Download The Digestive System

The Digestive
System
Chapter 24
Digestive Quiz
• 1 – 10 questions
• Comes straight from power point. Know the function of each
section/part of the digestive system and the fluids secreted in
each section.
• The function of those fluids secreted
– EX: Bile is secreted by the ________ and breaks down ________
which is located in the __________.
Overview
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Functions of the Digestive System
Anatomy of Digestive System
Phases of Digestive
Digestive Organs
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–
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–
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Mouth (Mechanical and Chemical Digestion)
Esophagus (Mechanical Digestion)
Stomach (Mechanical and Chemical Digestion)
Pancreas (Chemical Digestion)
Liver and Gallbladder (Chemical Digestion)
Small intestine (Mechanical and Chemical Digestion)
Large intestine (Mechanical and Chemical Digestion)
The Digestive System
• gastrointestinal system
• alimentary canal
– extends from mouth to anus
– mouth, pharynx, esophagus,
stomach, small intestine,
large intestine (colon)
• accessory organs
– teeth, tongue, salivary glands,
liver, gallbladder, pancreas
The Digestive System
• Functions:
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–
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–
–
–
ingestion
secretion
digestion
mixing and propulsion (motility)
absorption
defecation
The Digestive System
• Mechanical Digestion
– mastication
– swallowing
– mixing :
• increase the contact of food and digestive
chemicals
– peristalsis:
• intermittent contraction of muscles within
the GI tract facilitate the movement of food
The Digestive System
• Chemical digestion – water is used to break chemical bonds
(hydrolysis)
– Fats are broken down into fatty acids and glycerol
– Carbohydrates are broken down from complex polysaccharides into
monosaccharides
– proteins are broken down into polypeptides and amino acids
The Digestive System
• Responsible for facilitating the body’s metabolic processes
– Catabolism: larger molecules are broken into smaller molecules
(digestion)
– Anabolism: smaller molecules are used as building blocks for larger
molecules (liver)
Digestive System Anatomy
• the walls of the GI tract have the same 4-layered arrangement
of tissues
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–
–
–
mucosa
submucosa
muscularis
serosa
“Brain of the Gut”
ENTERIC NERVOUS SYSTEM
To ANS and
CNS neurons
Myenteric plexus: regulates motility
Extrinsic influence
Autonomic NS:
- Parasympathetic
stimulates ENS
- Sympathetic inhibits ENS
Interneuron
GI reflex pathways
Submucosal plexus:
controls secretions
Motor neuron
Motor neuron
Regulate secretions
and motility based on
contents of the lumen
Sensory neuron
chemoreceptors
stretch receptors
Longitudinal and
circular smooth muscle
layers of the muscularis
Mucosal epithelium
Mechanical Digestion
Chemical Digestion
Digestion
• 3 phases
– cephalic phase
– gastric phase
– intestinal phase
Digestion – Cephalic Phase
• smell, sight, thought, initial taste of food
• activates the cerebral
cortex, hypothalamus
and brain stem to
prepare for digestion
– facial and glossopharyngeal nerves stimulation saliva
secretion
– vagus nerves stimulate gastric juice secretion
Digestion – Gastric Phase
• this phase begins once food reaches the stomach
– secretion of gastric juice
– increased gastric motility
• Neural regulation
– stretch and chemoreceptors
• Hormonal regulation
– gastrin – secreted in from cells in the stomach
Digestion – Intestinal Phase
• Begins once food enters the
small intestine and
promotes the continued
digestion of foods
• neural regulation
– distension of the intestine
• hormonal regulation
– stimulates further breakdown of proteins and fatty acids
The Mouth
• Oral/buccal cavity
• mechanical digestion:
– mastication breaks the food down
• mixes food with saliva
• formation of a soft bolus
The Mouth
• chemical digestion:
– saliva starts the digestive process
– 99.5% water, dissolved ions, bacteriolytic lysosomes
• Digestive enzymes:
– salivary amylase: begins the breakdown of starch
• polysaccharides broken down into mono, di, and tri-saccharides
