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Swallowing Mechanisms, Digestion, Salivation, Hunger and Satiety,…

- - - - Prevents food from entering the nasal cavity
    - - Seal the airway to prevent aspiration
  - - - Prevent backflow into the pharynx
    - - Allows the bolus to enter the stomach, and then prevents refluxing into the esophagus
- - - - Targets
        
        Mucosal secretory cells
        
        GI endocrine cells
        
        Smooth muscle cells → increase motility
        
        GI microvasculature
        
        Immunomodulatory and inflammatory gut cells
    - - Sympathetic
        
        Releases norepinephrine and epinephrine → inhibit intestinal smooth muscle
        Inhibitory effect on enteric nervous system
      - Parasympathetic
        
        Vagus and pelvic splanchinic nerves → Acetylcholine (induces GI contractions) → Binds to M3 & M2 receptors → Ca²⁺ influx → depolarization
        Inhibitory neurotransmitter -> NO and VIP
  - - - Glucose & galactose: cotransported with Na⁺/K⁺ pump via SGLT1
      - Fructose: transported via GLUT5
      - All sugars enter the bloodstream via GLUT2
  - - - Peptidases on the brush border help finalize breakdown
      - Proteins → peptides → amino acids (by proteases)
      - Proteases break down proteins.
    - - Secreted as proenzymes (inactive) and activated later.
        Activation:
        Enzymes are secreted as inactive forms (proenzymes) and activated by low pH or trypsin.
      - Proenzymes and Activation
        
        Trypsinogen → (by enteropeptidase and trypsin) → Trypsin
        
        Chymotrypsinogen → Chymotrypsin
        
        Proelastase → Elastase
        
        Procarboxypeptidases A & B → Carboxypeptidases A & B
      - Action and result:
        Endopeptidases break into oligopeptides (2–6 aa)
        Exopeptidases release single amino acids
      - Gastric chief cells secrete pepsinogen, activated to pepsin in acidic stomach (pH 1.5–3.5), digesting ~10–15% of proteins.
  - - - Gastric lipase begins digestion in the stomach. Lipases hydrolyze lipids.
      - Pancreatic enzymes
        
        Lipase: Triglycerides → monoglycerides + fatty acids
        
        Cholesterol esterase: Cholesteryl esters → cholesterol + fatty acids
        
        Phospholipase A₂: Phospholipids → lysophosphatidylcholine + fatty acids
    - - Micelles (formed by bile salts) help absorb hydrophobic products using bile salts
      - Micelles enter epithelial cells, where lipids are packaged into chylomicrons and transported via lymph to blood
  - - - Vitamin D promotes calcium absorption (mainly in the ileum) via a protein (calbindin)
      - Inhibitors: oxalic acid, tannins, magnesium.
      - Stimulants: acidity, vitamin D, estrogen, lactose.
    - - Fe³⁺ (ferric) must be reduced to Fe²⁺ (ferrous) to be absorbed.
      - Binds to apoferritin inside enterocytes, then to transferrin in the blood.
  - - - Water reabsorption
      - Fermentation of undigested materials. Gut microbiota ferment undigested carbohydrates, producing gases and short-chain fatty acids.
      - Colonic motility: haustration, peristalsis, high-amplitude contractions.
    - - Stool stretches rectum → activates myenteric plexus and PNS.
      - NO and VIP relax internal anal sphincter.
      - Voluntary control of external anal sphincter enables defecation.
- - - - Peristaltic contractions (mix food with gatric secretions to form chyme)
      - Antral pumping (push chyme towards duodenum)
      - Receptive relaxation (fundus relaxes to accomodate food)
      - Retropulsion (chyme pushed backwards into stomach for more mixing)
  - - - Myenteric Plexus
        
        Location: between circular and longitudinal muscle layers
        
        Function: controls motility
      - Meissner Plexus
        
        Function: controls secretion
        
        Location: submucosa
    - - Stimulates ENS
    - - Inhibits ENS
  - - - Liquids empty right away
      - Solids need to be broken down to particles <2mm in size
        
        If not ----- less contraction ----- more time for digestion
      - Duodenal Feedback
        
        Chyme to fatty ----- send it back to stomach
        
        Chyme to acid ----- sends it back into stomach
        
        Signals stomach to slow down
    - - Accelerate Emptying
        
        Gastrin
      - Slow Emptying
        
        High acidity
        
        Large volume
        
        CCK (high fat content)
        
        High osmolarity
- - - - Alpha (α) cells: secrete glucagon → increases blood glucose
      - Delta (δ) cells: secrete somatostatin → inhibits both insulin and glucagon
      - Beta (β) cells: secrete insulin → lowers blood glucose
      - PP cells: secrete pancreatic polypeptide → regulates other secretions
  - - - Trypsinogen (→ protein digestion)
      - Amylase (→ carbohydrate digestion)
      - Lipase (→ fat digestion)
    - - secrete bicarbonate (HCO₃⁻) to neutralize stomach acid
  - - - 1. Cephalic
      - 2. Gastric
      - 3. Intestinal
- - - - Hepatocytes
        
        Bile flows via hepatic and cystic ducts to be stored and concentrated
        
        Helps with digestion, fat absorption and excretion
        
        Composition
        
        Bile salts
        
        Phospholipids
        
        Cholesterol
        
        Converted in hepatocyte by 7a-hydroxylase into
        
        1 more item...
        
        Bile pigments (e.g: Bilirubin)
        
        Bilirubin
        
        1 more item...
      - Storage of Bile
        
        Concentrates
        
        Links to cystic duct
        
        CKK ( secreted by I cells)
        
        Contracts Gallbladder
        
        Bile gets out of Gallbladder
        
        Relaxes Sphincter of Oddi
        
        Bile and Bicarbonate rich fluid flow through it to reach duodenum
        
        Absorbs sodium and chloride and water follows osmotically
- - - - Saliva secretion
      - Uses ACh to act on muscarini receptors
    - - Saliva composition modification