The Digestive System
Objectives
• Name and locate the digestive organs and the
accessory organs of digestion.
• List the general function of the digestive system
and the function of each organ.
• Define mechanical and chemical digestion and
tell where each takes place.
• Define terms related to the digestive system.
• Name the origins and functions of the various
digestive enzymes.
Objectives
• Correctly label a diagram of the digestive
system
• State the purposes of HCl
• Name and locate the sphincters of the
digestive tract and describe the function of
each.
Digestion
• The process by which food is broken down
mechanically and chemically and
converted into absorbable forms.
• The main goal of digestion is to break
down macronutrients into molecules that
are readily absorbed.
• This is accomplished through mechanical
and chemical means
Mechanical Digestion
• GI motility: Beginning in the mouth,
muscles and nerves in the tract coordinate
their actions to provide motility, an
automatic response to the presence of
food.
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Mechanical Digestion, cont’d
• Muscles
– Muscle tone/tonic contraction: Ensures
continuous passage of the food mass and
valve control along the way
– Periodic muscle contraction and relaxation:
Rhythmic waves that mix the food mass and
move it forward
7
Mechanical Digestion, cont’d
• Nerves
– Specific nerves regulate muscle action along
the GI tract
– The intramural nerve plexus is the network of
nerves in the GI wall extending from the
esophagus to the anus
8
Chemical Digestion
• Digestive enzymes: Break down nutrients
• Hydrochloric acid and buffer ions: Produce the correct
pH necessary for enzyme activity
• Mucus: Lubricates and protects the GI tract tissues and
helps mix the food mass
• Water and electrolytes: Carry and circulate the products
of digestion through the tract and into the tissues
• Bile: Divides fat into smaller pieces to assist fat enzymes
9
End Products of Digestion
• Carbohydrates
– Starches, disaccharides digested to
monosaccharides (e.g. glucose, fructose,
galactose)
• Proteins digested to amino acids
• Fats digested to fatty acids and glycerol
• Vitamins, minerals and water
Three essential processes
• Secretion: Delivery of enzymes, mucus,
ions and the like into the lumen, and
hormones into blood.
• Absorption: Transport of water, ions and
nutrients from the lumen, across the
epithelium and into blood.
• Motility: Contractions of smooth muscle in
the wall of the tube that crush, mix and
propel its contents.
The Alimentary Canal
•
•
•
•
Long tube
Begins at the mouth
Ends at the anus
Food moved via
peristalsis
• Mechanical digestion
• Chemical digestion
•
•
•
•
•
•
•
Mouth: Foodstuffs are broken down mechanically by chewing and
saliva is added as a lubricant. In some species, saliva contains
amylase, an enzyme that digests starch.
Esophagus: A simple conduit between the mouth and stomach clearly important but only marginally interesting compared to other
regions of the tube.
Stomach: Where the real action begins - enzymatic digestion of
proteins initiated and foodstuffs reduced to liquid form.
Liver: The center of metabolic activity in the body - its major role in
the digestive process is to provide bile salts to the small intestine,
which are critical for digestion and absorption of fats.
Pancreas: Important roles as both an endocrine and exocrine organ
- provides a potent mixture of digestive enzymes to the small
intestine which are critical for digestion of fats, carbohydrates and
protein.
Small Intestine: this is where the final stages of chemical
enzymatic digestion occur and where almost all nutrients are
absorbed.
Large Intestine: water is absorbed, bacterial fermentation takes
place and feces are formed
The Gastrointestinal System
14
Digestion in the Mouth and
Esophagus
• Mechanical digestion
–
–
–
–
Mastication breaks down food.
Food is swallowed and passes down esophagus.
Muscles at tongue base facilitate process.
Gastroesophageal sphincter at stomach entrance
relaxes, allowing food to enter, then constricts to
retain food.
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Digestion in the Mouth and
Esophagus, cont’d
• Chemical digestion
– Salivary glands secrete material containing salivary
amylase or ptyalin.