• salivary amylase is inactivated by stomach acids
– lingual lipase: begins the breakdown of fats
– not active until the stomach reaches a specific acidity level
– triglycerides broken down into diglycerides)
The Mouth
• Salivary regulation is under the control
of the autonomic nervous system
– parasympathetic stimulation
• promotes secretion of saliva
– sympathetic stimulation
• decreases salivation
• touch, smell, taste, and psychological
factors increase salivation
The Esophagus
• upper esophageal sphincter
– skeletal muscle
– regulates movement from mouth to
esophagus
– voluntary
• lower esophageal sphincter
– smooth muscle
– regulates movement from esophagus
into stomach
– involuntary
Deglutition (swallowing)
Nasopharynx
Hard palate
Soft palate
Bolus
Uvula
Oropharynx
Tongue
Epiglottis
Laryngopharynx
Larynx
Esophagus
(a) Position of structures before swallowing
(b) During pharyngeal stage of swallowing
Deglutition
• Esophageal phase
• peristalsis: coordinated
contractions of the circular
and longitudinal muscularis
layer pushes the bolus
onward
The Esophagus
• GERD: gastroesophageal reflux disease
– failure of the LES to close adequately after swallowing
– food can move from the stomach back into the lower portion of the
esophagus
• Heart Burn
– HCl from the stomach can irritate the esophageal wall
• Alcohol and smoking relax the LES
• foods that strongly stimulate HCl secretion
– coffee, chocolate, tomatoes, fatty foods, OJ, onions
The Stomach
Note the oblique layer of smooth muscle
in the gastric muscularis (limited to the
body of the stomach)
Function
Holds and mixes food until it can
pass into the small intestine
Mechanical Digestion in the Stomach
• Gentle, rippling, peristaltic movements called mixing waves
pass over the stomach every 15 to 25 seconds once food
enters the stomach.
– These waves macerate food, mix it with secretions of the gastric
glands, and reduce it to a soupy liquid called chyme.
• the mixing waves get stronger further down the stomach
Mechanical Digestion in the Stomach
• as a mixing wave reaches the pylorus ~3mL of
chyme enters the duodenum = gastric emptying
• the chyme enters the
duodenum to begin the
intestinal phase of digestion
The Stomach
• The strongly acidic nature of gastric juice kills microbes,
partially denatures proteins in food, and converts pepsinogen
into pepsin
– Pepsin is the only proteolytic enzyme in the stomach
– Gastric lipase splits triglycerides
– What other lipase is active in the stomach?
– Intrinsic factor (IF) is needed for absorption of vitamin B12 (essetial
for RBC production, brain and nervous system health).
The Stomach
• Disruption of the balance b/w HCl
production, pepsin secretion, and mucosal
defenses may erode the stomach's
epithelial lining
– excessive alcohol consumption,
or use of NSAIDS like aspirin or
ibuprofen may exacerbate this
condition
Peptic Ulcer Disease
• ulcers that develop in areas of the GI tract exposed to gastric
juice
• CAUSES:
– helicobacter pylori (most
common)
• produce an enzyme that shields
the bacterium from gastric
juice, but damages the
protective mucous layer
• NSAIDS (aspirin, ibuprofen)
– hypersecretion of HCl
• TREATMENT:
• Tums, Maalox
• Proton pump inhibitors (if HCl hypersecretion is the issue)
Vomiting: emesis
• STIMULUS
– irritation and distension of the stomach (strongest stimulus)
– unpleasant sights, general anesthesia, dizziness, drugs
• Control
– sensory nerve afferent impulses to the vomiting center in the medulla
oblongata
– motor efferents activate contraction of the GI organs, diaphragm and
abdominal muscles
• The stomach is squeezed between the diaphragm and abdominal
muscles
• Prolonged vomiting  alkalosis due to the loss of gastric juice
The Stomach
• epithelial cells of the stomach are impermeable to most
materials, and very little absorption takes place
– mucous cells absorb water, ions, short chain FA’s, some drugs
(aspirin), and alcohol
• Within 2 to 4 hours after eating a meal, the stomach has
emptied its contents into the duodenum
– Foods rich in carbohydrate spend the least time
– High-protein foods remain somewhat longer
– Emptying is slowest after a fat-laden meal containing large amounts
of triglycerides
Gastric Bypass
Gastric Sleeve
Gastric Band
Anatomy of the lower GI tract
The Pancreas
