The Digestive System
rev 12-12
Digestion is a process of breaking down large
molecules contained in food into basic building
blocks which our body can then use to build
nutrients.
• The digestive system includes:
– mouth
--small intestine
– pharynx
--large intestine
– esophagus
--rectum
– stomach
--anus
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• These organs are collectively called the
Gastrointestinal System or Tract. It is actually
a hollow tube extending from the mouth to the
anus.
– The space within the tube is called the lumen.
• The system also includes 4 accessory organs:
– Salivary glands
– Liver
--gallbladder
--pancreas
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Gastrointestinal (GI) Tract Layers
Structure of the system:
• Mucosa: innermost layer is composed of simple
columnar epithelial tissue lined with mucus secreting
cells. Dorsal to the columnar cells is connective
tissue which has capillaries to nourish the epithelial
cells and absorb nutrients
– Lymphatic capillaries in the digestive tract are called
lacteals. They play a major role in absorbing digested
fat from the intestine
• Submucosa- is made of connective tissue which
contains blood, lymphatic vessels, and nerves.
– This layer also has a lot of elastic fibers. This enable
the stomach to stretch with meals and regain its usual
shape after meals.
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Gastrointestinal (GI) Tract Layers
• Muscularis: the thickest part of the wall; is
responsible for motility;
– composed of 2 layers of smooth muscle.
Each layer runs in a different direction
• this layer is responsible for peristalsis (the
wavelike propulsion of material through the
digestive tract) and segmentation (the
back and forth mixing of food which occurs
primarily in the small intestine).
• Serosa: the outermost layer, is made up of
connective tissue covered with an epithelial
membrane and attaches the digestive system to
the walls of the body cavities
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• Sphincter: thick ring of circular smooth muscle.
When this contracts, it closes off the
passageway between the digestive organs.
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Digestive System Processes
Five processes accomplish digestion
• Mechanical Digestion: by chewing we break down
large food particles into smaller ones and we “mix”
the food. When we swallow, we move the food.
• Secretion: fluid, digestive enzymes and hormones,
bile, acid, alkali, mucus are secreted into the GIT.
• Chemical Digestion: breaking down food to
smallest absorbable units.
• Absorption: nutrient molecules pass through
mucosa, into blood or lymph vessels
• Elimination: undigested material eliminated
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The smooth muscle in the GIT produces 2 kinds of
movement: Peristalsis and Segmentation
• Peristalsis propels food forward.
– The bolus stretches a portion of the tract and
causes the muscle in front of it to relax and
the muscle behind it to contract.
– The contractions push the food forward.
– Occurs in all parts of the system but
especially in the esophagus
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• Segmentation mixes food.
– The sections of smooth muscle relax and
contract in a random manner causing a back
and forth mixing and presses food against the
mucosa so the body can absorb the nutrients.
– Occurs primarily in the small intestine
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Mouth: begins digestion with chewing
• Teeth
– Break down food into small pieces
– Mouth contain bacteria that live off the food
that remains on our teeth
–Bacteria release acid that can dissolve
tooth enamel and create cavities or
dental caries.
• Tongue: skeletal muscle, moves food in mouth,
contributes to sense of taste and necessary for
speech
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• Saliva moistens food
– Source: parotid, submandibular, sublingual
salivary glands
• Connect to the mouth via ducts
Composition:
– Mucin holds food particles together to make
swallowing easier
– Amylase begins digesting carbohydrates
– Bicarbonate maintains pH between 6.5-7.5; may
help protect teeth against acid producing bacteria
– Lysozyme inhibits bacterial growth
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Swallowing: Delivers Food to Stomach
Swallowing is coordinated with a temporary
breathing stoppage.
• Voluntary movement of the tongue and jaws
push bolus of food into pharynx
• Involuntary phase: swallowing reflex: receptors
in pharynx stimulated by presence of food
– Soft palate rises
– Larynx rises slightly
– Epiglottis closes opening to trachea
– Tongue pushes food further
– Food enters esophagus
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Pharynx and Esophagus
• Pharynx
– Common passageway for air and food
– Participates in swallowing
• Esophagus: muscular tube which connects
pharynx to stomach
– Structure: mix of skeletal and smooth muscle
– Mucus-secreting cells: assist passage of food
– Food motility: gravity and peristalsis
– Lower end of the esophagus: gastroesophageal
or cardiac sphincter opens briefly as food
arrives and lets food into the stomach
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Stomach Function
Is a muscular, stretchable sac
• Functions:
– Stores food until it can be digested
– Digestion: chemical digestion of proteins due
to acids and enzymes; most bacteria killed by
acids
– Muscular contractions mix secretions with
food and mechanically break apart food, push
food into small intestine
– Regulation of delivery of food into small
intestine
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• Gastric juices
– The mucosa contains gastric pits (small
openings) that lead to gastric glands.
• Glandular cells secrete gastric juices:
– Hydrochloric acid: produces a pH of about 2,
breaks down large bits of food
– Intrinsic factor: used to help absorb vitamin B12
– Mucus: protects stomach lining from acid
– Pepsinogen: when combined with acid from
stomach produces pepsin (a protein digestion
enzyme) which begins protein breakdown
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• Chyme is the name of the watery mixture of
partially digested food and gastric juices
• Pyloric sphincter: regulates rate of movement of
chyme into the small intestine
• The stomach generally does not absorb nutrients
because the inner lining is coated with mucus
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Small Intestine
• Functions:
– Digestion: neutralize acid from stomach
(occurs when sodium bicarbonate is
produced by pancreas), add digestive
enzymes and bile, digest proteins,
carbohydrates, and lipids to absorbable
materials, pancreas also adds enzymes
(lipase, amylase, and protease)
– Absorption: 95% of food absorbed here
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Small Intestine
• Structure
– 3 different sections: duodenum, jejunum, ileum
• Most chemical digestion occurs in the duodenum
• Most absorption occurs in the jejunum and ileum
– Mucosa has folds covered with projections
called villi and each villi has smaller projections
called microvilli. At the center of each villus are
capillaries and a lymph vessel called a lacteal.
