Download Digestive Processes

Unit I: Metabolism
Digestive Processes
Chapter 21 and 22
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
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
Swallowing / Deglutition
1. Buccal Phase
• Compression of the bolus
Soft palate
• Tongue then forces the bolus
Bolus
Esophagus
into the oropharynx
• Voluntary
2. Pharyngeal Phase
Tongue
Bolus
• Tactile receptors on the palatal
arches and uvula are stimulated.
• Swallowing center in the medulla
oblongata then coordinates:
• Elevation of the larynx
• Folding of the epiglottis
• Elevate the uvula and
soft palate
• Involuntary
Swallowing / Deglutition
3. Esophageal Phase
• Myenteric nerve plexis signals
peristaltic waves
• LES opens
Peristalsis
• Typical bolus = 9 seconds
• Liquids = 2 seconds
• A dry or poorly lubricated
bolus may need secondary
peristaltic waves
• Bolus continues to the stomach
Thoracic
cavity
Lower esophageal sphincter (LES)
Stomach
Peristalsis allows us to move a bolus regardless of our body position.
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
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
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.
Medulla Oblongata
Vagus nerve (N X)
• 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.
Submucosal
and myenteric
plexuses
Distension
Elevated pH
via
bloodstream
Submucosal
plexuses
Mucous
cells
Chief cells
Gastrin
Mucous
cells
Chief cells
Mucus
Gastrin
Pepsinogen
Parietal cells HCl
G cells
G cells
KEY
Stimulation
Chemoreceptors
Mucus
Pepsinogen
Parietal cells HCl
Stretch
receptors
Partly
digested
peptides
Mixing
waves
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 Enterogastric Myenteric
plexus
Purpose: Inhibit gastric activity reflex
Chief
• Stimulates contraction of
via bloodstream
cells
pyloric sphincter
Parietal
Duodenal
• Mucus production
cells
stretch and
chemoreceptors
CCK
GIP
Presence of
lipids and
carbohydrates
Secretin
Decreased pH
Peristalsis
KEY
Inhibition
Intestinal Hormones
•
Cholecystokinin (CCK)
1. Contraction of gallbladder
2. Secretions of pancreatic enzymes
3. Relaxation of hepatopancreatic sphincter
•
Secretin
•
Glucose dependent insulin peptide/ Gastric inhibitory peptide
Overall effect: reduce gastrin secretion and passage of chyme
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
Carbohydrate Digestion
• Salivary amylase  oligosaccharide
– 50% of dietary starch digested before it reaches small intestine
• Pancreatic amylase  oligosaccharide and maltose
• Brush border enzymes  monosaccharides
Carbohydrate Absorption
• 80% of glucose taken up by Naglucose transport proteins →
Disaccharides
Enzymes
• Facilitated diffusion → blood
capillaries of villus →
Simple sugars
(such as glucose
or fructose)
• Delivered to liver
Epithelial cell
Simple sugars
Protein Digestion
Proteins
ingested
½ of amino acids digested come
from dietary proteins
Protease pepsin breaks down
complex proteins into smaller
peptide and polypeptide chains
Pancreatic enzymes: Trypsin,
chymotrypsin, and
carboxypeptidase break down
proteins into a mixture of
dipeptides, tripeptides, and
amino acids.
amino acids
The epithelial surfaces
of the small intestine
contain several brush
border enzymes that
release individual
amino acids.
Dipeptidase
Facilitated diffusion
and cotransport
Amino acids
Epithelial cell
Facilitated diffusion
and cotransport
Amino acids
Protein Absorption
Behaves like monosaccharides:
•Taken up by Na-dependent transport proteins → epithelial cells →
•Facilitated diffusion → blood capillaries of villus → bloodstream
Lipid Digestion
1. Lipase – Lingual lipase, gastric lipase
Bile
2.
3.
Lipid Absorption
1.
2.
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
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)
Absorption and Motility of Large Intestine
Vitamin Absorption
Vitamin K
1500mL/day enters
Vitamin B5
Biotin
Over 1 L of water is
reabsorbed
through osmosis.
Organic waste products, such as
urobilinogen, and various toxins.
Only 200 mL of feces is ejected.
75% water, 25% solids
• 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
Stimulation of
somatic motor
neurons
Defecation
Stimulation of
motor neurons
in sacral spinal
cord
Stimulation of
myenteric plexus
Parasympathetic
Defecation Reflex
Intrinsic Defecation
Reflex
Increased
peristalsis
throughout large
intestine
Stimulation of
stretch receptors
Start
Increased local
peristalsis
DISTENSION
OF RECTUM
Relaxation of internal
anal sphincter
External sphincter
can be voluntarily
relaxed
Involuntary contraction
of external anal
sphincter
When external sphincter
is relaxed:
DEFECATION OCCURS
stimulates
inhibits