Digestion Physiology
Chapter 26
Test (Chapters 25 & 26) April
6th and 7th
(next block day)
March 28, 2011
Announcements:
•
•
•
•
Turn in last week’s warm ups to the box
Pick up new warm up sheet
Practical grades in gradebook
Practical make up:
– Today after school @ 3:15
– Tuesday morning 6:45
Warm Ups:
1.
2.
3.
4.
5.
In the large intestine longitudinal muscles are grouped into
strips called _______; circular muscles are grouped into
rings are form _______.
What is produced in the appendix?
The two lobes of the liver are connected by the:
What are gallstones?
Through what two pathways does blood enter the hepatic
lobules?
Primary Mechanisms of Digestive
System (Table 26-1)
• Ingestion – take food in
• Digestion – break down complex nutrients into
simpler nutrients
• Motility – movement of GI tract; aids in
digestion
• Secretion – enzymes are required for digestion
to take place
• Absorption – movement across GI mucosa into
the internal environment
• Elimination – process of eliminating unabsorbed
material
Mechanical Digestion
• All motility of the digestive tract that
cause the following changes:
– Change in physical state of the food for
large particles to smaller particles
– Churning of the GI lumen to mix particles
with digestive juices
– Propelling food forward ending with
elimination
Mastication
•
•
•
•
Mastication = chewing
Requires tongue, cheek and lips
Reduces particle size
Mix food with saliva
Deglutition
• Deglutition = swallowing
1. Oral stage
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•
•
Bolus is formed voluntarily
Tongue pushes bolus against the palate and into
the oropharynx
Soft palate acts as a valve to prevent food from
entering the nasopharynx
2. Pharyngeal stage
•
•
Involuntary reflexes push bolus toward esophagus
Epiglottis prevents food from entering trachea
•
Involuntary reflexes move bolus towards stomach
3. Esophageal stage
Motility
• Smooth muscle contractions take over
in the lower portion of the esophagus
• Peristalsis – progressive wavelike ripple
of the muscle layer of a hollow organ
– Bolus stretches the GI tract wall 
triggers contraction of circular smooth
muscle  bolus moves forward
Motility
• Segmentation – mixing movement; back
and forward movement within a single
region
– Mechanically breakdown food particles
– Mix food and digestive juices together
– Facilitate absorption
• Peristalsis and segmentation can occur
in alternating sequence to churn/mix
and progress food
Peristalsis & Segmentation
Regulation of Motility
• Gastric Motility
– Emptying the stomach takes approx 2-6
hours after a meal
– Food is churned with digestive juices to
form chyme
• Ejection every 20 seconds into the duodenum
• Controlled by hormonal and nervous mechanisms
Regulation of Motility
• Hormonal Control
– Fats and nutrients in duodenum cause
secretion of gastric inhibitory peptide
(GIP) from the intestinal mucosa into the
bloodstream
• Slows peristalsis in stomach; decreasing passage
of food into the duodenum
Regulation of Motility
• Nervous Control
– Nerve receptors in duodenum are sensitive
to acid and distention
• Sensory and motor fibers within the vagus
nerve (CN X) cause a reflex inhibition of gastric
peristalsis (enterogastric reflex)
Regulation of Motility
• Intestinal Motility
• Takes approx 5 hours for food to pass through
the small intestine
– Segmentation
• Mixes chyme and digestive juices from liver, pancreas and
intestinal mucosa
• causes contact with intestinal mucosa to increase absorption
– Peristalsis
• Continues in the jejunum to move food into the large intestine
• Stimulated by the hormone cholecystokinin-pancreozymin
(CCK)
– Secreted by intestinal endocrine cells in the presence of chyme
Mechanical Digestion
• Summarized in Table 26-2; page 775
Chemical Digestion
• Consists of all the changes in the
chemical composition of food
• Result of hydrolysis
– Compounds combine with water then split
into simpler compounds
– Enzymes catalyze the hydrolysis of foods
Chemical Digestion
• Six main types of chemical substances:
– Carbohydrates, proteins, fats, vitamins,
mineral salts, water
– Only carbohydrates, proteins and fats must
undergo chemical digestion to be absorbed
Digestive Enzymes Properties
• Extracellular enzymes
• Classified as hydrolases
• Function optimally at a specific pH
– Ex: amylase vs pepsin
• Continually destroyed or eliminated
• Most digestive enzymes are synthesized
and secreted as proenzymes
– Kinases convert proenzymes to active
enzymes
Carbohydrate Digestion
• Carbohydrates are saccharide
compounds
– Contain one or more saccharide groups
• Polysaccharides – starches & glycogen
• Disaccharides – sucrose, lactose and
maltose
• Monosaccharides – glucose, fructose &
galactose
Carbohydrate Digestion
• Polysaccharides are hydrolyzed by
amylases
– Present in saliva and pancreatic juice
• Sucrose, lactose and maltose are
hydrolyzed by sucrase, lactase, and
maltase
– Located on epithelial cells lining villi in small
intestine
– End result (usually glucose) is located at site
of absorption (“contact digestion”)
Protein Digestion
• Proteins are large molecules composed
of twisted chains of amino acids
• Proteases catalyze the hydrolysis of
