15 - Digestive System Flashcards
Describe the four processes of digestion
1) Ingestion
- food into oral cavity
2) Digestion
- large molecules broken down into smaller molecules
- 2 types:
a) mechanical
- motility e.g. chewing
b) chemical
- enzymes + acid secretions
3) Absorption
- end products of digestion enter blood or lymph
4) Defecation
- elimination of waste + undigested material
What are the 5 layers of the GI tract?
Mucosa Submucosa Muscularis Extrema Serosa (or adventitia) Enteric Nervous System
Describe the GI mucosa
1) Mucosa
- 3 layers:
a) epithelium with numerous goblet cells
- stratified squamous: mouth, pharynx, esophagus, anal canal
- simple columnar: stomach, small + large intestine
b) lamina propria = areolar CT
- contains blood, lymph vessels, lymph nodules/tissues (immune)
a) muscularis mucosa
- smooth muscle - allows movement of mucosa
Describe the GI submucosa
- areolar CT
- contains: blood + lymphatic vessels, submucosal nerve plexus
Describe the GI Muscularis Externa
- smooth muscle
- inner circular layer
- outer longitudinal layer
- myenteric nerve plexus between layers
- contractions cause motility (mixing + movement)
Describe the Enteric Nervous System
- nervous system of GI tract
- myenteric plexus
- controls muscularis externa - submucous plexus
- controls activity of mucosal glands and muscle
Describe the oral cavity
- includes:
a) lips
b) cheeks
c) palate
i) hard palate- 2 maxillae + 2 palatine bones
ii) soft - posterior to hard palate - skeletal muscle
- posterior projection = uvula
- rises to close the nasopharynx when swallowing
d) tongue
- 2 maxillae + 2 palatine bones
- attached to hyoid bone
- skeletal muscle
- projections of mucosa = papillae (taste buds)
Describe the salivary glands
- 3 pairs:
a) parotid - inferior and anterior to ears
- mumps = inflammation of 1 or both parotids
b) submandibular - floor of mouth
c) sublingual - below tongue on floor of mouth
- saliva
- 99.5% H2O
- 0.5% solutes (e.g. enzymes)
Describe the dentition
- in maxillae and mandible
- child dentition
- primary (1⁰) dentition - deciduous (“baby”) teeth
- adult dentition
- secondary (2⁰) dentition – permanent teeth
Classification child adult
Central Incisor 1 1 Lateral Incisor 1 1 Canine 1 1 Premolars 0 2 Molars 2 3 5 8 Total Teeth (# in each quadrant x4) = 20 32
- Tooth Structure:
a) crown - above the gum
- dentin = majority of tooth
- enamel overlay = acellular, highly calcified – hard!
b) root - dentin with cementum overlay
NOTE: dentin, enamel & cementum = similar to bone, but avascular
c) neck - enamel + cementum boundary (gums)
d) periodontal ligaments - attach root to bones
e) root canal extends to pulp cavity - contains CT, blood/lymph vessels, and nerves
Describe the Oropharynx and Laryngopharynx
- only muscularis externa (skeletal muscle) and mucosa (stratified squamous epithelium)
Describe the esophagus
- posterior to trachea
- passes through diaphragm to the abdominal cavity
- all 4 histological layers in GI tract from this point on
- exceptions in esophagus:
a) muscularis externa - upper 1/3 - skeletal muscle
- middle 1/3 – skeletal + smooth muscle (transition zone)
- lower 1/3 – smooth muscle
b) has adventitia in mediastinum (outermost layer)
= fibrous CT (no epithelium)
Describe the stomach
- has 4 regions
a) cardiac region (cardia) - attached to esophagus
b) fundus - above esophageal entrance
c) body
d) pyloric region (pylorus) - has pyloric sphincter
- greater and lesser curvatures
- converts food into chime (food + gastric juice)
- stomach mucosa:
- invaginations of epithelium form gastric glands (exocrine)
- secrete gastric juice to lumen
Gastric glands contain:
a) chief cells- secrete pepsinogen + gastric lipase (enzymes)
b) parietal cells - secrete HCl + intrinsic factor (for vit B12 abs in ileum)
c) goblet cells - mucus (surface epithelium also contains many goblet cells)
d) G cells (enteroendocrine cells) - secrete gastrin (hormone; to blood)
- secrete pepsinogen + gastric lipase (enzymes)
- rugae = folds of the mucosa + submucosa due to contraction of muscularis mucosa
- allow expansion without tearing mucosa
- muscularis externa
- function = churning
- 3 layers:
a) inner oblique
b) middle circular
c) outer longitudinal
Describe the small intestine
- end of pyloric sphincter to ileocaecal valve
- 3 segments:
a) duodenum
- first fold (short) - retroperitoneal
- extra glands here secrete alkaline mucus to protect against stomach acid
b) jejunum
– middle section
c) ileum
- has groups of lymph nodules = Peyer’s patches
- prevent infection of small intestine and bacteria from entering blood
- segments specialized to increase absorption surface area:
a) plicae circulares - submucosa thrown into folds
b) villi - projections of mucosa into lumen
- contains:
- blood capillaries
- lacteals (= lymph capillaries that absorb e.g. fats)
c) microvilli – brush border on enterocytes (simple columnar)
- extend into lumen - within epithelium, separate enteroendocrine cells secrete the hormones:
- secretin
- cholecystokinin
Describe the pancreas
- parts: head, body, tail
- contains:
i) exocrine portion
① acinar cells/ acini (most of pancreas)- secrete digestive enzymes (into ducts)
② duct cells - secrete alkaline fluid to neutralize stomach acid
→ digestive enzymes + alkaline fluid = pancreatic juice
ii) endocrine portion
= Islets of Langerhans (amid acini)- secrete hormones: insulin + glucagon to regulate blood sugar
- secrete digestive enzymes (into ducts)
- contains:
Describe the liver
- 4 lobes
- right, left, caudate, quadrate
- cells = hepatocytes
- filters material from GI tract (nutrients, toxins etc) before going to rest of body produce bile
- produces bile – for fat digestion
