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Digestion: breakdown of food molecules for
use by the cell
Food is broken down by the digestive
system into a form that can diffuse directly
through the wall of the digestive tract and
into the blood
Made of digestive tract (muscular tube)
and accessory organs (salivary glands,
gallbladder, liver, & pancreas)
Functions:
1) Ingestion: taking food into the mouth
2) Mechanical mixing: physical
manipulation of solid foods by tongue,
teeth, & muscles of tract
3) Digestion: chemical breakdown of food
into smaller molecules
4) Secretion: release of water, acids,
enzymes, and buffers
5) Absorption: movement of small organic
molecules, electrolytes, vitamins, & water
across the digestive epithelium & into the
interstitial fluid
6) Excretion: waste products (indigestible
substances & worn out cells) are
compacted and discharged by defecation
Histology
-lining is made of 4 major layers that
secrete, contract, contain blood vessels, &
protect the rest of the body against
enzymes, acids, & bacteria
1) Mucosa: inner lining
-epithelium covering a layer of loose
connective tissue (lamina propria)
-folded to increase surface area, form
villi (fingerlike projections) in small
intestine
-areas of mechanical stress (mouth
esophagus, & anus) have stratified
epithelium, rest has simple
columnar epithelium
-lamina propria contains secretory cells
-outer layer contains thin muscle that
contracts to move villi and folds
2) Submucosa: second layer of loose
connective tissue
-large blood vessels, lymph vessels, &
nerves
3) Muscularis externa: smooth muscle
arranged in circular & longitudinal layers
-peristalsis: contractions that move
contents thru tract
-segmentation: regions of small
intestine that churn materials to
continue mechanical breakdown &
mix secretions
4) Serosa: outer layer, serous membrane
-none in mouth, esophagus, & rectum
(covered by dense collagen fibers
(adventitia) that attach these to
surrounding structures)
-peritoneum: lines cavities
-mesentary: parietal & visceral
peritoneum that suspends tract to
stabilize organs & prevent
tangling/injury
Oral Cavity: part that receives food
1) analyzes material
2) mechanically processes material
3) lubricates material
4) begins digestion of lipids &
carbohydrates with salivary enzymes
-anterior boundaries = cheeks & labia
(lips)
-vestibule: space between cheeks/lips
& teeth
-gingivae: gums that surround teeth
base
-superior boundary = hard & soft palates
-inferior = tongue
-lingual frenulum: fold that connects
anterior tongue to underlying
tissues
-posterior =
base of tongue
& uvula
The Tongue
Tongue: manipulate food inside the mouth
-Functions
1) mechanical processing by
compression, abrasion, & distortion
2) manipulate to help chewing &
swallowing
3) sensory analysis by touch, taste, &
temperature receptors
Salivary Glands
-3 pairs
1) Parotid salivary glands
2) sublingual salivary glands
3) Submandibular salivary glands
-produce 1 – 1.5 liters of saliva daily
-made of 99.4% water with ions, buffers,
waste products, metabolites, &
enzymes
-lubricates & dissolves chemicals to
stimulate taste buds when eating
-mucus reduces friction when swallowing
-continuous production flushes oral cavity
& produces immunoglobulins &
lysozymes to kill bacteria
-production failure would cause
infection & erosion of teeth & gums
Secretions
-salivary amylase: produced by parotid,
breaks down starch
-other gland products contain less enzymes
but more buffers & mucus
-max. production is 7ml/minute
-submandibular produces 70% of volume
-normal pH = 6.7, shifts to 7.5 during
eating
Teeth
Mastication: chewing
-breaks down tough connective tissue &
plant fibers
-helps disperse saliva (mucus & enzymes)
PARTS
1. Neck: boundary between root & crown
2. Root: part surrounded by bone
3. Crown: exposed section
-covered by enamel: crystalline form of
calcium phosphate (hardest biologically
made substance)
-requires adequate calcium, phosphates,