proteins into smaller compounds
– Proteoses  peptides  amino acids
• Proteins have varying peptide bonds
holding amino acids together =
increased need for varying proteases
Protein Digestion - Proteases
• Pepsin – gastric juice
• Trypsin and chymotrypsin – pancreatic
juice
• Peptidases – intestinal brush border
Proteoses
Fat Digestion
• Fats are insoluble in water – must be
emulsified prior to digestion
– Emulsify = dispersed as small droplets
– Lecithin and bile salts emulsify oils and fats in the
small intestine by forming micelles (fig 26-8, page
778)
– Lecithin mixes with fat to form micelles
• Fats broken down by mechanical digestion are
further broken down by lipase
• Action of lipase is enhanced by colipase
(released from the pancreas)
Residuals of Digestion
• Certain compounds cannot be digested in
humans b/c we lack the enzyme required
for hydrolysis
• These compounds are excreted in the
feces
• Cellulose (dietary fiber), connective
tissue from meat (collagen), undigested
fats combined with calcium and
magnesium, bacteria, pigments, water,
mucous
Chemical Digestion
• Summarized in Table 26-3, page 779
Secretion
• Release of substances from exocrine
glands in the GI tract
– Saliva, gastric juice, bile, pancreatic juice,
intestinal juice
Saliva
• Secreted from salivary glands
• Water component helps liquefies food  chyme
– Allows enzymes to mix with food particles
• Mucus lubricates food to protect mucosa lining
• Amylase – chemically digest starch and glycogen
• Lipase (small amounts) – digest lipids
– Decreased function when fat are not emulsified
• Sodium bicarbonate (NaHCO3)
– Dissociated in water
– Bicarbonate ions bind with H+ to increase pH
Control of Salivary Secretion
• Controlled by reflex mechanisms:
– Olfactory & visual stimuli send impulses to
centers in the brainstem  efferent
impulses to salivary glands
– Chemical and mechanical stimuli comes from
the presence of food in the mouth
Gastric Juice
• Secreted by gastric glands surrounds by
gastric pits
• Chief cells – secrete enzymes of digestive
juices
– Pepsin (proenzyme = pepsinogen)
• Pepsinogen is activated by hydrochloric acid (HCl)
• Parietal cells
– Secrete HCl
• Decreases stomach pH; increases blood pH
– Secrete intrinsic factor
• Binds to molecules of vitamin B12 to facilitate absorption
in the small intestine
Control of Gastric Secretion
•
Gastric secretion is controlled by 3
phases:
1. Cephalic phase (“psychic phase”)
–
–
–
Sight, smell, taste, thought of food activate
control centers in medulla oblongata
Parasympathetic fibers of the vagus nerve
conduct impulses to gastric glands
Vagal impulses stimulate production of
gastrin
•
Gastrin stimulates gastric secretion
Control of Gastric Secretion
2. Gastric phase:
– Gastrin secretion is further stimulated by
the presence of products of protein
digestion & distention
– Gastrin continue to stimulate the secretion
of gastric juices (high pepsinogen and HCl
content)
Control of Gastric Secretion
3. Intestinal phase:
– Gastric inhibitory peptide (GIP) in secreted in the
small intestine in the presence of fats and
carbohydrates
• Decrease gastric motility and secretion
– Secretin secreted in the small intestine in the
presence of acid, digested proteins and fats
• Inhibit gastric secretion
• Simulate secretion of pancreatic enzymes
• Stimulate ejection of bile into small intestine
– CCK
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•
•
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Secreted in the small intestine in the presence of chyme
Stimulates ejection of bile from gallbladder
Stimulates secretion of pancreatic juices
Opposes action of gastrin; raises pH of gastric juice
Pancreatic Juice
• Secreted by exocrine acinar cells of the
pancreas
• Mostly water
• Enzymes:
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–
–
–
Trypsin and chymotrypsin (proteases)
Lipases
Nucelases (RNA and DNA digesting enzymes)
Amylase (starch digesting enzyme)
• Secrete bicarbonate into the GI lumen and H+
into the blood to buffer the effects parietal
cell secretion (fig 26-10 and fig 26-11)
Control of Pancreatic Secretion
• Secretin
– Stimulates the secretion of pancreatic
fluid high in bicarbonate to neutralize
acidity of chyme in the small intestine
– See notes under “Control of Gastric
Section – Intestinal phase”
• CCK – see above
Bile
• Secreted by liver and stored in gallbladder
• Lecithin and bile salts
– Emulsify fats by creating a hydrophilic “shell”
around tiny fats droplets
• Sodium Bicarbonate – increase pH of chyme in
small intestine
• Excretions:
– Cholesterol, products of detoxification, bilirubin
(product of hemolysis)
Control of Bile Secretion
• Controlled by CCK and secretin
• See Table 26-5, page 782
Intestinal Juice
• Mucus – provides lubrication
• Sodium bicarbonate – increases pH to
allow intestinal enzymes to function at
optimal level
• Water – carries mucus and NaHCO3
**Study These Tables**
Table 26-4: Digestive Secretions
Table 26-5: Actions of Digestive
Hormones
Both on page 782
Wednesday/Thursday 3/30-3/31
Warm Up:
1. Name the 5 components of saliva.
2. What is the proenzyme of pepsin? What is
needed to activate this proenzyme?