• The pancreas secretes enzymes that digest food in the small
intestine, and bicarbonate which buffers the acidic chyme
leaving the stomach
• Pancreatic juice secreted into the pancreatic duct which takes
it to the small intestine
Duodenum
SUPERIOR
Common bile
duct
Body of
pancreas
Major
duodenal papilla
Pancreatic duct
Head of pancreas
MEDIAL
Tail of pancreas
(e) Anterior view
LATERAL
Pancreatitis
• inflammation of the pancreas
• CAUSES: alcohol abuse, chronic gallstones, cystic fibrosis,
hypercalcemia, hyperlipidemia
• excessive trypsin secretion results in the digestion of the
pancreatic cells
Pancreatic Cancer
•
•
•
•
•
usually affects people over the age of 50, more common in males
Few symptoms until the cancer is in an advanced stage
High mortality rate
4th most common cause of death from cancer
Research suggests it is
linked to high fatty
food diet, high alcohol
consumption, genetics,
smoking, and chronic
pancreatitis
Liver and Gallbladder
• The liver is the body’s
largest gland and second
largest organ
• The liver is made up of
lobules that house
– hepatocytes
– bile canaliculi
– hepatic sinusoids
The Liver and Gallbladder
• Hepatocytes are the major functional cells of the liver
• hepatocytes secrete bile into the bile canaliculi
• bile = an excretory product that helps emulsify fats for the watery
environment of small intestine digestive juices.
Hepatic
sinusoids
Bile canaliculi
The Liver and Gallbladder
• Bile: an alkaline solution
– consists of water, bile salts, cholesterol, and bile pigments
• Bile salts are used in the small intestine for the
emulsification and absorption of lipids
• Without bile salts, most of the lipids in food would be passed out in feces,
undigested.
• The dark pigment in bile is called bilirubin and comes from
the catabolism of old red blood cells.
The Liver and Gallbladder
• Surgical removal of the gall bladder (cholecystectomy) results
in severe indigestion if the person eats a large meal high in
fat content.
Hepatocyte Functions
Non-metabolic Functions
• Phagocytosis of old or worn-out cells (in sinusoids)and secretes
them into bile (bilirubin = heme in RBC’s)
• Detoxification of certain drugs and alcohol
• Modifies vitamin D to its active form
• Storage for glycogen, vitamins and minerals
Gallstones
• insufficient bile salts or excessive cholesterol content of bile
may cause cholesterol to crystallize and form gallstones
• may obstruct the bile flow into the duodenum
• TREATMENT: dissolving drugs, shock-wave therapy, surgery
– cholecystectomy may be an option for recurring gallstones
Jaundice
• CAUSE: buildup of
bilirubin
• SIGNS: yellowish
coloration of the eyes,
skin, and mucous
membranes
• CATEGORIES
– prehepatic jaundice: excessive bilirubin production
– hepatic jaundice: liver disease
– extrahepatic jaundice: blockage of bile drainage
The Small Intestine
• The small intestine is
divided into 3 regions:
– The duodenum (10 in)
– The jejunum (8 ft)
– The ileum (12 ft)
• In the small intestine,
digestion continues, while
the process of absorption
begins
Mechanical Digestion Small Intestine
• Regulated by the myenteric plexus
• Segmentations: localized mixing contractions where the
intestine is distended
– used to mix chyme and bring it into contact with the mucosa for
absorption
The Small Intestine
• circular folds in the mucosa and submucosa encourage the
turbulent flow of chyme
Mechanical Digestion in the Small
Intestine
• Migrating motility complexes – peristalsis
– begins once absorption is complete and the intestine is less
distended
– begins in lower stomach and pushes chyme forward
– takes 90-120 min to move from stomach to ileum, then another
begins
– food stays in SI for 3-5 hrs
Histology of the small
intestine
Circular folds
(plicae circulares)
• circular folds: folds in the
mucosa and submucosa layer
Circular folds
• Villi: multicellular finger-like
projections within the mucosa
• microvilli: fingerlike projections
on the apical surface of the
absorptive cells (brush border)
• All increase the surface area
for absorption
Villi
Submucosa
muscularis
Serosa
Histology of the Small Intestine
• the mucosa contains intestinal glands that secrete intestinal
juice (water, mucous, and HCO3-)
• intestinal juice along with
pancreatic juice facilitate the
