Download The Basics of Digestion

Chapter 3
The Basics of Digestion
Slide Show was developed by:
Richard C. Krejci, Ph.D.
Professor of Public Health
Columbia College 9.14.15
Objectives for Chapter 3
1. Define digestion, absorption, and nutrient transport.
2. Describe the organs involved in digestion and their primary functions.
3. Explain the roles of the gallbladder, liver, and pancreas in digestion.
4. Explain the function of peristalsis and segmentation in the movement
of food through the GI tract.
5. Explain the role of enzymes in digestion.
Objectives for Chapter 3
6. List the main carbohydrate-, protein-, and fat-digesting enzymes and
the tissues that secrete them.
7. Identify the hormones involved in digestion, including their primary
action and their source of origin.
8. Explain the role of the small intestine, villi, and microvilli in digestion.
9. Explain how the circulatory and lymphatic systems transport
absorbed nutrients throughout the body.
10. Describe the symptoms and causes of the most common digestive
disorders.
What Makes Eating So Enjoyable?
Hunger and thirst: physical needs which drive
how much and how often we eat
Appetite: another powerful drive, but unreliable
 Influenced by food preferences and psychological
stimulation
 May eat without needing nourishment
Animation: Control of Appetite:
Hunger and Satiety
We Develop a Taste for Certain Foods
Taste: Five basic categories
 Detected by at least 10,000 taste buds mostly on
tongue, but also throat and elsewhere in mouth
1. Sweet: tip of tongue
2. Salty: sides of tongue
3. Sour: sides of tongue
4. Bitter: back of tongue
5. Savory (umami): throughout mouth;
not all people are sensitive to this
taste sensation
We Develop a Taste for Certain Foods
 Humans share an innate preference for sweet
(pleasure), salty (electrolytes), and fatty (rich textures
and aromas) foods
 Sometimes food preferences and nutritional needs
conflict and make changing food choices challenging
 Brain recognizes taste: food dissolves in saliva,
contacts the tongue surface, taste cells send nerve
impulses to brain, translated into taste
sensations
Aromas and Flavors Enhance the
Pleasure of Eating
Both mouth and nose contribute to tasting of foods
 Aromas detected by olfactory cells in nasal cavity as
food odors enter both nose and mouth (move to back
of throat and into nasal cavity)
 Flavor refers to both taste and aroma
 Example: Chocolate tastes sweet but its
flavor is “chocolate”
What Is Digestion and Why Is It
Important?
Digestion: a multi-step process of breaking down
foods into absorbable components using
mechanical and chemical means in the
gastrointestinal (GI) tract
 Gastrointestinal tract consists of:
 Mouth
 Esophagus
 Stomach
 Small and large intestines
 Other organs (liver, pancreas,
gall bladder)
What Is Digestion and Why Is It
Important?
 The three main roles of the GI tract are to:
1. Break down food into smallest components
2. Absorb nutrients
3. Prevent microorganisms or other harmful compounds
in food from entering tissues of the body
 The adult GI tract is about 23 feet long with
extensive surface area for nutrient absorption
 Cells lining GI tract are replaced every three to five days
Animation: Overview of Digestion
and Absorption
Animation: Basic Absorption
Mechanisms
Digestion Is Mechanical and Chemical
Mechanical digestion: chewing, grinding food
to aid swallowing
 Peristalsis: the forward, rhythmic muscular
contraction that moves food through GI tract
Chemical digestion: digestive juices and
enzymes break down food into absorbable
nutrients
Peristalsis
Figure 3.1
Digestion Allows Us to Absorb
Nutrients from Foods
After digestion, nutrients are absorbed through the
walls of the intestines into the body’s two transport
systems:
 Circulatory system (blood)
 Lymphatic system (lymph nodes)
 Sent to the liver for processing before delivery to
the body’s cells
 GI tract is highly efficient: 92 to 97 percent of
nutrients from food are digested and absorbed
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What Are the Organs of the GI Tract and
Why Are They Important?
Digestion begins in the mouth
 Saliva released: contains water, electrolytes, mucus
and a few enzymes
 Softens, lubricates, dissolves food particles
 Bolus (food mass) moves into pharynx, is swallowed,
and enters the esophagus
 Epiglottis closes off trachea during swallowing to
prevent food from lodging in the windpipe
From Cells to Organs and Organ Systems
Figure 3.2
The Epiglottis
Figure 3.3
The Stomach Stores, Mixes, and
Prepares Food for Digestion
 Esophagus propels food into the stomach
 Bolus is pushed down esophagus by peristalsis
 Lower Esophageal Sphincter (LES): at
bottom of esophagus relaxes and allows
food into stomach
 LES then closes to prevent backflow of
hydrochloric acid (HCl) from stomach
 “Reflux” of stomach acid causes “heartburn”
(irritation of esophagus lining) (GERD)
The Stomach Stores, Mixes, and
Prepares Food for Digestion
 Stomach’s mechanical action: churning mixes food with
digestive juices for several hours
 Stomach produces powerful digestive secretions:
 HCl: activates enzyme pepsin, enhances absorption of minerals,
breaks down connective tissue of meat
 Mucus protects stomach lining from damage
 Digestive enzymes, intrinsic factor (for vitamin B12 absorption),
stomach hormone called gastrin
 Chyme: semi-liquid substance of partially
digested food and digestive juices
Anatomy of the Stomach
Pyloric sphincter
Figure 3.4
The Stomach Stores, Mixes and
Prepares Food for Digestion
 Gastrin: stimulates digestive activities and secretion of HCl;
increases gastric motility and emptying
 Liquids, carbohydrates, low-fiber, and low-calorie foods exit stomach
faster
 High fiber, fat, and protein foods exit slower, keep you feeling full
longer
 Lower esophageal sphincter: located between the
esophagus and stomach prevents a backflow of fluids
(acids).
