Digestion and
Absorption
Chapter 3
Appendix E
1
Overview of Digestion and Absorption
• Chapter 3
• Gastrointestinal tract
• Overview of Digestion (details discussed with each
nutrient lecture)
– Mechanical
– Chemical
• Overview of Absorption (details discussed with each
nutrient lecture)
• Supporting organs and functions
• Regulation and influences on Digestion and
Absorption
• Circulation of nutrients
• GI disorders
2
Chemosenses: Taste & Smell
What is flavour?
• Smell: olfactory cells stimulated by odors
• Taste: gustatory cells located in surfaces of
mouth
• Common Chemical Sense
• Texture
All of these contribute to activation of cephalic
3
Gastrointestinal Functions
•
•
•
•
•
ingestion
transport & movement
secretion
absorption
elimination (excretion)
4
Organization of the Digestive Tract
• long hollow tube
• structures include:
– mouth
– esophagus
– stomach
 small intestine
 large intestine
 rectum
• rings of muscle separate the structure of the
tube from each other
• accessory organs: salivary glands, liver,
gallbladder, and pancreas
5
Mouth:
• chews and mixes
food with saliva
Salivary Glands:
• secrete enzymes
& mucous
Pharynx & Esophagus:
• passages for food
Liver:
• manufactures bile
(required for fat
digestion)
Gallbladder:
• stores and
releases bile
Small Intestine:
• primary site of
digestion and
absorption
ES
Pyloric
Sphincter
Stomach:
• adds acid & enzymes
• mixes, churns and grinds
food
• regulates entry of food
into small intestine
Pancreas:
• secretes enzymes
and buffers
• secretes hormones
Large Intestine:
• reabsorbs water and minerals
• contains bacteria which digest some
food and synthesize some vitamins
7
Overview of Digestion
Definition
• the mechanical and chemical breakdown of
foods into smaller units that can be taken
across the intestinal epithelium into the body
1. Mechanical (Physical) Digestion
2. Chemical Digestion
8
Physical Digestion
• food is propelled in an anterograde direction
(from mouth to anus) by PERISTALSIS.
• SEGMENTATION divides and mixes
chyme by alternating forward and backward
movements.
• rate of propulsion is regulated to optimize
time for digestion and absorption (important
in the small and large intestine)
9
Peristalsis
Segmentation
10
Chemical Digestion: Enzymes
Enzyme definition:
• proteins that act as CATALYSTS to facilitate
chemical reactions
• change molecules while remaining unchanged
themselves
• digestive enzymes break down molecules into
smaller components via HYDROLYSIS
“hydro” = water
ENZYME
“lysis” = to burst or
break apart 11
12
Chemical Digestion: Enzymes cont.
Examples
prefix indicates
molecule the
enzymes acts on
amylase
sucrase
lactase
maltase
digest
carbohydrates
lipase
digests
lipids
suffix “ase” indicates an
enzyme
pepsin
trypsin
peptidase
digest
proteins
13
Absorption
- movement of small units of food particles (after
digestion) from the interior of the gut (lumen) into
the blood stream or lymphatic system.
- Requires particles to be broken down by digestion, a
large surface area to interact with particles and in
some cases specialized transportation mechanisms
to move particles across cell membranes
14
Absorption
• where does most absorption occur?
• what characteristics help optimize for
absorption?
15
Mechanisms of Absorption
1. Passive (simple)
diffusion
• the unassisted
movement of
substances in or out of
cells down a
concentration gradient
16
Mechanisms of Absorption
2. Facilitated diffusion
• carrier proteins
(transporters) in the cell
membrane help move
substances in or out of
cells down a
concentration gradient
17
Mechanisms of Absorption cont.
3. Active transport
• the movement of
molecules into or out of
cells against a
concentration gradient
using a transport protein
• requires energy (ATP)
18
Mechanisms of Absorption cont.
