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Functions of the Digestive System
- Break down nutrients into a form that the body can use
- Absorb nutrients into bloodstream
- Eliminate parts of food that can't be digested
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Ingestion
Materials enter digestive tract
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Digestion
- Mechanical: Materials are crushed, sheared, and churned to allow them to fit through digestive tract and to increase surface area
- Chemical Break down food
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Absorption
Nutrients, vitamins, electrolytes move into the interstitial fluid of the digestive tract, then into the blood
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Compaction
Water and indigestible residue consolidated into feces
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Defecation
Feces is eliminated
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Digestive System Anatomy
- Digestive tract (alimentary canal): mouth, pharynx, esophagus, stomach, small intestine, large intestine and GI tract includes stomach and intestines
- Accessory Organs: include teeth, tongue, salivary glands, live, gallbladder, pancreas
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Mucosa
Lines the lumen and consists of inner epithelium, lamina propia (loose connective tissue), and Muscularis mucosa (thin layer of smooth muscle)
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Submucosa
- Thicker layer of loose connective tissue
- Contains blood vessels, lymphatic vessels, nerves, and mucous glands
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Muscularis Externa
- Two layers of muscle
- Inner circular layer: sometimes thickens to form valves
- Outer longitudinal layer: responsible for moving food through tract
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Serosa
Thin layer of tissue between end of esophagus and beginning of rectum
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Adventitia
Fibrous connective tissue in pharynx, esophagus, and rectum
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Peritoneal Cavity
- Fluid-filled sac
- Visceral peritoneum: serosa layer of GI tract
- Parietal peritoneum: lines inner surface of the body cavity
- Membrane secrete fluid for lubrication
- Intraperitoneal organs are enclosed by mesentery (stomach, liver, parts of small intestine and colon)
- Retroperitoneal organs lie against posterior body wall and are only covered by mesentery on anterior side (duodenum, pancreas, parts of colon)
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Mesenteries
- Connective tissue sheets that suspend the stomach and intestines from the parietal peritoneum
- Allows freedom of movement while still holding abdominal viscera in proper place
- Prevents tangling (twisting) of intestines
- Holds blood vessels and nerves in contact with GI tract
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Lesser Omentum
- Mesentery that extends from the liver to the stomach
- Stabilizes the stomach
- Provides route for blood vessels entering and leaving the liver
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Greater Omentum
- Pouch of adipose tissue that hangs from the stomach and over the small intestine
- Provides protection and insulation
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Mesentery Proper
Provides support for the small intestine
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Mesocolon
- Mesentery fused to posterior body wall
- Provides support for the colon
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Enteric Nervous System
- Nervous system network in the esophagus, stomach, and intestines
- Regulates tract motility, secretion, and blood flow
- Can function independently of the CNS, but influenced by it
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Submucosal Plexus
- In submucosa
- Controls secretion from glands in mucosa
- Controls movement of muscularis mucosa
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Myenteric Plexus
- Parasympathetic fibers between layers of muscularis externa
- Controls contractions of these muscles
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Sensory Neurons
Monitor tension in gut wall
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Short (myenteric) Reflexes
Stretch or chemical stimulation activates contractions in nearby muscularis externa
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Long (vasovagal) Reflexes
Parasympathetic activation regulates digestive motility and secretion
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Hormonal Control
Gastrin and secretin regulate digestion
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Paracrine Control
Chemical messengers produced by digestive tract diffuse to nearby target cells
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Oral Cavity Functions
- Senses taste of food
- Mechanically processes food
- Lubricates food with saliva
- Begins chemical digestion of carbohydrates and lipids
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Cheeks and Lips
- Hold food in mouth and push it between teeth
- Essential for sucking and blowing
- Consist of subcutaneous fat, buccinator muscle of cheek, and orbicularis oris of lips
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Cutaneous Area of Lips
Colored like rest of face, has hair follicles and sebaceous glands
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Red Area of Lips
- Red, hairless region where lips meet
- Blood vessels very close to epidermal surface
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Labial Mucosa
Inner surface of lips
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Labial Frenulum
Fold that connects lip of gum
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The Tongue
- Manipulates food between teeth
- Covered in nonkeratinized stratified squamous epithelium
- Body: anterior 2/3 in oral cavity
- Root: posterior 1/3 in oropharynx
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Vallate Papillae
Row of papillae marking boundary between body and root
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Terminal Sulcus
Groove behind vallate papillae