3. Explain the role of bicarbonate in the GI
tract.
4. The control of gastric secretion can be
broken up into 3 phases. Name these phase
and briefly describe each one.
5. Name and give the function of the 5
enzymes found in pancreatic juice.
Absorption
• Passage of substances (digested foods,
vitamins, salts, water) across the
mucosa into the blood
• Majority of absorption takes place in
small intestine where surface area is
increased
Mechanisms of Absorption
• Some substances (water) are absorbed via
diffusion
• Secondary Active Transport (ex: Sodium)
– Na+ is actively transported from the basal
(backside) of epithelial cells lining the lumen of the
small intestine into blood capillaries
– Creates a low intracellular sodium concentration
– Na+ ions diffuse passively from the lumen into
epithelial cells
– Fig 26-14, page 785
Mechanisms of Absorption
• Sodium cotransport
– Glucose is very large and hydrophilic
– Requires carrier to cross intestinal mucosa
– Carriers bind sodium and glucose together to
passively transport out of lumen
• Amino Acid absorption
– Transported by passive carriers on luminal and basal
surfaces of absorptive cells
– Brush border enzymes can also act as carriers
– Polypeptides can diffuse into absorptive cells,
hydrolyze into amino acids, diffuse into blood
Mechanisms of Absorption
• Fatty acid/monoglyceride absorption
– Bile salts and lecithin form micelles
– Fat digestion takes place within these tiny
spheres
– At the intestinal brush border, micelle
contents can diffuse into absorptive cells
– Inside triglycerides reform within
chylomicrons
– Water soluble chylomicrons allow fats to be
transported through lymph and into
bloodstream
Mechanisms of Absorption
• Absorption of vitamins
– Vitamins A, D, E, K are fat-soluble
• Depend on bile salts for absorption
– B Vitamins require carrier-mediated
transport
• Ex: Vit B12 and intrinsic factor
Summary of Absorption
• See table 26-6, page 787
Elimination
• Expulsion of digestive residuals from
the digestive tract in the form of feces
– Defecation
– Normally rectum is empty
– Massive peristalsis of feces into the
rectum stimulate receptors and relax the
external anal sphincter
– Voluntary control
Elimination
• Constipation
– Contents move through large intestine at a
slower rate
– Increased water absorption occurs
resulting in hardened feces
• Diarrhea
– Result of increased motility of the small
intestine
– Water absorption does not occur
GI Tract Disorders
• Common S/S
– Gastroenteritis
• Gastritis – stomach inflammation
• Enteritis – intestinal inflammation
– Anorexia: chronic loss of appetite
– Nausea: feeling of needing to vomit; may progress
to vomiting
– Emesis: vomiting
– Diarrhea: elimination of liquid feces; abdominal
cramps may also be present
– Constipation: decreased motility of colon; difficulty
in defecating
GI Tract Disorders
• Ulcers
– In stomach or duodenum
– Cause pain and may lead to perforation of
the wall of the GI tract
– Bleeding  anemia
– Causes:
• Hyperacidity
• H. phylori bacterium
GI Tract Disorders
• Stomach cancer
– Linked to excessive alcohol use, chewing
tobacco, eating heavily preserved foods
– Early signs:
• Heartburn, belching, nausea
– Later signs:
• Chronic indigestion, vomiting, anorexia, stomach
pain, blood in feces
GI Tract Disorders
• Diverticulosis – presence of abnormal
sac-like projections on the large
intestine (diverticula)
– When inflamed causes diverticulitis
– S/S: pain, tenderness, fever
GI Tract Disorders
• Colitis – inflammation of the large
intestine
– s/s: diarrhea, abdominal cramps,
constipation, bleeding, intestinal ulcers
– Crohn’s Disease: autoimmune colitis
– Treatment: surgical removal of affected
portions of the intestine
GI Tract Disorders
• Irritable bowel syndrome (IBS)
– “spastic colon”
– Noninflammatory condition usually caused
by stress
– Diarrhea or constipation
GI Tract Disorders
• Colorectal cancer
– Occurs after 50
– Associated with low-fiber, high-fat diet
– Early signs: change in bowel habits, fecal
blood, rectal bleeding, abdominal pain,
unexplained anemia, weight loss, fatigue
Liver Disorders
• Hepatitis – inflammation of the liver
– S/S: jaundice, liver enlargement, anorexia,
abdominal discomfort, gray-white feces,
dark urine
– Causes: alcohol or drug abuse; bacterial or
viral infection
• Cirrhosis – degenerative liver condition
– Tissue can no longer regenerate