 Pyloric sphincter: located between the stomach and small
intestine; allows about 1 tsp of chyme to enter the small
intestine every 30 seconds
 Prevents backflow of intestinal contents
Surface Area in the Small Intestine
Folds
Villi
Microvilli
Figure 3.5
Most Digestion and Absorption
Occur in the Small Intestine
Small intestine: long, narrow, coiled
 Three segments:
 Duodenum (10 inches)
 Jejunum (8 feet)
 Ileum (12 feet)
 Interior surface area tremendously
increased by circular folds, villi, microvilli
Most Digestion and Absorption Occur
in the Small Intestine
 Mechanical and chemical digestion in small
intestine:
 Peristalsis moves chyme through intestine
 Segmentation mixes chyme with chemical secretions
 Pendulum-like movements enhances nutrient absorption
 Chyme moves though at rate of 1 centimeter per
minute
 Total contact time 3 to 10 hours,
depending on amount and type of food
Large Intestine Absorbs Water and Some
Nutrients
 Ileocecal sphincter: prevents backflow of fecal
matter into ileum
 Most of nutrients in chyme have been absorbed
when it reaches large intestine
 Large intestine has three sections: cecum, colon,
rectum
 About 5 feet long, 2.5 inches in diameter
 Absorbs water and electrolytes
 No digestive enzymes; chemical
digestion done by bacteria
Anatomy of the Large Intestine
Figure 3.6
Large Intestine Absorbs Water and
Some Nutrients
 Intestinal matter passes through colon in
12 to 70 hours depending on age, health,
diet, fiber intake
 Bacteria in colon produce vitamin K and biotin and
break down fiber and undigested carbohydrates,
producing methane, carbon dioxide, hydrogen
gas, and other compounds
 Stool stored in rectum
 Anus controlled by two sphincters: internal and
external
 Final stage of defecation is under voluntary control
Enzymes, Hormones, and Bile Aid
Digestion
 Complete digestion of chyme requires:
1. Enzymes: break down food into absorbable nutrient
components
 Secreted by stomach, small intestine, pancreas
 Amylases, lipases, proteases
2. Hormones: pepsin, gastrin, insulin, glucagon,
secretin, cholecystokinin
 Control digestive secretions and regulate enzymes
The Liver, Gallbladder, and Pancreas
Are Accessory Organs
Liver: largest gland in body
 Produces bile needed for fat digestion
 Metabolism of carbohydrates, fats, and protein
 Stores nutrients: vitamins A, D, B12, E, copper, iron,
glycogen (glucose storage form)
 Detoxifies alcohol (cirrhosis)
Gallbladder: concentrates
and stores bile
 Released into GI tract when
fat is ingested
Image taken from WebMD, 2011.
The Liver, Gallbladder, and Pancreas
Are Accessory Organs
Pancreas:
 Produces hormones: insulin and glucagon regulate
blood glucose
 Sodium bicarbonate neutralizes acidic chyme, protects
enzymes from inactivation by acid
 Digestive enzymes:
 Amylase: digests carbohydrate
 Lipase: digests fats
 Trypsin, chymotrypsin, and
carboxypeptidase: digest protein
Image taken from WebMD, 2011.
Overview: Process of Digestion and
Organs of the GI Tract
Figure 3.7
Organs of the GI Tract
Table 3.2
Digestive Secretions
Table 3.1
What Other Body Systems
Affect the Use of Nutrients?
A. Nervous system stimulates your appetite

Hormone ghrelin signals brain to eat when stomach is empty
B. Circulatory system transports nutrients, oxygen, waste
products through your blood
C. Lymphatic system distributes fat and fat-soluble nutrients
through your lymph
D. Endocrine system hormones regulate growth,
reproduction, metabolism, and the cell’s use of nutrients
E. Excretory system eliminates wastes from circulatory
system via the urine
The Circulatory and Lymphatic Systems
Figure 3.9
The Excretory System
Figure 3.10
The Endocrine System
What Are Some Common Digestive
Disorders?
Disorders of the mouth and esophagus:
 Gingivitis and periodontal disease
 Swallowing problems: dysphagia
 Esophageal problems
 Heartburn (acid reflux) may be caused by
weak LES
 Certain foods, smoking, drinking alcohol, being
overweight or obese, tight-fitting clothes,
reclining after eating, large evening meals may
worsen the condition
What Are Some Common Digestive
Disorders?
Disorders of the stomach:
 Gastroenteritis
 Peptic ulcers
Gallbladder disease:
 Gallstones
What Are Some Common Disorders of
the Digestive System?
Disorders of the intestines:
 Flatulence
 Constipation and diarrhea
 Hemorrhoids
Other Common Digestive Disorders
More serious intestinal disorders: (table 3.3)
 Irritable Bowel Syndrome (IBS)
 Stress management
 Celiac disease (page 84)
 Gluten free diet
 Crohn’s disease
 Inflammatory condition
 Only can be managed
 Colon cancer
 Colonoscopy at 50
The End
Slide Show developed by:
Richard C. Krejci, Ph.D.
Professor of Public Health
Columbia College
All Rights Reserved