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Lingual Frenulum
Fold that attaches tongue to floor of mouth
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Lingual Tonsils
Contained in the root of the tongue
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Intrinsic Muscles
- Entirely within tongue
- Produced subtle tongue movements
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Extrinsic Muscles
- Insert into tongue
- Produce strong tongue movements
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Lingual Glands
- Serous and mucous glands among extrinsic muscles
- Secrete some saliva
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The Palate
Separates oral cavity from nasal cavity
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Hard Palate
Anterior part supported by maxillae and palatine bones
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Palatine Rugae
Ridges that help tongue hold food
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Soft Palate
- Posterior part composed of muscle and glands
- Contains uvula, which keeps food in the mouth until swallowing occurs and prevents food from entering nasal cavity
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Chewing
- Masticate food into smaller pieces
- Makes it easier to swallow
- Increases surface area for enzymes to digest it
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Teeth
- 32 adult teeth, 20 deciduous (baby) teeth
- 16 in mandible, 16 in maxilla
- From midline to rear-
- 2 incisors to bite food
- 1 canine to puncture/shred
- 2 premolars and 3 molars to crush and grind
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Deciduous Teeth
- Teeth develop beneath gums to erupt in predictable order
- Erupt from 6-30 months
- Replaced by permanent teeth from ages 6-25
- Root of baby teeth dissolves as permanent tooth develops
- Wisdom teeth (3rd molars) can be crowded against neighbors so that they can't erupt (impacted)
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Alveolus
Tooth socket in the bone
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Periodontal Ligament
Anchors tooth in alveolus
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Gingiva (Gum)
Covers alveolar bone
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Neck
Where crown, root, and gum meet
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Gingival Sulcus
Groove between tooth and gum
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Dentin
Hard, yellow tissue
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Enamel
Hard covering for crown formed before tooth erupts
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Cementum
- Hard covering for root
- Dentin and cementum can regenerate but enamel can't
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Root Canal
Tube for nerves and blood vessels to reach pulp cavity in crown
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Tooth Disease
Plaques of sugars and bacteria left on the teeth can erode enamel and dentin to for caries (cavities)
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Root Canal Procedure
- Endodontic Therapy required if cavity extends into pulp
- Pulp is removed and replaced with new material
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Functions of Saliva
- Moistens mouth
- Begins digestion of starch and fat
- Cleanses teeth
- Inhibits bacterial growth
- Dissolves tastants to stimulate taste buds
- Moistens and binds food for swallowing
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Saliva Components
Mostly water
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Salivary Amylase
begins starch digestion
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Lingual Lipase
activated by stomach acid and digests fat after food is swallowed
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Mucus
binds and lubricates food
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Immunoglobulin A:
inhibits bacterial growth
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Intrinsic Salivary Glands
Small glands that constantly secrete saliva
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Extrinsic Salivary Gland
Large glands with ducts
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Parotid Gland
Anterior to earlobe
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Submandibular Gland
Along Mandible
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Lingual Gland
Along tongue
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Salivary Glands
- Mucous cells secrete mucous
- Serous cells secrete thin fluid full of amalyse and electrolytes
- The fluids combine to salivary ducts
- Salivatory nuclei in medulla and pons regulate saliva production
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Pharynx
- Three pharyngeal constrictors force food downward during swallowing
- When not swallowing, inferior constrictor remains closed to prevent air from entering esophagus
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Esophagus
- Extends from pharynx to cardiac orifice of stomach
- Passes through esophageal hiatus in diaphragm
- Lower esophageal sphincter prevents stomach contents from entering esophagus and prevents stomach acid from eroding esophagus (heartburn)
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Swallowing
- Bolus of food pushed down through mouth, pharynx, and esophagus
- Swallowing center in medulla coordinates muscles of pharynx and esophagus through trigeminal, facial, glossopharyngeal and hypoglossal nerves
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Buccal Phase of Swallowing
- Under voluntary control
- Tongue collects flood, forms bolus, pushes it posteriorly
- Food accumulates in oropharynx, epiglottis moves, and bolus enters laryngopharynx
- Bolus activates tactile receptors that trigger phase 2
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Pharyngoesophageal Phase of Swallowing
- Involuntary
- Tongue, soft palate, and epiglottis/larynx block food from mouth, nasal cavity, and larynx
- Food driven downward by pharyngeal constrictors
- Bolus triggers peristalsis once it enters esophagus
- Lower esophageal sphincter relaxes to let bolus enter stomach
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