Chapter 3: Digestion, Absorption

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Chapter 3Digestion, Absorption Transport
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Why Do We Want to Eat?

• Food is intimately connected to our sense of
  taste, but also stimulates our senses of
• Sight
• Smell
• Touch
• Hearing

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Why Do We Want to Eat?

• Appetite psychological desire to eat certain
  foods.
• Strong cravings even when were not hungry are
  due to appetite.
• Hunger physiological sensation that prompts us
  to eat.
• Satiety the feeling of being full.

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Why Do We Want To Eat?

• The signals that prompt us to eat include
• Nerve receptors in the stomach send signals to
  the brain to indicate if the stomach is full or
  empty.
• Blood glucose levels trigger the release of
  hormones called insulin and glucagon
• The hypothalamus region of the brain receives
  these signals.

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Why Do We Want To Eat?
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Why Do We Want To Eat?

• Hormones chemicals produced in specialized
  glands that travel in the bloodstream to target
  organs in other parts of the body.
• Some hormones stimulate food intake.
• Some hormones produce a feeling of satiety.

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Why Do We Want To Eat?

• Foods have differing effects on our feelings of
  hunger
• Proteins have the highest satiety value
• Carbohydrates have the lowest satiety value
• Bulky foods (fibre) provide a sense of satiety
• Solid foods are more filling than semi-solid
  foods or liquids

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What Happens to the Food We Eat?

• The food we eat undergoes three processes
• Digestion
• Absorption
• Elimination
• These processes occur in the gastrointestinal
  tract.

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Breaking Down Food

• Digestion
• Process by which foods are broken down into their
  absorbable units
• Mechanical digestion the physical breakdown of
  food.
• Chemical digestion enzymatic reactions that
  break down large food molecules.
• Absorption
• Physiological process by which molecules of food
  are taken across the GI tract walls into the
  body.

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• GI tract is a flexible muscular tube from the
  mouth, through the esophagus, stomach, small
  intestine, large intestine, and rectum to the
  anus.

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Breaking Down Food

• Mouth
• Teeth chewing breaks food into small pieces mix
  with fluids/saliva for swallowing
• Tongue
• Taste movement
• 4 basic tastes sweet, sour, bitter, salty
• umami
• Aroma, texture temp also influence flavour

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Breaking Down Food

• Mouth
• Swallowing food passes through pharynx
• Tube shared by digestive respiratory system
• Epiglottis closes off airway during swallow
• Food now known as bolus

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Breaking Down Food

• Esophagus
• Upper esophageal sphincter muscle opens.
• When food reaches end of esophagus, cardiac or
  lower esophageal sphincter relaxes allowing food
  to enter stomach.

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Muscular Action of Digestion

• Peristalsis
• Entire GI tract covered in muscle (circular
  longitudinal)
• Wavelike muscular contractions of GI tract that
  push its contents through.
• Begins when bolus enters esophagus.
• Segmentation
• Circular muscles rhythmically contract squeeze
  contents
• Mix chyme, promote close contact with digestive
  juices absorptive surface

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Breaking Down Food

• Stomach
• Thickest strongest muscles includes diagonal
  muscles
• Secretes gastric juices
• Mixes churns food until it becomes chyme
• Pyloric sphincter regulates chyme release into
  small intestine

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Intestinal Route

• Small intestine
• Duodenum (Common bile duct enters)
• jejunum
• Ileum (ileocecal valve)
• Large intestine
• bypass appendix
• ascending colon
• transverse colon
• descending colon
• sigmoid colon
• Rectum
• Anus

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Secretions of digestion

• Secretions breakdown food into small nutrients so
  body can absorb it
• Contain water enzymes.
• P. 77 Glossary of digestive enzymes
• P. 78 - Glossary of digestive glands their
  secretions.

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Enzymes

• Proteins that facilitate chemical reactions
• Make molecule, break molecule, change arrangement
  exchange parts of molecules
• Remain unchanged in the process - catalyst
• How can you identify an enzyme?
• End in ase
• Name states What it acts on e.g. lipase
  (lipids) or protease (protein)
• Where it is secreted from
• e.g. pancreatic or gastric lipase
• Ex gastric lipase

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Secretion Producing Organs

• Salivary glands
• Stomach
• Pancreas
• Liver (via gallbladder)
• Small intestine

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Digestive Secretions

• Saliva
• Secreted by salivary glands in mouth.
• Contains water, salts, mucus enzymes that
  initiate breakdown of CHO (salivary amylase)
• Contains lysozyme enzyme with antibacterial
  properties
• Protects tooth surface tissue in mouth,
  esophagus stomach.

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Digestive Secretions

• Gastric Juice
• Secreted from stomach cells
• Contains water, enzymes (pepsin, gastric lipase),
  intrinsic factor HCL
• I.F. binds to Vit B12
• HCL acts primarily in protein digestion kills
  bacteria pH 2
• Denatures protein
• Activates pepsinogen to pepsin (protein digestive
  enzyme - hydrolysis)

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Digestive Secretions

• Mucus
• Sites of production mouth, stomach, small
  intestine, large intestine.
• Protects cells, lubricates.

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Digestive Secretions

• Pancreatic Juice Intestinal Enzymes
• Pancreas add digestive juice through ducts
  leading to the duodenum.
• Pancreatic amylase, lipase protease
• Intestinal cells also provide enzymes
• i.e. lactase
• Pancreatic juice also contain sodium bicarbonate
  neutralizing the acidic chyme from the stomach.

