Digestion & Absorption
The body’s breakdown and uptake
of nutrients
We are what we eat!

Biochemically, this
is true
 However, most of us
don’t look like this
guy
 To build the body
out of the materials
in food, we must first
break foods down
From Atoms to Organisms
Digestive System Terminology


Digestion: process of breaking food into
components small enough to be absorbed by the
body
Absorption: process of taking substances into
the interior of the body
 Gastrointestinal tract: hollow tube consisting
of the mouth, pharynx, esophagus, stomach,
small intestine, large intestine and anus
 Transit time: amount of time it takes food to
pass the length of the GI tract
 Feces: body waste, including unabsorbed
food residue, bacteria and dead cells
The Digestive System
Anatomy of the Digestive Tract

Gastrointestinal (GI) tract
 Flexible and muscular
 Path
 Mouth → esophagus → stomach →
small intestine → large intestine →
rectum → anus
 Lumen
 Continuous
 Considered outside the body
Digestive Secretions

Digestion inside the GI tract is assisted by
digestive secretions
 Mucus: viscous material produced by goblet
cells
– Moistens, lubricates and protects the GI tract

Enzymes: protein molecules that speed up
chemical reactions
– Enzymes are catalysts
– In digestion, speed up breakdown of nutrients
Turning Digestion On

Cephalic phase
– Sensory stimuli
– Thoughts of food
– Tasting and chewing

Gastric and intestinal phases
– Presence of food in digestive organs sends
signals to brain and the organs themselves

Both phases are mediated by nerves
and hormones
Control
of
Digestion
 Process of
digestion begins
here
 Mastication
 Saliva
 Enzymes
 4 taste sensations
 #5: savory or
umami
 Aroma, texture,
and temperature
 Movement to
pharynx:
swallowing reflex
 Bolus
The Mouth
Pharynx and Epiglottis
Heimlich Maneuver
Passage Through the Esophagus
The Esophagus and Stomach
Esophagus
Muscular tube; passes through diaphragm
Two sphincters
Upper esophageal sphincter
Lower esophageal sphincter
Stomach
Storage, mixing, and digestion
Chyme
Pyloric sphincter

Small intestine
 Three segments
 Duodenum, jejunum,
ileum
 Common bile duct
 Pancreas and gall
bladder
 Absorption!

Large intestine (colon)
 Ileocecal valve
 Absorption of water,
some vitamins, &
minerals
 Rectum and anus
Structure
of the
Gut Wall
Absorption in the Small Intestine
Muscular Action of Digestion

Peristalsis
 Circular and longitudinal muscles working
together

Stomach action
 Circular, longitudinal, and diagonal muscles
 Pyloric sphincter: times the release of chyme

Sphincter contractions
 Periodically open and close
 Control pace of GI tract contents
Secretions of Digestion

Five organs
 Salivary glands, stomach, pancreas,
liver, small intestine

Secretions
 Water
 Enzymes
 Protein facilitator of chemical reactions
 Ex: Lipase, protease, carbohydrase
(amylase)
 Bile: emulsifies fat
Enzymes and Digestion
Secretions of Digestion

Saliva
 Moistens food - what does this
accomplish?
 Protective role
 Carbohydrate digestion

Gastric juice
 Protein digestion
 Mucus
 Low pH (very acidic)
The
pH Scale
Secretions of Digestion

Pancreatic juice
 Released via ducts into duodenum
 Enzymes act on all three energyyielding nutrients
 Contains sodium bicarbonate:
neutralizes acidic chyme

Bile
 Produced in liver
 Stored in gallbladder
Signals
and
Secretions
Large Intestine

No further digestion occurs here, except bacterial
breakdown of digestive residues
 Slow transit
 Water absorption
 Some vitamin and mineral absorption
 Undigested residues
 Exercise intestinal muscles
 Fibers: retention of water

Feces stored in rectum prior to defecation
 2 sphincters: internal and external

Intestinal bacteria
 Synthesize some B-vitamins and vitamin K
 From fiber: short-chain fatty acids
Nutrient Breakdown in Digestion
Nutrient Absorption

Most absorption happens in the small
intestine
Nutrient Transport Pathways

Bloodstream
 Water-soluble nutrients, smaller products of
fat digestion
 Sugars, amino acids, small lipid molecules, watersoluble vitamins
 Liver: the gatekeeper
 Stores some nutrients; allows others to flow to rest
of body

Lymphatic system
 Larger fats and fat-soluble vitamins
 Form large particles called chylomicrons
 Lymphatic capillaries in small intestine are
called lacteals
Absorption in Villi of Small
Intestine
Absorptive
Routes
The Vascular Route

Vascular system
 Closed system of vessels
 Heart is the pump

Blood
 Delivers oxygen and nutrients
 Removes carbon dioxide and wastes
 Blood flow
 Special routing for digestive system
 Hepatic portal vein
The Lymphatic Route

Lymphatic system: one-way route
 No pump
 Returns fluid in tissues to bloodstream
 Houses immune cells
 Chylomicrons: too large to enter capillaries
 Taken up by lacteals
 Bypass liver at first and enter bloodstream
directly via thoracic duct
Health of the GI Tract: Gut Flora

Gastrointestinal bacteria – flora
 300-500 species – about 10x the number of cells
in the body! – totaling about 3 pounds
 Most are beneficial

Probiotic bacteria: microorganisms that
confer a benefit on host when consumed in
adequate amount
 Prebiotics: indigestible carbohydrates in
some foods (fruits, vegetables, and whole
grains) that support growth of healthy
bacteria
 Dysbiosis: imbalance of gut bacteria
 What are some probiotic-containing foods?
Direct Immune Function of
Probiotic Bacteria
Benefits of Healthy Gut Flora










Break down indigestible substances (fibers, complex proteins)
Improve digestion and absorption
Support health of intestinal tissues – growth, development, and
nourishment of cells
Produce vitamin K and some B-vitamins
Improve lactose digestion in lactose-intolerant people
Prevent diarrhea related to antibiotic use; shorten duration of
diarrhea from illness
Promote mucin production
Enhance immune function
Positively associated with healthy body weight
Evidence indicates that they may also alleviate constipation,
reduce allergy symptoms, and reduce risk of colon cancer