– Ebner’s glands at the back of the tongue secrete a
lingual lipase.
– Salivary glands also secrete a mucous material to
lubricate and bind food particles, facilitating the
swallowing of the food bolus.
– Secretions from the mucous glands in the esophagus
help move food toward the stomach.
16
The Mouth
Medial Section of a Canine Tooth
Salivary Glands
Structural Layers of the Alimentary
Canal
• Four basic layers
–
–
–
–
Mucosa
Submucosa
Muscle
Serosa
• Exact structure may
vary
Stomach
• Five major functions
– Temporary food storage
– Control the rate at which food enters the
duodenum
– Acid secretion and antibacterial action
– Fluidisation of stomach contents
– Preliminary digestion with pepsin, lipases etc.
Stomach: Four Regions with
Different Functions
•
•
•
•
Fundus
Body
Pylorus
Cardiac
The Stomach
The Stomach
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Digestion in the Stomach
• Mechanical digestion
– Under sphincter control, the food enters the upper portion
of the stomach as individual bolus lumps.
– Stomach muscles knead, store, mix, and propel the food
mass forward.
– By the time the food mass reaches the lower portion of the
stomach, it is a semiliquid acid/food mix called chyme.
– Chyme is released slowly into the first section of the small
intestine (duodenum) by the pyloric valve.
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Digestion in the Stomach, cont’d
• Chemical digestion: three types of gastric
secretions
– Hydrochloric acid: Parietal cells in the stomach lining
secrete acid to promote gastric enzyme activity.
– Mucus: Secretions protect the stomach lining from the
erosive effect of the acid and also bind and mix the food
mass and help move it along.
– Enzymes: Pepsinogen is secreted by stomach cells and
activated by acid to become pepsin, a protein-splitting
enzyme.
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The Stomach: Secretions
• Mucous cells secrete mucous
• Chief cells secrete pepsinogen
• Parietal cells produce HCl and secrete
intrinsic factor
• HCl converts pepsinogen to pepsin
• Acid environment kills microorganisms
• G cells secrete gastrin
Proton Pumps
• Parietal cells
• Hydrogen ion—potassium ion exchange
Small Intestine
•
•
•
•
•
•
1 inch diameter
20 feet long
Duodenum
Jejunum
Ileum
Peyer’s patches
Digestion in the Small Intestine
• Mechanical digestion
– Peristaltic waves slowly push food mass forward.
– Pendular movements sweep back and forth.
– Segmentation rings chop food mass into successive
soft lumps and mix them with secretions.
– Longitudinal rotation rolls food in a spiral motion,
exposing new surfaces for absorption.
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Digestion in Small Intestine,
cont’d
• Pancreatic enzymes
– Carbohydrate: Pancreatic amylase converts starch to
maltose and sucrose.
– Protein: Trypsin and chymotrypsin split large protein
molecules into small peptide fragments and
eventually into single amino acids.
– Fat: Pancreatic lipase converts fat to glycerides and
fatty acids.
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Absorption in the Small Intestine
• Three absorbing structures
– Mucosal folds: Surface of small intestine piles
into folds
– Villi: Small, finger-like projections cover the
mucosal folds, increasing the area of exposed
intestinal surface
– Microvilli: Smaller projections cover each villi
(look like bristles on a brush)
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Intestinal Wall
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Absorption Processes
• Simple diffusion: The force by which particles move
outward in all directions—from areas of greater to lesser
concentration.
• Facilitated diffusion: Similar to simple diffusion but uses
a protein channel to carry larger items.
• Active transport: The force by which particles move from
areas of greater to lesser concentration using a carrier to
“ferry” particles.
• Pinocytosis: Penetration of larger materials by attaching
to the cell membrane and being engulfed by the cell.
