Transcript Digestion Physiology Chapter 26

Digestion Physiology
Chapter 26
Test (Chapters 25 & 26) –
Tuesday, 3/30 (Tentative Date )
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
• Includes all motility of the digestive
tract that cause the following changes:
– Change in physical state of the food from
large particles to smaller particles
– Churning of the GI lumen to mix particles
with digestive juices
– Propelling food forward ending with
elimination
Mastication
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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
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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 vs. 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
• Vagus nerve (CN X) causes inhibition (stops)
gastric peristalsis
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
Properties of Digestive Enzymes
• Extracellular enzymes
• Classified as hydrolases
• Function optimally at a specific pH
– Ex: amylase vs pepsin
• Continually destroyed or eliminated
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 product (usually glucose) is absorbed in
the small intestine
Protein Digestion
• Proteins are large molecules composed
of twisted chains of amino acids
• Proteases catalyze the hydrolysis of
proteins into smaller compounds
– Proteins  proteoses or peptides  amino
acids
• The peptides bonds holding amino acids
together can vary = increased need for
varying proteases
Protein Digestion - Proteases
• Pepsin – gastric juice (stomach)
• Trypsin and chymotrypsin – pancreatic
juice (pancreas)
• Peptidases – intestinal brush border
(small intestine)
Proteoses
Fat Digestion
• Fats are insoluble in water so they 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)
• Micelles are further broken down by
lipase
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
– Examples: 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 (breakdown)
complex carbohydrates
• 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 afferent
impulses to centers in the brainstem 
efferent impulses to salivary glands
– Chemical and mechanical stimuli come from
the presence of food in the mouth
Gastric Juice
• Gastric juice = water, mucous, enzymes &
acid
• Secreted by gastric glands in the
stomach
• Chief cells – secrete enzymes of
digestive juices
– Pepsin
• Parietal cells
– Secrete HCl
• Decreases stomach pH
Control of Gastric Secretion
•
Gastric secretion is controlled by 3
phases:
1. Cephalic phase (“psychic phase”)
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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 continues to stimulate the
secretion of gastric juices (pepsin and HCl)
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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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 portion of the pancreas
• Mostly water
• Enzymes:
– Trypsin and chymotrypsin (proteases)
– Lipases
– 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 & CCK
– 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”
Bile
• Secreted by liver and stored in gallbladder
• Bile contains:
– 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
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
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