• Capillaries absorb water soluble nutrients
• Lacteals absorb fat soluble nutrients
– Named “small” intestine because its diameter is
smaller than the “large” intestine (1inch vs 2
inches)
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Accessory Organs: Aid Digestion and Absorption
• Pancreas: exocrine functions
– Secretes, into the duodenum, sodium
bicarbonate and digestive enzymes for
proteins, carbohydrates and fats
• Liver
– Produces bile to break down lipids (fats). Bile
emulsifies lipids in the small intestine—bile
breaks fats into small droplets-there is no
chemical change to the bile or to the lipids
– Hepatic portal system: drains blood from
digestive tract directly to liver before going to
general circulatory system
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• Liver functions:
• Storage of fat soluble vitamins and glucose
 Converts glucose to glycogen
• synthesis and chemical processing of nutrients
 Excess amino acids are removed and converted into
urea
• Inactivates (detoxifies) many chemicals including
alcohol, hormones, drugs and poisons
 Processes ammonia (waste product of metabolism)
into urea
• Destroys worn out RBC
• Is gatekeeper between intestines and general
circulationscreens blood in hepatic portal system so
blood leaving will be close to normal composition
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Gallbladder: stores and concentrates bile;
after meals, secretes bile into the small
intestine via the bile duct
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Large Intestine: Structure and Function
Most nutrients have been absorbed by the time the
digestive contents reach the large intestine; contents are
more solid than liquid
• Functions: absorbs nutrients and water and stores waste
until it can be eliminated
– Secretes mucus to help wastes move along
• Feces contain about 30% bacteria; many of these can eat
the leftover material in the colon; some produce vitamin K
(important for blood clotting), others produce gas
• Structure:
– Cecum (first area to receive chyme), appendix
– Colon: ascending, transverse, descending, sigmoid
– Rectum, anus (internal and external sphincters)
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Endocrine and Nervous Systems Regulation of
Digestion
• Regulation dependent on volume and content of
food
– Nervous system: stretch receptors in stomach
– Hormones:
• Gastrin: stimulates stomach to release gastric juice
• Secretin: released by small intestine; stimulates
pancreas to secrete a watery bicarbonate-rich
pancreatic juice to neutralize chyme and liver
cells to release bile
• Cholecystokinin (CCK): intestinal hormone
released when fatty chyme enters the
duodenum; causes gallbladder to contract
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Total time for food digestion: approximately 24-72
hours
Mouth and esophagus to stomach: 0-1 minute
Stomach:
30 min to 2 hours
Small intestine:
2-6 hours
Large intestine:
ascending colon: ~ 5 hours
transverse colon: ~6 hours
descending colon: ~ 9 hours
sigmoid colon: ~ 12-24 hours
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Disorders of the Digestive System
• Diverticulosis: weakness of the intestinal wall
so that there are small sacs produced when the
mucosa protrudes through the other layers of
the intestinal wall; diverticula can become
infected.
• Colon polyps: non-cancerous growth that
projects from a mucus membrane
• Esophageal varices: enlargement of veins at
lower end of esophagus;
– caused by portal hypertension, which can be
caused by cirrhosis and chronic hepatitis
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• Cirrhosis: destruction of liver cells resulting in
impaired liver function
• Hepatitis: inflammation of the liver; generally
viral: Hepatitis A, B, and C
– Hepatitis A is transmitted by contaminated
food or water; causes a brief illness before
complete recovery
– Hepatitis B travels in blood or body fluids;
usually passed via contaminated needles,
blood transfusions or sexual contact.
• If not treatedliver failure
• Vaccine available
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• Hepatitis C transmitted in infected blood also;
blood banks now test blood so the risk of getting
this type of hepatitis is decreased.
• Can remain dormant but still damages liver
• Gallstones: excessive cholesterol in the bile
may precipitate out of solution forming
gallstones. If they obstruct bile flow, cause
tremendous pain
– Treatment: drugs to try to dissolve them,
treatments to try to break them, surgery to
remove gallbladder
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Disorders of the Digestive System
Inflammatory Bowel Disease—IBD
• Crohn’s Disease
– Can affect any area of the GI tract, but most
commonly affects the ileum
– All layers of intestine may be involved and healthy
bowel may be found between sections of diseased
bowel
– Cause: unknown, possibly heredity, autoimmune,
environment
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– Symptoms similar to other intestinal
disorders  irritable bowel syndrome and
ulcerative colitis (inflammation and ulcers only
in the submucosa layer of the intestine)
• Abdominal pain, diarrhea, rectal bleeding,
intestinal ulcers, weight loss, arthritis, skin
problems, fever, eye inflammation, fatigue
– Treatment may include medication, nutritional
supplements, surgery; There is no cure
– Drugs can include anti-inflammatory medications,
steroids, immune system suppressors, antibiotics (if
secondary infection present), anti-diarrheal and fluid
replacement, pain relievers
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– Surgery-bowel diversion surgery
• Diet: no special diet has proven effective for
preventing or treating
• Stress: no evidence showing that stress causes
this;many people say stress causes flare-ups
Eating disorders:
–Anorexia nervosa: a person diets excessively or
stops eating completely
• Can cause death due to severely decreased
weight and lack of nutrients for the body to use
– Bulimia: binge and purge condition in which person
eats and then forces self to vomit
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