Download Chapter 14

Human Biology Concepts and Current Issues
Seventh Edition
Michael D. Johnson
14
The Digestive
System and
Nutrition
© 2014 Pearson Education, Inc.
Lecture Presentations by
Robert J. Sullivan
Marist College
The Digestive System Brings Nutrients Into the Body
 The digestive system includes
– Gastrointestinal (GI) tract
– Includes mouth, pharynx, esophagus, stomach, small
intestine, large intestine, rectum, anus
– Accessory organs
– Salivary glands, liver, gallbladder, pancreas
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Figure 14.1
ACCESSORY ORGANS:
ORGANS:
Mouth
• Teeth chew food
• Tongue positions and tastes food
Pharynx
• Passageway for food and air
• Participates in swallowing
Esophagus
Salivary glands
•
•
•
•
Saliva moistens food
Bicarbonate maintains pH
Amylase digests starch
Lysozyme inhibits bacteria
Liver
• Produces bile
• Performs various functions
associated with processing
and storing nutrients
Gallbladder
• Stores and concentrates
bile
Pancreas
• Secretes digestive enzymes
into small intestine
• Secretes bicarbonate into
small intestine to neutralize
stomach acid
Appendix
• No known digestive
function
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• Moves food from pharynx to stomach
Stomach
• Stores and mixes food
• Begins chemical digestion of protein by
enzymes and acid
• Regulates delivery to the small intestine
Small intestine
• Digests proteins, fats, and carbohydrates
• Absorbs most of the water and nutrients
• Secretes digestive hormones
and enzymes
Large intestine
• Absorbs the last of the water
and nutrients
• Stores waste material
Sigmoid colon
• Stores feces
Rectum
• Passageway for feces
Anus
• Expels undigested material
The Walls of the GI Tract Are Composed of Four
Layers
 Common layers throughout the system
1. Mucosa: innermost layer, mucous membrane in
contact with the lumen
2. Submucosa: layer of connective tissue containing
blood vessels, lymph vessels, nerves
3. Muscularis: two or three layers of smooth muscle,
responsible for movement, motility
4. Serosa: outermost layer
 Sphincters: thick muscular rings that separate
some of the organs
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Figure 14.2
Vein
Artery
Nerve
Lymph vessel
Serosa
• Connective tissue
outer covering
• Protects and anchors
the digestive tract
Longitudinal
layer
Circular
layer
Muscularis
Lumen
Mucosa
• Mucous membrane layer
• Lines the digestive tract
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• Two layers of smooth muscle
• Responsible for motility of the
digestive tract
Submucosa
• Connective tissue layer
• Contains blood vessels, lymph vessels,
and nerves
Five Basic Processes Accomplish Digestive System
Function
1. Mechanical processing and movement
– Includes chewing and mixing
2. Secretion of the following:
– Fluid
– Digestive enzymes and hormones
– Bile, acid, alkali, mucus
3. Digestion
– Breaking down food to smallest absorbable units
– Includes chemical and mechanical breakdown
Absorption
– Occurs through mucosa
– Nutrients and water move into blood or lymph vessels
5. Elimination
– Undigested material is eliminated as feces
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Figure 14.3
Esophagus
Bolus of
food
Stomach
Peristalsis.
Segmentation.
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Two Types of Motility Aid Digestive
Processes
Peristalsis: propels food forward
Segmentation: mixes food
The Mouth Processes Food for Swallowing
 Teeth: bite and chew food
– Types: incisors, canines, premolars, molars
– Children: 20 teeth
– Adults: 32 permanent teeth
– Structure: crown, root
 Tongue: positions and tastes food
–
–
–
–
Skeletal muscle
Moves food in mouth
Taste receptors are located on the tongue
Important for speech
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Figure 14.4
Molars
(12)
Premolars
(8)
Lower jaw
Canines (4)
Incisors
(8)
Upper jaw
Locations and types of adult human teeth.
Enamel
Dentin
Gingiva (gum)
Crown
Pulp cavity
Root canal
Periodontal
membrane
Root
Bone
Anatomy of a tooth.