& Vitamin D in diet for complete coating
& decay resistance
4. Dentin: mineralized matrix that makes up
most of tooth
-does not contain living cells
5. Pulp cavity: central area
-cells extend into dentin
-receives blood vessels & nerves through
root canal
6. Alveolus: socket in bone where root
extends
7. Periodontal ligament: collagen fibers
that connect root dentin to bone
8. Cementum: layer covering root dentin
-gives protection & anchors root to
periodontal ligament
-softer, can be easily eroded
if gum disease allows
bacterial infection
Types of Teeth
1. Incisors: used for clipping/cutting
-most anterior
2. Cuspids: Canines, used for tearing &
slashing
-conical w/ ridgeline & pointed tip
2. Bicuspids (premolars) & Molars:
-flattened crowns w/ large ridges
-used for crushing, mashing, & grinding
Tooth Development
Deciduous teeth: first teeth to appear
-20 total
-periodontal ligaments & roots erode &
teeth fall out
Secondary teeth: permanent dentition
-permit processing of a wider variety of
food
-32 total
-impacted: develop in locations that don’t
allow proper eruption
Pharynx
com
-common passageway for food, air, & liquid
-food passes thru oropharynx &
laryngopharynx
-muscles help initiate swallowing
Esophagus
-muscular tube connecting pharynx &
stomach
-lies posterior to trachea, passes thru
mediastinum & esophageal hiatus:
opening in diaphragm
-25 cm long, 2cm diameter
-lined w/ stratified squamous epithelium
for protection & mucous glands to
lubricate
Deglutition: Swallowing
-food must have proper texture &
consistency
-tongue creates
bolus: small mass
-takes 9 seconds to
complete process
-Phases
1) Oral phase: voluntary
-bolus compressed against hard palate
& is forced into oropharynx while
elevating the soft palate
2) Pharyngeal phase: involuntary
swallowing reflex
-larynx elevates & epiglottis folds to
close trachea
-pushes bolus thru upper esophageal
sphincter into esophagus
3) Esophageal phase: involuntary
-bolus pushed thru esophagus to lower
esophageal sphincter & into
stomach by peristalsis
Esophagitis: inflammation of the esophagus
due to weak/relaxed esophageal sphincter
that allows acid reflex
Heartburn: occasional occurrence
Hiatal (Diaphragmatic) Hernia: abdominal
organs slide into thoracic cavity thru
esophageal hiatus
-severity depends on location, size, &
number of herniated organs
-mild condition is common & mostly
unnoticed
Stomach
-muscular, J-shaped organ that
receives food from esophagus
-4 functions
1) temporary storage of ingested food
2) mechanical breakdown of resistant
materials
3) chemical breakdown by acids &
enzymes
4) production of intrinsic factor needed for
vitamin B12 absorption
-chyme: viscous, acidic mixture of partially
digested food created in stomach
-4 regions
1) cardia: surrounding esophagus
opening, smallest
2) fundus: bulge superior to cardia
3) body: large central area
4) pylorus: narrow inferior area, where
stomach connects to small intestine at
pyloric sphincter: regulates flow of
chyme into small intestine
-can expand to hold ~1.5 L
-muscularis externa is thick & contains
longitudinal, circular, & inner oblique
layers to add strength & mix/churn
contents
-rugae: large ridges & folds in mucosa that
allow for distension
-visceral peritoneum
a) greater omentum: extends inferior &
creates pouch that hangs over &
protects abdominal organs
b) lesser omentum: extends from lesser
curvature to liver
-lined by mucous epithelium that creates a
protective alkaline mucous
-gastric pits: shallow depressions that open
onto gastric surface
-end with gastric glands
-secrete 1500 ml daily
-2 gland types
1) Parietal cells
-secrete intrinsic factor: facilitate
vitamin B12 absorption across
intestinal lining
-secretes hydrochloric acid to
lower pH to 1.5 – 2
-kills microorganisms, breaks
cell walls/connective
tissue, & activates
enzymatic secretions
2) Chief Cells:
-secrete pepsinogen: inactive
enzyme, converted into pepsin
(proteolytic enzyme) by
hydrochloric acid
