Digestive Pathway
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Gastrointestinal (GI) tract
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Alimentary canal
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The alimentary canal or
gastrointestinal (GI) tract digests and absorbs food
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Alimentary canal – mouth, pharynx,
esophagus, stomach, small intestine, and large intestine
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Accessory digestive organs –
teeth, tongue, gallbladder, salivary glands, liver, and pancreas
Digestive Process
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The GI tract is a “disassembly”
line
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Nutrients become more available to
the body in each step
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There are six essential
activities:
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Ingestion, propulsion, and
mechanical digestion
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Chemical digestion, absorption,
and defecation
Steps of Digestion
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Ingestion
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Taking food into the digestive
tract
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Mixing and propulsion
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Propulsion – swallowing and
peristalsis
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Peristalsis – waves of contraction
and relaxation of muscles in the organ walls
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Digestion
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Mechanical digestion - chewing,
mixing, and churning food
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Chemical digestion - catabolic
breakdown of food
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Absorption
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Movement of nutrients from the GI
tract to the blood or lymph
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Defecation
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Elimination of indigestible solid
wastes
Peristalsis and Segmentation
Peritoneum and Peritoneal Cavity
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Peritoneum – serous membrane of
the abdominal cavity
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Visceral – covers external surface
of most digestive organs
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Parietal – lines the body wall
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Peritoneal cavity
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Lubricates digestive organs
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Allows them to slide across one
another
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Mesentery – double layer of
peritoneum that provides:
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Vascular and nerve supplies to the
viscera
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Hold digestive organs in place and
store fat
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Retroperitoneal organs – organs
outside the peritoneum
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Peritoneal organs
(intraperitoneal) – organs surrounded by peritoneum
HISTOLOGY
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Mucosa
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Moist epithelial layer that lines
the lumen of the alimentary canal
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Three major functions:
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Secretion of mucus
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Absorption of end products of
digestion
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Protection against infectious
disease
Mucosa
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Consists of three layers: a
lining epithelium, lamina propria, and muscularis mucosae
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Simple columnar epithelium
and mucus-secreting goblet cells
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Lamina Propria
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Loose areolar and reticular
connective tissue
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Muscularis mucosae
– smooth muscle cells that produce local movements of mucosa
Other Sublayers
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Submucosa – dense connective
tissue containing elastic fibers, blood and lymphatic vessels, lymph nodes, and
nerves
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ENS
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Muscularis externa – responsible
for segmentation and peristalsis
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Serosa – the protective visceral
peritoneum
Buccal Cavity
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Oral mucosa
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Vestibule
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Gingivae
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Hard palate
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Soft palate
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Uvula
Salivary Glands
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3 major glands
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Parotid
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Sublingual
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Submandibular
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Produce and secrete saliva that:
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Cleanses the mouth
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Moistens and dissolves food
chemicals
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Aids in bolus formation
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Contains enzymes that break down
starch
Saliva: Source and Composition
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Secreted from serous and mucous
cells of salivary glands
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97-99.5% water, hypo-osmotic,
slightly acidic solution containing
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Electrolytes – Na+, K+,
Cl–, PO42–, HCO3–
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Digestive enzyme – salivary
amylase
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Proteins – mucin, lysozyme,
defensins, and IgA
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Metabolic wastes – urea and uric
acid
Tooth Structure
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Periodontal ligament
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Anchors the tooth in the alveolus
of the jaw
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Forms the fibrous joint called a
gomaphosis
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Crown – exposed part of the tooth
above the gingiva
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Neck – constriction where the
crown and root come together
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Root – portion of the tooth
embedded in the jawbone
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Enamel – acellular, brittle
material composed of calcium salts and hydroxyapatite crystals; the hardest
substance in the body
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Encapsules the crown of the tooth
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Dentin – bonelike material deep to
the enamel cap that forms the bulk of the tooth
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Pulp cavity – cavity surrounded by
dentin that contains pulp
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Root canal – portion of the pulp
cavity that extends into the root
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Apical foramen – proximal opening
to the root canal
Teeth
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Primary and permanent dentitions
have formed by age 21
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Primary – 20 deciduous teeth that
erupt at intervals between 6 and 24 months
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Permanent – enlarge and develop
causing the root of deciduous teeth to be resorbed and fall out between the ages
of 6 and 12 years
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All but the third molars have
erupted by the end of adolescence
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Usually 32 permanent teeth
Deciduous Teeth
Permanent Teeth
Tooth Structure
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Teeth are classified according to
their shape and function
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Incisors
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Chisel-shaped teeth for cutting or
nipping
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Cuspids or canines – fanglike
teeth that tear or pierce
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Premolars (bicuspids) and molars –
have broad crowns with rounded tips; best suited for grinding or crushing
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During chewing, upper and lower
molars lock together generating crushing force
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Mastication
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Bolus
Pharynx
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From the mouth, the oro- and
laryngopharynx allow passage of:
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Food and fluids to the esophagus
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Air to the trachea
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Lined with stratified squamous
epithelium and mucus glands
ESOPHAGUS
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Muscular tube going from the
laryngopharynx to the stomach
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Adventitia
SWALLOWING
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Deglutition
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Food is ingested
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Mechanical digestion begins
(chewing)
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Propulsion is initiated by
swallowing
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Salivary amylase begins chemical
breakdown of starch
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The pharynx and esophagus serve as
conduits to pass food from the mouth to the stomach
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Buccal phase – bolus is forced
into the oropharynx
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Pharyngeal-esophageal phase
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Peristalsis moves food through the
pharynx to the esophagus
STOMACH
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Chemical breakdown of proteins
begins and food is converted to chyme
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Mechanical processing
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Chyme
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Storage
