(continues)
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BMR
• Greater in males than females
• Increases during growth and fever
• Decreases with age and during starvation
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Calculating BMR
• Harris-Benedict equation
– Used by dietitians for people over age 18
– Uses height, weight, and age
– Female BMR
• 655 + (9.6 × weight in kg) + (1.8 × height in centimeters [cm]) –
(4.7 × age)
– Male BMR
• 66 + (13.7 × weight in kg) + (5 × height in cm) – (6.8 × age)
(continues)
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Calculating BMR
• Another method used to estimate BMR:
– Convert body weight from pounds to kg
– Multiply kg by 24 (hours per day)
– Multiply answer by 0.9 for females and by 1.0 for males
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Stop and Share
• Consider the following questions:
– What is the BMR for a female weighing 110 pounds?
– What is the BMR for a male weighing 170 pounds?
(continues)
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Stop and Share
• 110-pound female
– 110 pounds ÷ 2.2 (pounds per kg) = 50 kg
– 50 kg × 24 hours = 1,200 calories
– 1,200 calories × 0.9 = 1,080 calories
(continues)
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Stop and Share
• 170-pound male
– 170 pounds ÷ 2.2 (pounds per kg) = 77.27 kg
– 77.27 kg × 24 hours = 1,854 calories
– 1,854 calories × 1.0 = 1,854 calories
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Conclusion
• Food broken down through processes of
mechanical and chemical digestion into
nutrients that can be absorbed
• Enzymes break down nutrients
• Absorption occurs mostly in small intestines
(continues)
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Conclusion
• During metabolism, carbohydrates and
proteins combine with oxygen
– Oxidation
• Energy released during oxidation measured in
calories
• Person’s energy requirement can be measured
in part by estimating BMR
Copyright © 2011 Delmar, Cengage Learning. ALL RIGHTS RESERVED.