Ch 14: Digestive Physiology -...
Transcript of Ch 14: Digestive Physiology -...
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Ch 14: Digestive Physiology
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The GI tract is a hollow tube
• ~30 ft, mouth to anus
• Transports food/liquid/chime
• Accessory organs (liver, pancreas) & glands outside
tubular system secrete substances that aid digestion.
• Allows for absorption of nutrients across intestinal
membrane to blood.
• Packages un-used substances for excretion as solid waste.
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GI tract:Mouth, teeth, salivary
glandsPharynxEsophagusStomachSmall intestine
- duodenum- Jejunum- ileum
Accessory to small Intestine:
- Liver- Pancreas
Large intestineRectumAnus
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4 basic GI processes• Motility- muscles mix &
move food through successive compartments
• Digestion- ingested food broken into small molecules
• Absorption- molecules move from GI lumen into blood stream
• Secretion- movement of substances into the lumen, e.g. enzymes, HCO3
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GI Tract LayersFour layers (a.k.a. tunics):
1. Mucosa:
2. Submucosa:
3. Muscularis:
4. Serosa:
1. Mucosa: - innermost layer with:
- mucus-secreting (goblet) cells or other exocrine cells
- endocrine cells
- absorptive cells
- lymph nodes (immune function)
- may be folded (“plicae”) to surface area
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2. Submucosa: - layer with:
- blood supply & lymph vessels
- nerve supply= Miessner’s plexi
– sensory neurons &
– motor neurons transmitting vagus nerve commands
to muscularis,
parasympathetic = ____________________
sympathetic = __________________________
3. Muscularis: - layer with:
- 2 muscle layers (circular layer for diameter, & longtitudinal for length)
- nerve supply= Auerbach’s plexis
4. Serosa: outermost layer of connective tissue, continuous with
peritoneum and mesentery.
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Upper GI tract begins mouth
• mastication, saliva, amylase
• Pharynx or throat- common passage for air & food
• Epiglottis covers opening to trachea (glottis) when swallowing.
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Saliva is a cocktail produced by several glands
• HCO3-
• mucus
• amylase
• lysozyme: lyses bacteria
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Esophagus is a muscular tube
• Beginning is skeletal muscle, lower portions all smooth muscle.
• Each end controlled by circular sphincter muscle
• Passes thru hole in diaphragm: esophageal hiatus
• Failure of lower esophageal sphincter allows gastric reflux- backflow of acidic stomach contents = ______________________________
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Barrett’s esophagus is pre-cancerous condition
• Chronic acid damage
• Misplaced glandular cells replace normal squamous epithelium
• Higher risk of adenocarcinoma (cancer of glandular tissue)
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Barrett’s endoscopy
Pink- normal esophageal
Squamous epithelium
Red- Barrett’s columnar
epithelium
Esophageal cancer:
• On the rise- top 10 most common cancers worldwide
• Risk factors: alcohol, smoking, acid reflux
• Very poor 5 yr survival rate for advanced stages (<40%)
• 90% of cases diagnosed in advanced stages
• Therapy- surgery, radiation, chemo
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• Neoplasm- any new growth, designated by the suffix “-oma”
• Carcinoma- cancerous cells arising from epithelial tissue
• Adenocarcinoma- cancerous cells with glandular (secretory) properties that begin growing in the cells that line some internal organs
Cancer terminology
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Esophageal cancer
• On the rise- top 10 most common cancers worldwide
• Risk factors: alcohol, smoking, reflux
• Very poor 5 yr survival rate for advanced stages (<40%)
• 90% of cases diagnosed in advanced stages
• Therapy- surgery, radiation, chemo
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Stomach is J-shaped muscular sac
• Folded lining (rugae) contains gastric glands
• Contractile activity mixes gastric juice w/ food chyme
• pyloric sphincter controls emptying
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Gastric pits, in fundus & body, have gastric cells:
• Neck cells: make mucus
• Parietal cells: secret HCl, & intrinsic factor for B12 absorption
• Chief cells: pepsinogen
• G-cells: hormone gastrin,stimulates parietal cells
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Peptic ulcer disease results from erosion of stomach lining
• Stomach mucus protects against pH~2!• Open lower layers to acid & pepsin attack• Affects ~10% of US population• Many are caused by H. pylori infection
Perforated ulcer
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Helicobacter pylori: an acidophile• Cause immune response that destroys tissue
• Treated w/: 1) antibiotics2) histamine receptor (H2) antagonists
3) proton pump blockersEx.__________________
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Sm Intestine - site of digestion for most foods
• 3 regions: duodenum, jejunum, ileum (Most nutrient absorption completed in first 20%, before reaching ileum)
• Duodenum receives chyme from stomach, pancreatic enzymes & bicarbonate, & bile from liver.
