Digestive System
Anatomy & Physiology
Function & Organization
Function
• The breakdown of food into small
enough particles to be absorbed
Organization
• Alimentary Canal aka GI tract
• Accessory Organs
Digestive Processes
• Ingestion: bringing food into the
system
• Mechanical Digestion: the
mechanical breakdown of food
(chewing, mixing, churning)
• Propulsion: movement of food
through system
Digestive Processes
• Chemical Digestion: breakdown of
large food molecules by enzymes
• Absorption: transport of digested
food materials to the blood or lymph
• Defecation: elimination of
indigestible material from the body
Peritoneum
• The largest serous membrane of the
body lining all organs of the
abdominal cavity and the cavity wall
– Parietal peritoneum: lines walls
– Visceral peritoneum: lines organs
– Peritoneal cavity: potential space
between membranes; produces serous
fluid
Extensions of the
Peritoneum
• Falciform ligament: connects liver to
anterior abdominal wall and
diaphragm
• Lesser omentum: attaches to medial
stomach, attaching it to anterior wall
Extensions of the
Peritoneum
• Greater Omentum: extends off
stomach; contains lymph nodes and
holds fatty deposits
• Mesentary: suspends coils of small
intestine and attaches it to posterior
abdominal wall
Wall Structure of
Alimentary Canal
• 4 layers
–
–
–
–
Mucosa/mucus membrane
Submucosa
Muscularis
Serosa
Wall Structure of
Alimentary Canal
1. Mucosa: lines lumen of alimentary
canal
– Protects from microorganisms
– Absorbs digested food materials
– Secretes mucous & digestive enzymes
Wall Structure of
Alimentary Canal
2. Submucosa
-Rich in blood vessels, lymphatic vessels
and nerve endings
-Provides nourishment for tissues and
carries away absorbed materials
Wall Structure of
Alimentary Canal
3. Muscularis
- smooth muscular layer (has 2
parts)
•
•
•
inner layer runs longitudinally
outer layer (constrictor layer) runs
circularly
induces a propelling action called
peristalsis
Wall Structure of
Alimentary Canal
4. Serosa
- visceral peritoneum
-outermost covering of alimentary
canal
Alimentary Canal: Mouth
o Function: receives food and starts
digestion and prepares it for swallowing
- Teeth: performs mastication
(chewing)
- Uvula: fingerlike extension from
archway; works with soft palate to
close opening to nasal cavity upon
swallowing
Mouth continued
o Salivary Glands: accessory organs
around mouth; secretes saliva
- saliva: 99.5% water, 0.5% solutes
- Lysozome: destroys bacteria
- Salivary amylase: begins chemical digestion
of carbohydrates
- Mucus: lubricates and binds food particles
to form a bolus
Alimentary Canal:
Pharynx
o Function: transports food from oral
cavity to esophagus
-uvula and epiglottis assist
pharynx in transporting bolus to
the esophagus ONLY
Alimentary Canal:
Esophagus
Function: 10 inch muscular tube that
transports bolus to stomach from
pharynx
- Peristalsis: alternate waves of
muscle contraction & relaxation;
moves bolus along alimentary canal
Esophagus continued
- Mucosa: made of stratified squamos
epithelium to resist abrasion
- Cardio-esophageal sphincter: found
at distal end; acts as a valve to
prevent stomach contents from
entering esophagus
Alimentary Canal:
Stomach
o Function: c-shaped organ acting as a
temporary storage site for food; performs
mechanical and chemical digestion
-Rugae: deep folds formed by inner
lining of stomach when empty
-Full stomach: can hold up to 1 gallon
of food
- Pyloric sphincter: valve that controls
movement of food from stomach into
small intestine
Stomach Wall
• 4 basic layers: mucosa, submucosa,
muscularis and serosa
• Mucosa & muscularis is functionally
and structurally different
– Mucosa: gastric pits work to produce
and secrete gastric juices
• HCL, digestive enzymes and mucus
Stomach Wall
– Muscularis
• Stomach has an additional layer of muscle
fibers