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Bacteria on a tooth’s surface (approx.  8,000).
The Mouth Processes Food for Swallowing
 Saliva
– Secreted by salivary glands
– Parotid glands
– Submandibular glands
– Sublingual glands
– Composition of saliva
– Water
– Mucin
– Salivary amylase: begins digestion of starch
– Lysozyme: inhibits bacterial growth
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Figure 14.5
Parotid gland
Sublingual gland
Submandibular gland
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Figure 14.6
Swallowing
Voluntary phase: tongue pushes bolus of food into pharynx
(throat)
Involuntary phase: swallowing reflex
Pharynx: common passageway for air and food
Food moves through esophagus with the help of peristalsis
Food passes through lower esophageal sphincter into the stomach
Nasal cavity
3
1
Hard palate
Swallowing begins
as tongue pushes
bolus into pharynx
Soft palate
Bolus of food
Tongue
2
Before swallowing,
muscles keep
esophagus closed
Voluntary phase.
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Soft palate rises
to close passage
to nasal cavity
Pharynx
4
Epiglottis bends
to close off
trachea and open
esophagus
Epiglottis
Trachea
(airway) open
Involuntary phase.
The Stomach Stores Food, Digests Protein, and
Regulates Delivery
 Food storage
 Digestion of proteins
 Regulation of delivery of partially digested food into
the small intestine
 Chyme: watery mixture of partially digested food
and gastric juice that is delivered to the small
intestine
 Requires 2–6 hours for stomach to empty
completely after a meal
 Stomach does not absorb nutrients
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Gastric Juice Breaks Down Proteins
 Specific cells secrete gastric juice, which contains
– Hydrochloric acid
– Produces a pH of about 2
– Intrinsic factor
– Made by same cells that make acid; needed to absorb
vitamin B12
– Mucus
– Protects stomach lining from acid
– Pepsinogen
– Converted to pepsin by acid
– Begins protein breakdown
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Stomach Contraction Mix Food and
Push It Forward
Stomach
Lower
esophageal
sphincter
Pyloric
sphincter
Key:
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 Location and direction of peristalsis
 Movement of stomach contents
The Small Intestine Digests Food and Absorbs
Nutrients and Water
 Functions of small intestine
– Digestion
– Neutralizes acid from stomach
– Adds digestive enzymes and bile
– Breaks down proteins, carbohydrates, and lipids to
absorbable materials (monomers)
– Absorption
– Almost all nutrients absorbed in small intestine
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The Small Intestine Digests Food and Absorbs
Nutrients and Water
 Large surface area for absorption
– Regions
– Duodenum: most digestion occurs here
– Jejunum: absorption
– Ileum: absorption
– Mucosa adaptations to increase surface area to
facilitate absorption
– Large circular folds
– Villi: microscopic projections of mucosa containing
capillaries
– Microvilli: projections on epithelial cells of villi
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Figure 14.9
Folds
Epithelial
cell
The wall of the small intestine
contains numerous folds that
increase its surface
area.
A single cell with its outer
membrane of microvilli.
Villi
Mucosa
Mucusproducing cell
Submucosa
Arteriole
Venule
Muscularis
Serosa
Lymph
vessel
Each fold is covered
with smaller folds
called villi.
Microvilli
Lacteal
A close-up view of a villus showing the single layer of
mucosal cells covering the surface and the centrally
located lymph vessel (lacteal) and blood vessels.
Human intestinal villi ( 270).