Gastritis: inflammation of the gastric
mucosa
-causes: drugs, alcohol, aspirin, severe
emotional/physical stress, bacterial
infection, or ingesting highly
acidic/alkaline chemicals
Peptic ulcer: acids & enzymes erode thru
mucous & stomach lining, or beginning of
small intestine
-caused by excessive acid production or
inadequate alkaline mucous production
-bacteria infection causes 80%
Phases of gastric regulation
1) Cephalic phase:
-begins with sight, smell, taste, or thought
of food
-prepares stomach
-gastric juice production reaches 500 ml/hr
2) Gastric phase:
-begins as food reaches stomach
-stretch receptors & chemoreceptors
cause release of hormone gastrin into
blood
-causes parietal & chief cells to increase
production
-may continue for several hours
-stomach contracts to churn/mix contents,
pylorus contractions squirt small
amounts of chyme thru phyloric
sphincter
3) Intestinal phase:
-controls gastric emptying rate
-inhibitory to reduce gastric juices so small
intestine has time to deal with &
neutralize acidic chyme
Digestion in Stomach
-preliminary protein digestion
-carbohydrates broken down by salivary
amylase until pH reaches 4.5 (1-2 hrs)
-no absorption occurs
-thick mucous layer
-epithelial cells not specialized for
transport
-gastric lining impermeable to water
-digestion not complete (protein &
carbohydrates only partially digested)
Small Intestine
-90% of nutrient absorption
-6m (20ft) long, diameter 2.5 – 4 cm
-3 subdivision
1) Duodenum: 25cm (1ft) long
-receives chyme from stomach &
secretions from pancreas & liver
2) Jejunum: starts at 1st bend, 2.5m (8ft)
-most of chemical digestion & nutrient
absorption occurs
3) Ileum: 3.5m (12ft) long
-ends at ileocecal valve, sphincter that
leads to cecum of large intestine
-fits into relatively small space because it’s
well-packed & stabilized by mesentaries
Intestinal wall
-plicae: series of transverse folds
-villi: fingerlike projections
-covered with simple columnar
epithelium & microvilli
-folds increase surface area for absorption
611x
-capillaries carry respiratory gases &
absorbed nutrients to liver
-lacteal: lymph vessel in each villus
-transport materials that can’t cross into
capillaries (e.g. fatty acids)
-intestinal glands lie at villi base
-secrete watery intestinal juice
Intestinal Movements
-segmentation mixes chyme w/ secretions &
enzymes to prepare for absorption
-weak peristaltic contractions move material from
duodenum thru ileum in about 5 hours
Intestinal juice: 1.8 L secreted into lumen
each day
-moistens contents, buffers acids, &
dissolves enzymes & products of
digestion
Products
-food is broken into simple sugars, fatty
acids, & amino acids which are absorbed
with most of water
Pancreas
-elongated, pink/gray organ
-15 cm (6 in.) & 80 g (3 oz.)
-lumpy texture w/ soft tissue that is easily
torn
Pancreatic juice: secretion of digestive enzymes
& buffers & carried thru ducts that converge at
pancreatic duct which funnels juice to
duodenum
-1000 ml secreted daily
-make-up
-buffer (sodium bicarbonate) with pH of
7.5 to 8.8 to neutralize chyme
-pancreatic amylase: carbohydrates
-pancreatic lipase: lipids
-nucleases: nucleic acids
-proteases: proteins, 70% of total volume
-enzymes are secreted as proenzymes,
inactive form that are activated by
enzymes in the small intestine to protect
lining of pancreatic ducts
Liver
-largest organ, 3.3 lbs, 2.5% of total body
weight
-essential metabolic & synthetic functions
-tough fibrous capsule
-4 lobes
-large right & left lobes separated by
faliciform ligament
-smaller caudate & quadrate lobes
-Histology
-liver lobules: basic functional unit
-~100,000
-lined with Kupffer cells: phagocytic
macrophages that engulf pathogens,
cell debris, & damaged blood cells
-as blood flows past, hepatocytes adjust
levels by selective absorption (glucose)
& secretion
-secrete bile (digestive fluid) into
common hepatic duct as it leaves
-can flow into
a. common bile duct: empties
into duodenum