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Large Intestine
•
•
•
•
•
AKA: the colon
2.5 inches in diameter
5 feet long
Joined to the ileum at the ileocecal valve
Functions: absorption of water, minerals,
vitamins and elimination of waste
• Appendix
Absorption in Large Intestine
• Water is taken up by the large intestine
– Most water in chyme is absorbed in the first
half of the colon
– Only a small amount remains to form and
eliminate feces
• Dietary fiber is not digested
– Contributes bulk to food mass
– Helps form feces
42
Absorption and Transport
• Carbohydrates: Reduced to simple sugars
(glucose, fructose, galactose)
• Fats: Changed into fatty acids and glycerides
• Proteins: Changed into single amino acids
• Vitamins and minerals: Liberated from food
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Transport
• Nutrients must be transported to cells
• Vascular (blood circulatory) system
– Veins and arteries
– Transports waste, such as carbon dioxide and
nitrogen, to lungs and kidneys for removal
• Lymphatic system
– Route for fatty materials, which are not water soluble
– Fat molecules pass into lymph vessels in villi
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Accessory Organs of Digestion
• Liver
• Gall bladder
• Pancreas
Liver
Liver
Liver Functions
•
•
•
•
•
•
•
•
•
Carbohydrate metabolism
Amino acid metabolism
Lipid metabolism
Synthesis of plasma proteins
Formation of biliruben
Phagocytosis
Storage
Detoxification
Formation of bile
The Biliary System
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Gall Bladder
• Sac 3 to 4 inches long
• Undersurface of the
right liver lobe
• Concentrates bile by
absorbing water
• Not a gland, just a
storage sac
Pancreas
• Upper left abdominal quadrant
• Endocrine and exocrine functions
• Exocrine glands: acini
– Lipase
– Amylase
– Trypsinogen
Pancreatic Juice
•
•
•
•
•
•
•
pancreatic amylase: This enzyme digests carbohydrates. pancreatic lipase: This enzyme digests a variety of lipids. It works especially well on
triglycerides, which are the fats and lards that we normally think about in our diets.
trypsinogen/trypsin: Trypsinogen is what is actually secreted by the pancreatic acinar
cells, but then an enzyme in the small intestines changes trypsinogen into its active
enzymatic form, trypsin. The enzyme that converts trypsinogen into trypsin is called
enterokinase, and I'll describe this more on the regulation webpage. But trypsin is an
enzyme that digests proteins.
chymotrypsin: This is an enzyme that digests proteins.
carboxypeptidase: This is an enzyme that digests proteins.
– Why do we need three enzymes to digest proteins? Because proteins are made from a
pool of 20 amino acid building blocks that can come together in any order. And each
enzyme only cuts through the connection between specific amino acids. So the more
enzymes for breaking down protein, the more places the protein can be split and the
smaller the pieces we can derive from the breakdown.
nucleases: These are enzymes that digest nucleic acids. The word "nuclease" represents
any nucleic acid breaker, and there are a few different ones that the pancreas secretes.
.
Factors Influencing GI Tract
Secretions
•
•
•
•
Nervous control
Conditioned reflexes
Oral reflexes
Physical contact
66
Hormones involved in Digestion
• The presence of food in the stomach stimulates stretch
receptors which relay this information to the medulla
oblongata. The medulla stimulates endocrine cells in the
stomach to secrete the hormone gastrin into the
circulatory system. Gastrin stimulates the stomach to
secrets gastric juice.
Secretin
Secretin is produced by cells of the duodenum.
It’s production is stimulated by acid chyme from stomach.
It stimulates the pancreas to produce sodium bicarbonate,
which neutralizes the acidic chyme. It also stimulates the
liver to secrete bile.
Choleycystokinin
• CCK production is stimulated by the
presence of food in the duodenum.
• It stimulates the gallbladder to release bile
and the pancreas to produce pancreatic
enzymes.
•
• GIP (Gastric Inhibitory Peptide)
• Food in the duodenum stimulates certain
endocrine cells to produce GIP.
• It has the opposite effects of gastrin; it inhibits
gastric glands in the stomach and it inhibits the
mixing and churning movement of stomach
muscles. This slows the rate of stomach
emptying when the duodenum contains food .