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Holds ingested food
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Gastric juice
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Chemical action
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Gastrin secretion
STOMACH
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Preliminary digestion
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No nutrient absorption
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Some chemicals can be absorbed
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Alcohol, aspirin
STOMACH
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Fundus
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Dome-shaped region beneath the
diaphragm
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Body
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Midportion of the stomach
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Pylorus
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Continuous with the duodenum
through the pyloric sphincter
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Rugae
STOMACH
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Gastric glands
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Gastric pits
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Gastric pits contain gastric
glands that secrete gastric juice, mucus, and gastrin
Gastric Glands
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Mucous cells
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Surface & neck
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Secrete acid mucus
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Enteroendocrine cells
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Parietal cells
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Secrete HCl and intrinsic factor
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Chief cells
Parietal Cells
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Intrinsic factor
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Vitamin B12
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Hydrochloric acid (HCl)
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Proton pumps
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Carbonic acid
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Alkaline tide
Chief Cells
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Pepsinogen
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Pepsin
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Rennin
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Gastric lipase
Regulation of Gastric Secretion
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Neural and hormonal mechanisms
regulate the release of gastric juice
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Stimulatory and inhibitory events
occur in three phases
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Cephalic (reflex) phase: prior to
food entry
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Gastric phase: once food enters
the stomach
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Intestinal phase: as partially
digested food enters the duodenum
Gastric Regulation
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Cephalic phase
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Sight or thought of food
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Stimulation of taste or smell
receptors
Gastric Regulation
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Gastric phase
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Stomach distension
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Activation of stretch receptors
(neural activation)
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Activation of chemoreceptors by
peptides, caffeine, and rising pH
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Release of gastrin to the blood
Gastric Regulation
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Intestinal phase
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Low pH; partially digested food
enters the duodenum and encourages gastric gland activity
Gastric Contractile Activity
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Most vigorous peristalsis and
mixing occurs near the pylorus
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Chyme is either:
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Delivered in small amounts to the
duodenum or
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Forced backward into the stomach
for further mixing
Small Intestine
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Site of most nutrient absorption
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Runs from pyloric sphincter to the
ileocecal valve
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Has three subdivisions:
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Duodenum
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Jejunum
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Ileum
Small Intestine
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Structural modifications of the
small intestine wall increase surface area
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Intestinal villi
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Villi – fingerlike extensions of
the mucosa
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Microvilli – tiny projections of
absorptive mucosal cells’ plasma membranes
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Brush border
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Goblet cells
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Lacteal
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Intestinal juice
Absorption in Small Intestine
PANCREAS
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Exocrine function
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Secretes pancreatic juice which
breaks down all categories of foodstuff
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The pancreas also has an endocrine
function
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Release of insulin and glucagon
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Pancreatic duct
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Duodenal ampulla
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Accessory pancreatic duct
PANCREAS
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Pancreatic islets
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Pancreatic acini
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Pancreatic juice
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Bicarbonate
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Amylase
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Pancreatic lipase
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Nucleases
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Proteases
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Peptidases
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Proenzymes
LIVER
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The largest gland in the body
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Superficially has four lobes –
right, left, caudate, and quadrate
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The falciform ligament:
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Separates the right and left lobes
anteriorly
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Suspends the liver from the
diaphragm and anterior abdominal wall
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Hexagonal-shaped liver lobules are
the structural and functional units of the liver
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Composed of hepatocyte (liver
cell) plates radiating outward from a central vein
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Portal triads are found at each of
the six corners of each liver lobule
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Portal triads consist of a bile
duct and
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Hepatic artery – supplies
oxygen-rich blood to the liver
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Hepatic portal vein – carries
venous blood with nutrients from digestive viscera
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Liver sinusoids – enlarged, leaky
capillaries located between hepatic plates
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Kupffer cells – hepatic
macrophages found in liver sinusoids
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Hepatic ducts
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Bile leaves the liver via:
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Bile ducts, which fuse into the
common hepatic duct
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The common hepatic duct, which
fuses with the cystic duct
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These two ducts form the bile duct
Microscopic Anatomy of the Liver
Gallbladder and Associated Ducts
Liver Functions
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Metabolic regulation
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Nutrient extraction/storage
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Removes waste products
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Vitamin & mineral storage
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Drug inactivation
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Hematological regulation
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Bile production
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Bile salts
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Emulsification
The Gallbladder
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Thin-walled, green muscular sac on
the ventral surface of the liver
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Stores and concentrates bile by
absorbing its water and ions
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Releases bile via the cystic duct,
which flows into the bile duct
Large Intestine
Is subdivided into
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Cecum
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Ileocecal valve
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The ileum joins the large
intestine at the ileocecal valve
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Vermiform appendix
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Colon
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Rectum
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Anal canal
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Sphincters
Large Intestine
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Other than digestion of enteric
bacteria, no further digestion takes place
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Reabsorption of water
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Absorption of vitamins
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Vitamin K, Biotin, B5
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Its major function is propulsion
of fecal material toward the anus
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Storage of feces