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Accessory Organs Aid Digestion and Absorption
 Pancreas
– Exocrine functions
– Secretes digestive enzymes and sodium bicarbonate
Role of bicarbonate: neutralize stomach acids
– Digestive enzymes include
– Proteases( protein digesting ): trypsin, chymotrypsin
– Pancreatic amylase: digests starch
– Lipase: digests lipids
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Table 14.1
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Accessory Organs: Aid Digestion and Absorption
 Liver
– Produces bile, which emulsifies lipids
– Hepatic portal system- drains blood from the digestive
tract to the liver
– Metabolic functions of liver
– Storage, synthesis, and chemical processing
 Gallbladder
– Concentrates and stores bile
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Figure 14.10
Liver
• Produces bile
(water and electrolytes,
cholesterol, bile salts,
lecithin, and
pigments)
Esophagus
Stomach
Common
bile duct
Pancreatic duct
Gallbladder
Pancreas
• Stores and
concentrates bile
• Delivers bile to the
duodenum via the
common bile duct
Duodenum
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• Secretes enzymes
(proteases, amylase, lipase)
• Produces sodium
bicarbonate
• Delivers these products to
• the duodenum via ducts
Figure 14.11
Inferior vena cava
Hepatic veins
Liver
Hepatic
portal vein
Spleen
Stomach
Pancreas
Small intestine
Large
intestine
The hepatic portal system.
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The Large Intestine Eliminates Wastes
 Functions
– Absorbs water & ions
– Temporarily stores and eliminates waste
 Structure
– Cecum, appendix
– Colon
– Ascending, transverse, descending, sigmoid
– Rectum, anus
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Figure 14.12
Transverse
colon
Ascending
colon
Small
intestine
Descending
colon
Ileocecal
valve
Cecum
Appendix
Rectum
Internal anal
sphincter
(smooth muscle)
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Sigmoid colon
External anal
sphincter
(skeletal muscle)
Anus
Anal canal
Nutrients Absorption
 Amino acids: (breakdown products of proteins) are
absorbed into blood capillaries
 Monosaccharides( mainly glucose) : (breakdown
products of carbohydrates) are absorbed into blood
capillaries
 Lipids: broken down to monoglycerides and fatty
acids, absorbed into lymph capillaries
 Water: absorbed by osmosis
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Animation: The Digestive System
Right-click and select Play
Endocrine and Nervous Systems Regulate Digestion
 Nervous system: stretch receptors in stomach
– Reflexes increase peristalsis and secretion of gastric
juice
 Hormones
– Gastrin: released by the stomach
– Secretin: released by the duodenum
– Cholecystokinin: released by the duodenum
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Endocrine and Nervous Systems Regulate Digestion
 Gastrin
– Causes release of gastric juice
 Secretin
– Triggered by presence of acid in chyme
– Stimulates pancreas to secrete water and bicarbonate
 Cholecystokinin (CCK)
– Triggered by fat and protein in chyme
– Signals pancreas to secrete digestive enzymes
– Signals gallbladder to release bile
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Figure 14.16
• Eat plenty of fruits,
vegetables, and
whole grains
• Choose a diet low
in cholesterol and
saturated fat
Fruits
Vegetables
Grains
Focus on fruits.
Vary your veggies.
Make at least
half your grains
whole.
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Protein
Foods
Go lean with
protein.
Dairy
Get your
calcium-rich
foods.
Carbohydrates: A Major Energy Source





Body’s primary source of energy
45–65% of caloric intake should be carbohydrates
Simple carbohydrates: sugars
Complex carbohydrates: starch, glycogen, fiber
Whole foods that provide complex carbohydrates are
preferred nutrient sources over simple carbohydrates
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Lipids: Essential Cell Components and Energy
Sources
 Saturated fats
– Meat, dairy products
– Tend to raise LDL (“bad”) cholesterol
– Increase risk of heart disease
 Unsaturated fats
– Plant sources, olive, corn, canola oils
– Certain cold water fish (omega-3 fatty acids)
– Linked to reduced risk of heart disease
 Trans fats
– Produced by hydrogenation of vegetable oils
– Raise LDL cholesterol
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Complete Proteins Contain Every Amino Acid
 20 different amino acids
– 12 can be made by the human body
– 8 essential amino acids must be ingested in foods
 Complete proteins: contain all 20 amino acids
– Most animal proteins
 Incomplete proteins: lack one or more essential
amino acids
– Many plant proteins are incomplete
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