CH 3: Digestion, Absorption, and Transport

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CH 3 Digestion, Absorption, and Transport

• The GI Tract

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Overview Digestive System

• Functions Digestion, Absorption, Elimination
• Digestion process of breaking down foods to
  release nutrients
• Goal is to break nutrients into absorbable units
• 2 types of digestions
• Mechanical
• Enzymatic (chemical)

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Overview Digestive System

• Absorption movement of nutrients out of GI
  tract into blood or lymph
• Water soluble nutrients ?
• Fat soluble nutrients ?
• Excretion elimination of undigested foods
  (feces)

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Overview Digestive System

• Structure Gastrointestinal (GI) Tract
• Continuous tube from mouth to anus
• See board and page 27

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GI Tract Anatomy
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Digestive Tract

• Layers of GI tract organs
• Serosa (outermost)
• Interface between GI tract and lymph blood
• Muscle Layers
• Longitudinal muscles
• Circular muscles
• Submucosal and Mucosal and layers (innermost)
• Nerves, blood and lymph vessels
• Cells of the mucosal layer produce secretions

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Mouth

• Structure teeth, tongue, salivary glands
• Secretions
• Saliva
• Salivary amylase ..
• Digestion
• Mechanical .
• Enzymatic/chemical .
• Swallow ? tongue pushes food ? pharynx ?

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Esophagus

• Structure 12 tube
• UES and LES
• Function
• Transports food from mouth to stomach
• Peristalsis and gravity aid food movement
• Secretions -- mucus
• Digestion
• Mechanical (limited)
• Enzymatic/chemical starch digestion continues

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Stomach

• Structure .page 73
• Secretions ..
• Digestion
• Mechanical
• Enzymatic/chemical

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• Stomach
• Structure muscular sack that can expand
• extra muscle layer to aid in the mechanical
  digestion of food (pg 73)

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Stomach Related Secretions

• Gastrin hormone that stimulates stomach to
  release secretions
• Hydrochloric Acid (HCl) -- unravels proteins,
  kills bacteria, activates pepsinogen
• Pepsinogen once activated, begins protein
  digestion
• Mucus secreted by goblet cells, protects
  stomach, moistens food

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Stomach - Digestion

• Mechanical
• Stomach muscles grind food into a paste called
  chyme
• Enzymatic/chemical
• Proteins uncoiled
• Protein digestion to polypeptides begins
• Starch digestion stops (why??)

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Small Intestine (SI)

• Function The SI is where
• the majority of digestion to absorbable units
  occurs
• Digestion of carbohydrates, fats, and proteins
• Vitamins and minerals do not need digestion
• Nutrients are absorbed into either capillaries or
  lacteals

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Small Intestine

• Structure see page 79
• Length -- 21-22 long when relaxed
• Layers ..
• Mucosal folds, villi, microvilli .
• Goblet cells and crypts create secretions
• Lacteals and capillaries .
• Sections
• Duodenum
• Jejunum
• Ileum

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Small Intestine
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The Small Intestinal Villi
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The Small Intestinal Villi
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Small Intestines

• Secretions of the SI
• Mucus
• Secreted by ____________ cells
• Digestive enzymes that finish the digestion of
  carbs, fats, and proteins
• Secreted from crypts

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Small Intestine

• Secretions of the SI, contd
• Hormones
• Secretin
• CCK .
• Gastric-inhibitory peptide

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Small Intestine

• Hormone Summary
• Secretin produced when chyme enters SI
• stimulates pancreatic secretions
• CCK produced when fat enters SI
• Stimulates _________to release ______
• Slows GI motility (slows peristalsis)
• Cholecystokinin
• Gastricinhibitory peptide produced when chyme
  enters SI
• Slows stomach secretions
• Slows GI motility

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Secretions into SI

• Pancreatic secretions
• Released in response to ________
• Sodium bicarbonate
• Neutralizes acidic chyme
• Digestive enzymes that begin the digestion of
  carbs, fats, and proteins

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Secretions into SI

• Liver and Gall Bladder
• Liver makes bile
• Gall bladder concentrates and stores bile
• Bile is released into SI in response to the
  hormone _______
• Function of bile

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Small Intestine

• Digestion - Mechanical
• Peristalsis pushes food through SI
• Segmentation mixes chyme with digestive enzymes
• Also breaks up food mass into smaller masses
• Bile emulsifies fats

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Peristalsis Segmentation
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Muscular Action of Digestion

• Segmentation

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SI

• Digestion Chemical/Enzymatic
• Pancreatic and SI enzymes digest carbs, fats, and
  proteins to absorbable units
• Duodenum
• Jejunum
• Ileum

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SI

• Absorbed into capillaries (blood)
• Digested carbohydrates and proteins
• Minerals
• Some require helper proteins/cells on walls of SI
  to be absorbed
• Water soluble vitamins
• Blood takes nutrients to the liver for processing
  after absorption (pg 81)

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SI

• Absorbed into lacteals (lymph)
• Digested fats
• Fat soluble vitamins
• Cholesterol
• Nutrients travel through lymph system to chest
  area where lymph and blood join
• Nutrients enter blood and travel to liver

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SI Review
What is the relationship between the structure of
the SI and its function?
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Large Intestine or Colon

• Undigested foods (fiber) enter into colon
• Unabsorbed nutrients pass into colon
• E.g. calcium, iron, cholesterol trapped in fiber

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Colon

• Structure
• 5-6 long
• Wider diameter than SI
• No villi or microvilli
• Appendix is a little pouch near beginning of
  colon
• Colon wraps around SI
• Page 82

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Colon

• Secretions
• Mucus
• Bacteria living in colon
• Digest small amounts of fiber and undigested
  nutrients
• Often produce...

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Colon

• Absorption
• Water, salts, vitamins made by bacteria are
  absorbed into __________
• Fiber attracts water
• Too little fiber in diet ?
• Too much fiber in diet ?

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Rectum

• Feces pass from colon into rectum
• Rectum stores feces until excretion occurs