b. common cystic duct: leads to
gallbladder
Liver Functions
-more than 200 identified
1) Metabolic regulation
-regulated composition of circulatory blood
-after absorption in intestines blood travels
to liver
a. extracts toxins & metabolic wastes
that will be inactivated/excreted
b. monitor & adjust organic nutrient
levels
-remove & store excess,
synthesize or release
deficiencies
e.g. blood sugar-breaks
down glycogen
 blood sugar-absorbs
glucose & builds
glycogen
-fat-soluble vitamins (A, D, K, & E)
absorbed & stored
2) Hematological regulation
-largest blood reservoir (25% of cardiac
output)
-Kupffer cells “clean” blood
-synthesize plasma proteins for osmotic
control, nutrient transport, & clotting
systems
3) Bile Production
-water, ions, bilirubin (pigment derived
from hemoglobin breakdown),
cholesterol, & bile salts (lipids)
-water & ions dilute & buffer acids
-bile salts emulsify (create tiny drops)
lipids that aren’t water-soluble &
normally form large blobs that are
difficult to breakdown
-increase surface area to allow lipases
to digest fats
Gallbladder
-pear-shaped muscular organ that stores &
modified bile
-nestled under right lobe of liver
-liver produces 1L of bile daily, sphincter to
common bile duct remains closed until
chyme reaches duodenum
-most bile travels to gallbladder via cystic
duct
-can hold 40-70 ml
-during storage bile loses water & becomes
more concentrated (if too much water is
lost, bile salts precipitate out forming
gallstones)
-wall contractions push bile into duodenum
- fat content in chyme  bile secretions
Large Intestine
-runs from end of ileum to anus
-main functions
1) water reabsorption & feces compaction
2) absorption of vitamins released by
bacterial action
3) storage of fecal material for defecation
-1.5m (5 ft) long & 3 in diameter
-3 parts
1) Cecum: expanded chamber where
contents enter from ileum thru ileocecal
valve
-veriform appendix: small thin pouch
attached to inferior area
-contains lymph tissue
-can become
infected if
bacteria
become
trapped &
divide rapidly
2) Colon: 4 regions, dominated by haustra:
pouches that permit distension &
elongation
-3 longitudinal muscle (taeniae coli) help
bunch to create haustra
-regions
a) ascending colon: from ileocecal
valve to bend
b) transverse colon: horizontal region
c) descending colon: inferior to bend
d) sigmoid colon: S-shaped, empties
into rectum
3) Rectum: end of digestive tract
-ends with anal canal
-boundary between simple columnar &
stratified squamous tissues
-anus: anal canal opening of keratinized
skin
-internal anal sphincter:
involuntary,
-external anal sphincter:
voluntary
Large Intestine Functions
1) Absorption
-1500ml of material passes thru ileocecal
sphincter daily
-1300ml of water is recovered, 200ml of
feces are defecated
-fecal content
-75% water
-5% bacteria
-20% indigestible material, inorganic
matter, & dead cells
a) bile salts-reabsorbed & transported to
liver for re-use
b) vitamins-organic molecules needed to
metabolic reactions (many assist
enzymes)
-generated by bacteria
1. Vitamin K-needed by liver to
create clotting factors,
bacteria produce 50% of daily
requirement
2. Biotin-glucose metabolism
3. Vitamin B5-pantothenic acid, help
produce steroid hormones & some
neurotransmitters
c) Bilirubin products-bacteria convert into
products that can be excreted in urine
(create yellow color), or others stay &
turn brown when exposed to oxygen
d) Toxins-peptide breakdown creates
ammonia, nitrogen-containing
compounds (most odor), hydrogen
sulfide, many are removed by liver &
excreted by kidneys
-Indigestible carbohydrates provide food
source for bacteria
2) Defecation: forcing feces out the sigmoid
colon
-defecation reflex:
-stretch receptors trigger peristalsis in
colon & rectum to move feces
toward anus
-increases peristalsis throughout large
intestine
-removal requires internal sphincter to
relax, which immediately causes the
external sphincter to shut until
conscious control opens it