• endocytosis
– the uptake of material
by a cell through
indentation and
pinching off of part
of the membrane to
form a vesicle
19
Assisting Organs of the Digestive Tract
Salivary Glands
– Saliva, mucous, enzymes
Liver
– Production of bile, detox centre
Gallbladder
– Storage of bile
Pancreas
– Enzyme secretions & hormone release
20
Journey along the GI Tract
•
•
•
•
mouth
stomach
small Intestine
large Intestine
21
Mouth
• digestive process begins here
• enzymes in saliva break down molecules:
– salivary amylase breaks down starch
– lingual lipase breaks down fat minimally
• bolus formed from food, saliva, fluids, and mucus
• moves rapidly through esophagus into stomach
22
Stomach
• Digestion
– produces secretions collectively called gastric juices to
churn and mix food. Gastric juices:
•
hydrochloric acid, mucus, pepsinogen, gastric lipase, gastrin,
intrinsic factor.
– churning moves chyme through the pyloric sphincter into
the small intestine
• Absorption
– some lipid soluble compounds and weak acids such as
alcohol and aspirin absorbed through stomach
23
Small Intestine
• digestive and absorptive workhorse of the gut
• approx. 3m long in 3 parts: duodenum, jejunum and
ileum
• Digestion
– Bicarbonate neutralizes chyme by signal from secretin
– Secretions from pancreas and gallbladder enter small
intestine
– Bile released through signal from Cholecystokinin (CCK)
to emulsify fats
24
Villi on surface of
intestinal folds
increase area of
intestine another 10
times
Small Intestine contd.
• Absorption
– takes place along entire length of small intestine
– most minerals (except for electrolytes) absorbed in
duodenum and upper part of jejunum
– CHO’s, AA’s and water soluble vitamins absorbed along
jejunum and upper ileum
– lipids and fat-soluble vitamins absorbed primarily in ileum.
• after 3-10hr journey through small intestine, chyme
passes through ileocecal valve to the large intestine
26
Large Intestine
• 1.5 m long and includes: cecum, colon,
rectum, and anal canal
• Digestion
– bacteria breaks down small amounts of fiber
– bacterial activity produces vitamins K, some B
vitamins and various gases
• Absorption
– water, sodium, chloride, potassium, vitamin K
27
Regulation of GI Activity
Nervous System
– enteric nervous system: local system of nerves
– parasympathetic nervous system: division of
autonomic nervous system
Hormone System
–
–
–
–
Gastrin
Gastric Inhibitory Peptide (GIP)
Secretin
Cholecystokinin (CCK)
28
Transport of nutrients
Some nutrients travel freely
in the blood (e.g. glucose,
proteins).
Others are bound to
protein transporters (e.g.
lipids, vitamins, and
minerals)
Fig 3.18
30
Transport of Nutrients
1. Vascular system (blood)
– direct absorption of glucose, fructose, galactose,
& proteins, and small lipid molecules
– travels directly to the liver
2. Lymphatic system
– larger lipid molecules are too large to enter the
blood directly
– absorbed first into the lymph
– enter the blood at the inferior vena cava near
heart
31
Influences on Digestion & Absorption
• Psychological
– taste, smell, presentation of food
• Chemical
– processing of foods and food preparation,
medications
• Bacterial
– some may cause gastritis, ulcers; others are
beneficial
32
Bugs in your gut?
Intestinal Microflora
• e.g. lactobacilli, bifidobacteria (probiotics)
–
–
–
–
–
–
improved digestion
intestinal regularity
enhanced GI immune function
improved lactose intolerance
reduced risk of allergies
reduced risk of colorectal cancer
• Prebiotics – nondigestible products that can
be fermented by GI bacteria, stimulating their
33
growth
Diverticulosis & Diverticulitis
• diverticula = bulging
pockets in weakened
areas of the intestinal
wall
• may trap feces and
become painfully
infected and inflamed
• can diet help?
34
Heartburn & GERD
• due to inappropriate relaxation of the LES
which allows hydrochloric acid from the
stomach to flow backwards into the
esophagus
• prolonged and repeated reflux contributes to
esophageal damage leading to cellular changes
referred to as Barrett’s esophagus, a precancerous stage of esophageal cancer
35
• can diet help?
Heartburn & GERD
36
Colorectal Cancer
• second leading cause of cancer-related deaths
in North America
• increased risk associated with high meat & fat
intake and low fruit, vegetable, folate,
calcium, & Vitamin D intake
• does a fiber-rich diet lower risk?
37