-- 4/23/10
Digestive System
- Gastrointestinal (GI) Tract
- 2 layers of smooth muscle (except in stomach which has 3)
- structure - common layers throughout system
- mucosa - innermost layer, nutrients pass through
- submucosa - connect tissue, lymph and blood vessels, nerves
- muscularis - 2 or 3 layers of smooth muscle responsible for motility in GI tract
- serosa - outermost later, connective tissue sheath
Digestive system processes
- 5 processes
- mechanical processing and movement - chewing, mixing
- secretion - fluid, digestive enzymes and hormones, bile, acid, alkali, mucus
- digestion - breaking down food to smallest absorbable units (not complete
breakdown)
- absorption - throughout mucosa, into blood or lymph vessels
- elimination - undigested material eliminated
Motility - peristalsis - like eating go-gurt - most obvious in esophagus
Motility - segmentation - most common in small intestine
The mouth - begins digestion
- teeth - incisors & canines - chopping; premolars & molars - grinding
- structure - crown, root
- tongue - skeletal muscle, moves food in mouth, taste
- saliva
- source - parotid, submandibular, sublingual salivary glands
- composition - mucin, salivary amylase (begins breakdowns of complex carbs in
mouth; hungrier = more salivary amylase), bicarbonate, lysozyme
swallowing - delivers food to stomach
- voluntary phase - tongue pushes bolus of food into pharynx
- involuntary phase/swallowing reflex - receptors in pharynx stimulated by presence of
food
- soft palate rises
- larynx rises slightly
- epiglottis closes opening to trachea
- tongue pushes food further
- good enters esophagus
pharynx
- common passageway for air and food
- participates in swallowing
esophagus
- structure - mix of skeletal and smooth muscle
- mucus-secreting cells - assist passage of food (by decreasing friction)
- food motility - gravity and peristalsis
stomach function
- functions - food storage, digestion, regulation of delivery
- gastric juice - specific cells secrete
- HCl (hydrochloric acid) - produces a pH of about 2, breaks down large bits of
food
- Intrinsic factor - made by same cells making acid, needed to absorb vitamin
B12
- mucus - protects stomach lining from acid
- pepsinogen - with acid, begins protein break-down - inactive until the production
of stomach acid
- stomach contraction - stomach contractions - blend food and propel forward
- structural adaptation - third muscularis layer
- direction - from lower esophageal sphincter to pyloric sphincter
- chyme - result of mixing, affects hormone secretions regulation peristalsis and
emptying of stomach
--4/26/10
small intestine - 10’10” long. most digestion occurs in 1st 10”; most absorption occurs in
last 10’
small intestine
- functions
- digestion - neutralize acid from stomach, add digestive enzymes and bile, break
proteins, carbohydrates, lipids, and nucleic acids to absorb materials
- absorption - 95% of food absorbed here
- structure
- regions - duodenum (1st 10”), jejunum & ileum (last 10’)
- mucosa adaptations - villi (& microvilli - increase absorption and digestion
surface area by 500x) containing blood capillaries (break down everything but
lipids) and lacteal capillaries
Enzymes - generally end with “-ase” - made in pancreas
- carbohydrases - break down carbs
- proteases - break down proteins
- nucleases - break down nucleic acids
- lipases - break down lipids
* enzymes break down food under a basic pH (> 10)
Accessory organs - aid digestion and absorption
- pancreas - exocrine functions
- secretes digestive enzymes and sodium bicarbonate
- also endocrine gland - helps control blood-sugar level (via insulin)
- liver
- produces bile - bile helps water and oil “mix” by