Digestive Processes

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Unit I Metabolism Digestive Processes
Chapter 21 and 22
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Mastication or Chewing

• ? surface area exposed to digestive enzymes
• Contact of food with sensory receptors triggers
  chewing reflex
• tongue, buccinator and orbicularis oris
  manipulate food
• masseter and temporalis elevate the teeth to
  crush food
• medial and lateral pterygoids swing teeth in
  side-to-side grinding action of molars

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Salivation

• Total of 1 to 1.5 L of saliva per day
• Cells filter water from blood and add other
  substances
• Food stimulates receptors that signal salivatory
  nuclei in medulla and pons
• parasympathetic stimulation ? salivary glands
  produce thin saliva, rich in enzymes
• sympathetic stimulation ? produce less abundant,
  thicker saliva, with more mucus

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Swallowing / Deglutition
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Swallowing / Deglutition

• Typical bolus 9 seconds
• Liquids 2 seconds
• A dry or poorly lubricated bolus may need
  secondary peristaltic waves

Peristalsis allows us to move a bolus regardless
of our body position.
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Gastric Motility

• Swallowing center signals stomach to relax
• Receptive-relaxation response
• Peristalsis
• contraction every 20 seconds
• stronger contractions after 30 minutes ejects 3
  ml
• typical meal emptied from stomach in 4 hours

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Vomiting

• Induced by
• excessive stretching of stomach, psychological
  stimuli or chemical irritants (bacterial toxins)
• Emetic center in medulla causes
• retching
• Lower esophageal sphincter to relax
• stomach and duodenum to contract spasmodically
• vomiting
• when abdominal contraction forces upper
  esophageal sphincter to open

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Regulation of Gastric Secretion
1. Cephalic Phase
2. Gastric Phase

• See, smell, taste, or think of food.
• Directed by the CNS
• Prepares the stomach to receive food
• Production of gastric juice accelerates
• reaching rates of about 500 mL/h
• lasts only minutes.

• Arrival of food in the stomach
• (1) distension of the stomach
• (2) an increase in the pH
• (3) the presence of undigested proteins
• Lasts 3-4 hours
• Gastrin stimulates contractions in the
• muscularis externa of the stomach.

Mucous cells
Chief cells
Parietal cells
Chief cells
G cells
Parietal cells
G cells
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Regulation of Gastric Secretion
3. Intestinal Phase

• Chyme first enters the small intestine
• Function control the rate of gastric emptying
• Enterogastric reflex
• Stimuli? stimulation of stretch receptors in
  stomach and
• ?stimulation of stretch
• receptors in intestine
• Purpose Inhibit gastric activity
• Stimulates contraction of
• pyloric sphincter
• Mucus production

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Intestinal Hormones

• Cholecystokinin (CCK)
• Contraction of gallbladder
• Secretions of pancreatic enzymes
• Relaxation of hepatopancreatic sphincter
• Secretin
• Glucose dependent insulin peptide/ Gastric
  inhibitory peptide
• Overall effect reduce gastrin secretion and
  passage of chyme

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Small Intestine Motility

• Segmentation -
• stationary ring-like constrictions
• not as in peristalsis
• Purpose
• mix
• churn
• Pacemaker cells in Muscularis externa
• Provides the most contact digestion

http//youtu.be/PfnKvErPwY4
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Carbohydrate Digestion

• Salivary amylase ? oligosaccharide
• 50 of dietary starch digested before it reaches
  small intestine
• Pancreatic amylase ? oligosaccharide and maltose
• Brush border enzymes ? monosaccharides

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Carbohydrate Absorption

• 80 of glucose taken up by Na-glucose transport
  proteins ?
• Facilitated diffusion ? blood capillaries of
  villus ?
• Delivered to liver

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Protein Digestion
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Protein Absorption

• Behaves like monosaccharides
• Taken up by Na-dependent transport proteins ?
  epithelial cells ?
• Facilitated diffusion ? blood capillaries of
  villus ? bloodstream

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Lipid Digestion

1. Lipase Lingual lipase, gastric lipase

Bile
2.
3.
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Lipid Absorption
1.
2.
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Nucleic Acids, Vitamins, and Minerals

• Nucleases
• brush border split them completely apart
• Vitamins are absorbed unchanged
• A, D3, E and K with other lipids
• B complex and C by simple diffusion
• B12 if bound to intrinsic factor
• Minerals are absorbed all along small intestine
• Na cotransported with sugars and amino acids
• Cl- exchanged for bicarbonate reversing stomach
• Iron and calcium absorbed as needed

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Water Balance

• Digestive tract receives about 9 L of water/day
• 0.7 L in food, 1.6 L in drink, 6.7 L in
  secretions
• 8 L is absorbed by small intestine and 0.8 L by
  large intestine
• Diarrhea
• feces pass through too quickly if irritated
• feces contains high concentrations of a solute
  (lactose)

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Absorption and Motility of Large Intestine

• Transit time is 12 to 24 hours
• reabsorbs water and electrolytes
• Haustral contractions occur every 30 minutes
• distension of a haustrum stimulates it to
  contract
• Mass movements occur 1 to 3 times a day
• filling of the stomach and duodenum

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Defecation
External sphincter can be voluntarily relaxed