the chemical digestive
process in the lumen of the
small intestine
• brush-border enzymes
complete chemical digestion
Chemical Digestion of Small Intestine Brush Border Enzymes
• Carbohydrate digestion - enzymes located on the brush
border of the absorptive cells
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–
–
–
α dextrinase: clips glucose molecules off α-dextrins
maltase: breaks maltose into 2 glucose molecules
sucrase: breaks sucrose into glucose and fructose
lactase: breaks lactose into glucose and galactose
• The small intestine only absorbs monosaccharides
Lactose Intolerance
• absorptive cells fail to produce enough lactase
• Undigested lactose in
chyme causes excess of
fluid retention in feces
• Bacteria ferment the
undigested lactase in the
colon producing gas
• Symptoms: diarrhea, gas,
bloating, abdominal
cramps
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Chemical Digestion of Small Intestine
• Protein digestion
– Stomach – pepsin (from pepsinogen)
– Pancreatic juice peptidases
• trypsin, chymotripsin, carboxypeptidase, elastase
– small intestine brush border enzymes
• aminopeptidase – cleaves amino acids off a peptide
• dipeptidase – splits dipeptides into single amino acids
Chemical Digestion of Small Intestine
• Lipid digestion
– In the stomach: lingual lipase,
gastric lipase
– In the small intestine: pancreatic
lipase
• TGs broken into FA’s and
monoglycerides
• Emulsification: bile salts break
the larger lipid globules into
small lipid globules
– Pancreatic lipase acts on the small globules
Intestinal Absorption
• The passage of digested nutrients into the blood or lymph
• 90% of absorption occurs in the small intestine
• Methods of absorption
– diffusion, facilitated diffusion, osmosis, active transport
Intestinal Absorption
INGESTED AND SECRETED
ABSORBED
Saliva (1 liter)
• Water
– ALL water
absorption occurs
via osmosis from
the intestinal
lumen to the
capillaries
Ingestion of liquids
(2.3 liters)
Gastric juice
(2 liters)
Bile (1 liter)
Pancreatic juice
(2 liters)
Intestinal juice
(1 liter)
Small intestine
(8.3 liters)
Total ingested
and secreted
= 9.3 liters
Large intestine
(0.9 liters)
Excreted in
feces (0.1 liter)
Total absorbed
= 9.2 liters
Fluid balance in GI tract
Alcohol absorption
• lipid-soluble  simple diffusion
• absorption is most rapid in the
small intestine
• the longer alcohol remains
in the stomach the slower
blood alcohol levels will rise
• alcohol dehydrogenase in the gastric mucosa detoxifies
alcohol
• females and asian males have low rates of activity of
alcohol dehydrogenase
The Large Intestine
• The large intestine is about 5 feet in length
• Starting at the ileocecal valve, the large intestine has 4 parts:
– The cecum
– The colon
•
•
•
•
ascending
transverse
descending
sigmoid
– The rectum
– The anal canal
Mechanical Digestion in the large
intestine
• Immediately after a meal the gastroileal
reflex intensifies peristalsis in the ileum
and forces any chyme there into the
cecum
• Haustral churning – the haustra of the
colon distend as chyme enters
– a specific degree of distension triggers
contraction and the chyme is squeezed into
the next haustra
• Mass peristalsis – a strong peristaltic
wave drives chyme from the transverse
colon into the rectum
Gastric distension initiates mass
peristalsis by the ANS
The Large
Intestine
• ~9 liters of fluid enter the
small intestine daily
– The small intestine absorbs
about 8 liters
– the remainder passes into
the large intestine, where
most of the rest of it is also
absorbed
• Only 100 mL/d of water is
excreted in the feces.
Chemical Digestion in the large
intestine
• No digestive enzymes are secreted
• However, bacteria (flora) in the intestine ferment any
remaining carbohydrates
– byproducts = hydrogen, carbon dioxide, methane
• Contribute to flatulence
The Large Intestine
• Though the human body consists of about 100 trillion
cells, we carry about ten times as many microorganisms
in the intestines. Bacteria make up most of the flora in
the colon and about 60% of the dry mass of feces.
• As these bacteria digest/ferment left-over food, they
secrete beneficial chemicals such as vitamin K, biotin (a
B vitamin), and some amino acids (they are our main
source of some of these nutrients.)