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Digestive Secretions

• Bile
• Produced in liver continuously
• Concentrated stored in gallbladder
• Flows into the duodenum
• Acts as emulsifier, not an enzyme, which brings
  fats into suspension in water so enzymes can
  break them down.

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Digestion the final stage

• 3 energy yielding nutrients disassembled ready
  to be absorbed
• Undigested residues continue through digestive
  tract creating semi-solid mass
• Exercises muscles
• Retains water to keep stool at proper consistency
• Carries some bile acids, minerals, additives
  contaminants

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Elimination the final stage

• Colon
• Intestinal bacteria ferment some fibres
• Produce water, gas, short chain fatty acids
  (energy for intestinal cells)
• Retrieves any reusable material like water
  dissolved salts
• Strong muscles of rectum anal canal hold waste
  until time to defecate
• Defecation muscles relax 2 anal sphincters
  open to allow passage of waste

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• http//www.medtropolis.com/VBody.asp

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Absorption
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Absorption

• Process starts within 3-4 hours of eating
• Absorb molecules from carbohydrate, fat and
  protein digestion as well as vitamins minerals
• A small amount of absorption occurs in the
  stomach, e.g. water, alcohol
• Most absorption takes place in the small
  intestine
• Surface area equivalent to tennis court

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Anatomy of absorption

• Walls of small intestine contain thousands of
  wrinkled folds
• Each fold contoured with thousands of fingerlike
  projections called villi
• Villi are covered by microvilli
• Crevices between villi known as crypts
• Tubular glands secreting intestinal juices
• Goblet cells secreting mucus

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Anatomy of Absorption

• To remove the molecules rapidly provide room
  for more, a rush of circulating blood
  continuously washes the underside of the surface
  of the small intestine

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Methods of Absorption

• Diffusion
• Cross intestinal cell membrane freely.
• Facilitated Diffusion
• Need a carrier to transport across the cell
  membrane.
• Active Transport
• Need a carrier energy to transport across an
  area of low concentration to an area of higher
  concentration of the cell membrane.

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Absorption

• Portions of GI tract are specialized to absorb
  certain nutrients
• Readily digested absorbed early on
• Disease or resection of intestine may compromise
  absorption

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AbsorptionSites
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What about food combining?

• Idea that we should not eat certain food in
  combination is a myth.
• Digestive system is able to cope well with mixed
  diet.
• Foods eaten together can actually enhance
  nutrients absorption.

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Transport

• Water-soluble nutrients smaller products of fat
  digestion
• Released directly into bloodstream.
• Larger fats fat-soluble vitamins
• Assembles in larger molecules clustered
  together with special proteins forming
  chylomicrons.
• Chylomicrons are released into the lymphatic
  system, eventually entering the bloodstream.

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Vascular System

• Arteries
• carry blood away from the heart
• Capillaries
• branches between arteries and veins
• site of exchange of nutrients, gases and wastes
• Veins
• carry blood back to heart

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Routing of blood

• Simple route
• heart to arteries to capillaries to veins back to
  heart.
• Special route past the digestive system
• heart to arteries to capillaries (in intestines)
  to veins (hepatic portal vein) to capillaries (in
  liver) to veins (hepatic vein) back to heart.

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Lymphatic system

• Organized system of vessels ducts
• No pump
• Lacteals in each villi to take up nutrients
• Nutrients from the GI tract that enter the lymph
  system enter the vascular system bypassing liver
  at first
• Thoracic duct to subclavian vein

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Regulation of digestion absorption

• Hormonal (endocrine) system
• Nervous system

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Hormones

• Chemical messengers secreted by glands.
• Each hormone travels to a targeted organ where it
  produces a specific response to maintain
  homeostasis.

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GI Hormones

• Gastrin
• produced by cells in the stomach wall that
  stimulates secretion of gastric juices (HCl) from
  gastric cells
• Secretin
• produced by cells in the duodenum wall that
  stimulates the pancreas to secrete pancreatic
  juices (bicarbonate)

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GI Hormones

• Cholecystokinin (CCK)
• produced by cells of the intestinal wall which
  tells the gallbladder to release bile and to slow
  down GI motility.
• Gastric-inhibitory Peptide
• produced by the intestine that tells the stomach
  to slow down secretion of gastric juices GI
  mobility.

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Protective Factors

• Neutral pH in small large intestine promote
  bacterial growth
• 10 trillion bacteria representing some 400
  species subspecies in healthy intestinal tract
• Bacteria help produce some vitamins
• E.g. vit K
• If normal intestinal bacteria are thriving,
  pathogenic bacteria have a difficult time
  surviving.
• Saliva, mucus, gastric acid digestive enzymes
  also defend against pathogens.

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Probiotics

• Live microorganisms found in, or added to,
  fermented foods that optimize the bacterial
  environment of intestine.
• In Canada, most foods that contain probiotics are
  fortified milk fermented yogurt.
• Also, supplement form.
• Most frequently used probiotics are species of
  Lactobacillus or Bifidobacterium.

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Factors influencing GI tract

• Physical maturity
• Physical activity
• Aging
• Illness
• Sleep
• Nutrition
• Mental state

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Common Digestive Problems

• Heartburn (ulcers Helicobacter pylori)
• Food allergies intolerances
• Celiac disease
• Irritable Bowel Syndrome (IBS)
• Crohns and colitis
• Diarrhea
• Constipation

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Next

• Carbohydrates