• Allows stomach to churn and mix stomach
contents
Stomach Functions
• Mechanical Digestion: churning and
mixing of stomach contents to aid
digestion
• Chemical Digestion: pepsin: breaks
down proteins (works only in acidic
environments)
Stomach Functions
• Absorption: limited absorptive abilities
(water, salts, glucose, alcohol, aspirin and
some lipid-soluble drugs)
• Propulsion: propels food into small
intestine in the form of CHYME (mixture
of food particles & gastric juice)
– Chyme presses against pyloric sphincter
causing it to relax and allowing chyme to pass
Alimentary Canal: Small
Intestine
• Completes mechanical and chemical
digestion & is the main site of nutrient
absorption
• Propels food through with peristaltic
waves
• 20 feet long, highly coiled
• Takes ~ 3-10 hours to move chyme through
• Suspended to posterior abdominal wall via
mesentary
Small Intestine
continued
3 Segments:
1. Duodenum: receives chyme from
stomach & digestive enzymes from
accessory organs; about 10 inches
long
2. Jejunum: site of absorption; about
8 feet long
Small Intestine
continued
3. Ileum: about 12 feet long; the end
of digestion and absorption of
nutrients
Ileocecal valve: the junction at which the
small & large intestine join; sphincter
muscle controlling the flow of material
between organs
Wall of Small Intestine
• Highly convoluted mucosa for better
absorption through increased surface
area
– Intestinal villi: tiny projections of
mucosa
– Microvilli: even smaller projections on
the intestinal villi
Wall of Small Intestine
• Within each villi is:
– Blood capillaries
– Lacteals (lymphatic vessels)
- Act to carry absorbed nutrients away
Alimentary Canal: Large
Intestine
• Final segment of alimentary canal
• ~5 feet long but larger in diameter
than small intestine
• Function: to dry out indigestible
material by absorbing water;
eliminate unwanted materials
Large Intestine
continued
• 5 main segments of large intestine
– Cecum: sac-like pouch receiving material from
ileum
– Vermiform appendix: worm-like extension
holding lymphatic tissue
– Colon: ascending, transverse, descending,
sigmoid
– Rectum
– Anus
Large Intestine
continued
Anus: has 2 sphincters
1. Internal: involuntary; told to relax
when fecal matter stimulates the
defecation reflex in the rectum
2. External: voluntary; if relaxed,
elimination occurs
Fecal Formation
• Fecal matter: 75% water, 25%
indigestible material, mucus and bacteria
• Color: produced by bile
• Gas: produced by the metabolism of
bacteria or from swallowing air
• Movement of matter is very slow ~ 18-24
hours
Carbohydrate Digestion
• Carbohydrate: nutrient; main source
of energy in the body
• Glucose: a simple sugar; the end
product of carbohydrate digestion
• Takes place in the mouth & small
intestine
Protein Digestion
• Protein: basic nutrient; used in
anabolism (tissue building)
• Amino Acid: the building blocks of
proteins; the end product of protein
digestion
• Takes place in the stomach & small
intestine
Fat Digestion
• Fat: basic food type; used for energy
• Fatty acids and glycerol: end products of
fat digestion
• Takes place in the small intestine
• Enzymes:
– Bile in duodenum: fat droplets into smaller fat
droplets
Time to Digest
• Carbohydrate-rich meal: moves
through the stomach rapidly
• Fat-rich meals: may take up to 6
hours to pass through the stomach
Digestion Mishaps
• Heartburn: the damage to esophageal
mucosa due to presence of stomach acids
because of a weak cardio-esophageal
sphincter
• Gastric ulcer: when the mucus layer of
the stomach is not strong enough to
withstand gastric acids; burns a hole in
stomach wall
Digestion Mishaps
• Diarrhea: the decreased absorption of
water and electrolytes in the small & large
intestine leading to watery stool
• Constipation: the increased absorption of
water and electrolytes in the large
intestine; leads to hard, impacted stool