• Sympathetic impulses decrease
contractions and peristalsis
• Epinepherine is the neurotransmitter
hormone of the sympathetic system
• Parasympathetic increases
Bioavailability
• Bioavailability refers to:
– Amount of nutrient present in the GI tract
– Competition between nutrients for absorption
– Form in which the nutrient is present
• All nutrients present in a food are not
absorbed because of differing
bioavailability.
– This is considered when determining dietary intake
standards
72
Metabolism
• Nutrients are converted to energy or
stored in the body
• Metabolism: The sum of body processes
that change our food energy from the
three energy nutrients
– Chemical reactions within cell to maintain life
– Occurs in mitochondrion of the cell
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Metabolism, cont’d
• Two metabolic processes
– Catabolism: Breaking down of large
substances into smaller units (e.g., breaking
down a protein chain into amino acids)
– Anabolism: Building of larger substances from
smaller particles (e.g., building a complex
protein from single amino acids)
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Metabolism, cont’d
• Metabolic processes ensure that the body has
energy in the form of adenosine triphosphate
(ATP).
• Metabolism of glucose from carbohydrates
yields less energy than metabolism of fat. Still,
glucose is the body’s primary source of energy.
• Protein can be an energy source, but it is
relatively inefficient.
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Metabolic Pathways
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Energy Storage: Glycogenesis
• Glycogenesis: Anabolic process of
converting extra glucose into glycogen
• Glycogen is stored in the liver and
muscles for quick energy to be used at a
later time
77
Nursing Assessment
•
•
•
•
•
Inspection
Auscultation
Percussion
Palpation
Questions
GI Risk Factors
•
•
•
•
•
•
•
•
•
•
Family history of GI disorders
Chronic laxative use
Tobacco use
Chronic alcohol consumption
Chronic high stress
Chronic NSAID use
Previous abdominal surgeries
Neurologic disorders
Medication use that can lead to constipation
Cardiac, respiratory, endocrine disorders
Genetic Disease
• Phenylketonuria
– Protein metabolism
• Galactosemia
– Carbohydrate metabolism
80
Lactose Intolerance
• Most common disaccharidase deficiency
• Lactase in insufficient amounts, not absent
• Causes abdominal cramping and diarrhea
81
Genetic Disease
• Phenylketonuria
– Protein metabolism
• Galactosemia
– Carbohydrate metabolism
82
Lactose Intolerance
• Most common disaccharidase deficiency
• Lactase in insufficient amounts, not absent
• Causes abdominal cramping and diarrhea
83
Summary
• Nutrients from food must be changed, released,
regrouped, and rerouted into forms the body can
use.
• The activities of digestion, absorption, and
transport ensure that key nutrients are delivered
to the cells so metabolic tasks can be
completed.
84
Summary, cont’d
• Mechanical digestion consists of spontaneous
muscular activity responsible for initial
mechanical breakdown and the movement of the
food mass along the GI tract by the motion of
peristalsis.
• Chemical digestion involves the enzymatic
action that breaks food down into smaller
components and releases nutrients for
absorption.
85
Summary, cont’d
• Absorption involves the passage of food
nutrients from the intestines into the mucosal
lining of the intestinal wall.
• Nutrients absorbed are transported throughout
the body by the circulatory system.
• Metabolism is the sum of the body processes
that change food energy taken in (carbohydrate,
protein, and fat) into various forms of energy
86
Summary, cont’d
• Metabolism is balanced by two types of
metabolic actions
– Catabolism
– Anabolism
87
Vocabulary for Digestion
•
•
•
•
•
•
•
lysozyme
Borborygmi
Gastrectomy
Endoscopy
Esphagoplasty
choleylithiasis
choleycystectomy
•
•
•
•
•
•
•
Gastritis
Stomatitis
Peyer’s patches
Deamination
Lacteal
Peritoneum
sublingual