• Sm intestine endocrine secretions: (REVIEW!)- secretin - stimulate water and bicarbonate secretion from pancreas- cholecystokinin – stimulates gallbladder contraction & stimulates pancreatic
enzyme secretion- gastric inhibitory peptide - slows gastric motility (slow down) & stimulates
pancreatic enzyme secretion
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Hepatic portal system
• Substances absorbed into blood from intestine go to liver
• “1st pass metabolism”- orally administered drugs are taken directly to liver
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Liver produces bile• Contains HCO3
-, phospholipids, cholesterol-like bile salts
• Excess bile stored in gall bladder- bile secreted thru common bile duct to duodenum.
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Gallstones or “cholelethiasis”
Bile salts precipitate out of bile and form stones.
10% of americans
TX:- Medication to dissolve stones
- Surgical removal of gallbladder (laproscopically)
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Liver: a most versatile organ• Glucose/glycogen metabolism
• Removal of old RBCs
• Clears hormones, medications, toxins from body
• Cori cycle
• Blood glucose regulation
• Amino acid metabolism
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Pancreas has endocrine & exocrine functions
• Acinar cells – exocrine secretions (digestive enzymes)Amylase for starch & glycogen
Lipases for fats
Proteases for proteins
Nucleases for nucleic acids
• Islet cells – endocrine secretionBeta cells - insulinalpha cells - glucagon
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Large intestine removes H2O from chyme• Lg intestine:
- Cecum- colon (ascending, transverse, descending)- rectum
• Transforms chyme into solid feces(water reabsorbed from chyme)
• Solid feces stored in rectum- has stretch receptors that signal for defecation
• Anus has involuntary and voluntary sphincters
• Blockage/inflammation of vermiform appendix =appendicitis
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Defecation involves voluntary & involuntary anal sphincters
• Colon contraction pushes feces into rectum
• As rectum fills, stretch receptors stimulate urge to defecate.
• To defecate or not defecate depends on state of the anal sphincters:
• Internal anal sphincter = smooth muscle (involuntary)
parasympathetic - internal sphincter relaxes, allowing feces to pass through anus.
sympathetic – internal sphincter contract, keep feces in (moves back into colon, more water resorbed)
• External anal sphincter = skeletal muscle (voluntary)
External sphincter
Internal sphincter
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Defecation Reflex
Stimulus:
Autonomic Response:
Delay defecation (hold it!): Defecation:
Rectum stretches
- Rectum & sigmoid colon contracts (push feces into anus)- Internal anal sphincter relaxes (allow anus to fill)- External anal sphincter contracts (keep it in temporarily)
- External anal sphincter contracts(keep feces in)
- Puborectalis muscle contracts(keep feces in)
- External anal sphincter relaxes (allow feces out)
- Puborectalis muscle relaxes (allow feces out)
- Straining (abdominal)
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Carbohydrate digestion
A. Simple carbohydrates
> Monosaccharides = single sugar unit. Ex.: Glucose, galactose, fructose
> Disaccharides = 2 sugar units. Ex.: Sucrose, lactose, maltose
B. Complex carbohydrates(polysaccharides)
- Made of more than 2 sugar units
Ex.:> Starch = storage form of sugar in plants (ex. Potatoes)
> Cellulose = indigestible polysaccharide in plant cell walls.
> Glycogen = storage form of glucose in muscles & liver.
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Different types of enzymes can break down different nutrients:
• Amylase enzymes break down starch (polysaccharides) into sugar.(secreted in saliva & by pancreas)
• Protease enzymes break down proteins into amino acids.(secreted by stomach and pancreas)
• Lipase enzymes break down fats into fatty acids and glycerol.(secreted by pancreas)
BUT lipid digestion 1st relies on bile to break down to size enzymes can be effective
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Proteases stored as zymogens
• Inactive, stored in vesicle: protect cells
from proteolytic digestion
• undergo proteolytic activation in lumen
of gut
• Enterokinase: sm intestine brush
boarder enzyme that activates
zymogens