Digestion and Absorption

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Digestion and Absorption

• Chapter 3
• Appendix E

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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

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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
  responses

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Gastrointestinal Functions

• ingestion
• transport movement
• secretion
• absorption
• elimination (excretion)

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Organization of the Digestive Tract

• long hollow tube
• structures include
• mouth
• esophagus
• stomach
• rings of muscle separate the structure of the
  tube from each other
• accessory organs salivary glands, liver,
  gallbladder, and pancreas

• small intestine
• large intestine
• rectum

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• Mouth
• chews and mixes food with saliva

• Salivary Glands
• secrete enzymes mucous

• Pharynx Esophagus
• passages for food

• Stomach
• adds acid enzymes
• mixes, churns and grinds food
• regulates entry of food into small intestine

• Liver
• manufactures bile (required for fat digestion)

ES

• Pancreas
• secretes enzymes and buffers
• secretes hormones

• Gallbladder
• stores and releases bile

Pyloric Sphincter

• Small Intestine
• primary site of digestion and absorption

• Large Intestine
• reabsorbs water and minerals
• contains bacteria which digest some food and
  synthesize some vitamins

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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
• Mechanical (Physical) Digestion
• Chemical Digestion

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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)

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Peristalsis
Segmentation
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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 lysis to burst or break apart
ENZYME
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Chemical Digestion Enzymes cont.

• Examples

prefix indicates molecule the enzymes acts on
amylase sucrase lactase maltase
pepsin trypsin peptidase
lipase
digest proteins
digest carbohydrates
digests lipids
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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

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Absorption

• where does most absorption occur?
• what characteristics help optimize for absorption?

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Mechanisms of Absorption

• 1. Passive (simple) diffusion
• the unassisted movement of substances in or out
  of cells down a concentration gradient

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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

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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)

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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

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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

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Journey along the GI Tract

• mouth
• stomach
• small Intestine
• large Intestine

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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

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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

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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

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Villi on surface of intestinal folds increase
area of intestine another 10 times
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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
• CHOs, AAs 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

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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

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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)

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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
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Transport of Nutrients

• Vascular system (blood)
• direct absorption of glucose, fructose,
  galactose, proteins, and small lipid molecules
• travels directly to the liver
• 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

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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

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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
  growth

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Diverticulosis Diverticulitis

• diverticula bulging pockets in weakened areas
  of the intestinal wall
• may trap feces and become painfully infected and
  inflamed
• can diet help?

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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 Barretts esophagus, a
  pre-cancerous stage of esophageal cancer
• can diet help?

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Heartburn GERD
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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?