Download File

Human Digestion and Absorption
Learning Outcomes
• Outline the roles played by the
gastrointestinal tract and the related
accessory organs (liver, gallbladder, and
pancreas) in digestion and absorption.
• Describe how foods are moved along the
digestive tract.
• Explain the 4 main types of absorption.
Learning Outcomes
• Identify the key enzymes and hormones
involved in digestion and absorption and their
functions.
• Identify major nutrition-related gastrointestinal
diseases and disorders and typical approaches to
prevention and treatment.
• Explain why diarrhea represents a serious health
challenge to infants and young children around
the world.
Organization of the Human Body
• Chemical Level
– Atoms combine to
form molecules
• Cell Level
– Molecules form
organelles
– Use ATP
• Tissue Level
– Similar cells make up
tissues
• Organ Level
– Different tissues
combine to form
organs
• Organ System Level
– Organs make up an
organ system
• Organism Level
– Organ systems make
up an organism
Human Tissues
• Epithelial
– Cells that cover surfaces inside and outside of the body
• Connective
– Support and protect the body by holding structures
together
• Muscle
– Contract and relax to permit movement
• Nervous
– Transmit nerve impulses
Digestive Organ System
• GI tract
– Mouth, esophagus, stomach, small
intestine, large intestine
• Liver, pancreas and gall bladder
Digestive System Overview
• Digestion
– Process of breaking down foods into a form the
body can use
• Absorption
– Uptake of nutrients from the GI tract into the
blood or lymph
• Excretion of waste matter
• Immune system function
Anatomy of the GI Tract
• GI tract = the alimentary canal
– 15’ long, hollow, muscular tube
• Four layers
– Mucosa
• Innermost layer: forms hollow area of the lumen
– Submucosa
• Contains blood vessels (carry nutrients), nerves, and glands
– Muscle (as double layers)
• Moves food forward
– Serosa
• Outside layer;-protects the tract
Sphincters
• Ring like muscles that control the flow of
contents in the GI tract
–
–
–
–
–
Lower esophageal sphincter
Pyloric sphincter
Sphincter of Oddi
Ileocecal sphincter
Anal sphincter
GI Motility:
Mixing and Propulsion
• Peristalsis
– Contractions
• Segmentation
– Back and forth movement
• Mass movements
– Peristalsis over widespread area
• Elimination
• Vomiting
Digestive System Secretions
•
•
•
•
•
•
•
Saliva
Mucus
Digestive enzymes
Hydrochloric acid
Bile
Bicarbonate ions
Hormones
Mouth (Oral Cavity)
• Chewing increases surface area
• Mixed with saliva food becomes a bolus
• Saliva
– Lysozyme
• Breaks down bacteria
– Mucus
• Lubricates and hold bolus together
– Amylase
• Breaks down starch
– Enhances perception of flavor
Taste and Smell
• Taste buds on the tongue and soft palate –
contain taste-receptor cells
–
–
–
–
–
Salty
Sour
Sweet
Bitter
Umami
• Olfactory cells in nose (smell)
– Stimulated with chewing
Esophagus
• Swallowing
– Moves bolus from the mouth to the esophagus
• Epiglottis
– Prevents food from entering the trachea
– Closes over the larynx
Stomach
• Food bolus enters stomach through lower
esophageal sphincter
• Holding & mixing tank
– Mixed with stomach secretions food becomes
chyme
Stomach Secretions
• Parietal Cells
– Hydrochloric Acid
•
•
•
•
Inactivates proteins
Destroys bacteria and viruses
Dissolves minerals to aid in absorption
Converts pepsinogen into pepsin
– Pepsinogen
• Protein digesting enzyme
• Gastrin
– Hormone that controls release of HCl and
pepsinogen
Other Stomach Secretions
• Chief cells
– Gastric lipase
• Mucus
– Protects the stomach from being digested
– Production relies on prostaglandins
Small Intestine
• Most digestion and absorption occurs here
• Pyloric sphincter allows chyme into the
small intestine
– Gastric inhibitory peptide (hormone) slows
release of chyme
• Sections
– Duodenum, jejunum and ileum
Small Intestine (2)
• Circular folds
• Villi – lined with:
– Goblet cells - make mucus
– Endocrine cells - produce hormones
– Enterocytes - produce digestive enzymes and
absorb nutrients
• Contain a brush border of microvilli covered with
glycocalyx
Liver, Gallbladder and Pancreas
• Liver
– Provides bile
– Enterohepatic circulation (recycling of bile)
• Gallbladder
– Bile storage
• Pancreas
– Produces sodium bicarbonate, lipases, proteases
and pancreatic amylase
Hormones of the GI Tract
• Gastrin
– Stomach release of HCl and pepsinogen
• Cholescystokinin (CCK)
– Release of bile
• Secretin
– Release of pancreatic bicarbonate
• Gastric Inhibitory Peptide
– Limits release of gastric juices
Absorption
• Primarily occurs in small intestine
• Passive diffusion
– Concentration gradient
• Facilitated diffusion
– Concentration gradient + carrier protein
• Active
– Carrier protein + energy (regardless of concentration)
• Endocytosis
– Engulfment of compounds or liquids
Moving Nutrients around the Body
• Cardiovascular
System
– Includes heart, blood
vessels and blood
– Water-soluble nutrients
transported via
capillaries in villi to
portal vein
• Lymphatic System
– Includes lymph
– Fat-soluble nutrients
and large particles
transported via lacteals
into the lymph vessels
to thoracic duct
Large Intestine
• Ileocecal valve
• Colon
– Cecum, ascending colon, transverse colon,
descending colon and sigmoid colon
• Rectum
• Anus
Functions of Large Intestine
• Absorption of water and electrolytes
• Formation and expulsion of feces
• Housing of bacteria (microbiota)
Microbiota (bacterial flora)
• Beneficial bacteria
– Control pathogenic bacteria
– Synthesize Vitamin K and Biotin
– Aid lactose digestion and fermentation of dietary fibers
• Probiotics
– Live bacteria in food and supplements
– Health benefits
• Prebiotics
– Non digestible carbohydrates in food that promote the
growth of bacteria– E.g. inulin, resistant starch
When Digestive Processes Go Awry
• Heartburn and
Gastroesphogeal
reflux disease (GERD)
– Foods that increase
reflux:
• Citrus, caffeine,
chocolate, fatty foods,
spicy foods, onion,
garlic and tomato based
foods
• (Peptic) Ulcers
– Causes: H. pylori and
NSAID medications
– Treatment:
• Medications
• Avoid foods that
increase symptoms
When Digestive Processes Go Awry (2)
• Food Intolerances
• Intestinal Gas
(Flatulence)
• Constipation
– Fiber, fluid and
exercise
– Laxative use
• Diarrhea
– Replace fluid and
electrolytes
• Irritable Bowel
Syndrome (IBS)
– Cause unknown
• Inflammatory Bowel
Disease (IBD)
– Ulcerative colitis and
Crohn’s disease
• Hemorrhoids
• Gallstones
Celiac Disease
• Intolerance to gluten
• 1 in 133 people affected
– Many undiagnosed
• Can affect many body systems
– GI and others
• Gluten-free diet for life
– Corn, rice, quinoa, and buckwheat OK