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Why is chemical digestion in the mouth of the horse minimal?
unlike other species, there is no amylase
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Describe the esophagus of the horse.
- proximal- striated muscle
- distal- smooth muscle
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Why don't horses regurgitate or vomit?
horses have a complete cardiac sphincter that is very strong
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What is the innervation of the esophagus?
CN X vagus nerve
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What are the 2 big portions of the equine stomach?
- non-glandular/squamous portion (1/3)
- glandular portion (2/3) separated by the margo plicatus
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What are the functions of the glandular portion of the stomach?
acid, enzyme, and buffer production
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What are the functions of the squamous portion of the stomach?
mixing and folding feed
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The gastric epithelium of the equine stomach is highly ___________ to _________; therefore, the contents have __________.
impermeable; water; high osmolality
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Describe the acid secretion of acid in the equine stomach. What is the clinical significance of this?
continuous secretion of acid and enzymes (made to graze constantly); therefore, if horses are not eating, they will be predisposed to gastric ulcers
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What are the transit times of liquids and particulates through the equine stomach?
- Liquids: 75% gone by 30 min
- Particulates: 75% gone by 90 min
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Compare the gastric mucosa of foals to adult horses.
foals have thinner squamous mucosa and higher resting pH than adults
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Function of parietal cells of stomach.
acid secretion via H+K+ATPase pump
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Function of chief cells of stomach.
pepsinogen--> pepsin (protease)
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Function of enterochromaffin cells of stomach.
histamine--> stimulate acid release
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Function of D cells of stomach.
somatostatin--> inhibits acid release
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Function of G cells of stomach.
gastrin--> stimulates acid release
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Function of goblet cells of stomach.
bicarb and mucous release
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What drugs are used in horses to increase stomach pH? (2)
Ranitidine- Zantac (H2 receptor blocker), Omeprazole (PPI)
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What pancreatic enzyme in horses has the highest activity?
lipase (followed by amylase)
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What is secreted by the pancreas? (7)
lipase, amylase, elastase, trypsin, chymotrypsin, bicarb, Cl- (Cl- reclaimed in ileum)
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Describe secretion of enzymes and buffers from the pancreas.
continuous secretion ALL THE TIME
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Describe the secretion of biliary secretions from the liver.
continuous secretion
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Function of bile. (2)
fat digestion and absorption, cholesterol--> cholic + chenodeoxycholic
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Bile salts form __________ with __________ for __________ of fat to TG and FFA.
micelles; lipid; emulsification
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Lipases assist with digestion and absorption into ____________ as _____________.
villus lacteals; chylomicrons
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90% of bile salts are reabsorbed in the _________ and go back to ____________.
ileum; enterohepatic circulation
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In proximal small intestine, most ___________ takes place; in the distal small intestine, most _____________ occurs.
digestion; absorption
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What is the innervation of the small intestine?
CN X- Vagus nerve
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Describe the epithelium of the small intestine.
columnar mucosal epithelium with villi and microvilli brush border
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Function of S cells in the SI.
secretin stimulates pancreatic and biliary secretion into the SI
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What stimulates S cells in the SI?
acid in the duodenum
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What is the function of I cells in the SI?
CCK stimulates pancreatic and biliary secretions into the SI, decrease gastric emptying and acid production
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What stimulates I cells of the SI to secrete CCK?
stimulated by Ach, FFA, CCK-release factor, AA, fat in SI
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What is the precursor molecule for most pancreatic enzymes?
trypsin
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Transcellular absorption is _________ and occurs by...
active; transporters across a cell membrane.
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Paracellular absorption is _________ and occurs by...
passive; concentration-dependent diffusion between cells.
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If there is a dysfunction of ___________ enzyme, polysaccharides and sodium will stay in the lumen, leading to __________.
lactase; diarrhea
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The crypts of Lieberkuhm (intestinal crypts) _________ volume to keep __________.
volume; chyme fluidity
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What are the parts of the intestinal crypts? (4)
stem, goblet, Paneth, endocrine cells
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Intestinal crypts secrete __________ so _________ will follow; this secretion is mediated by __(2)__.
Cl- and Na+; water; enteric nervous system and prostaglandins
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What is the major role of intestinal crypts in diarrhea development?
prostaglandin-mediated hypersecretory function
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What are Paneth cells of the intestinal crypts?
functionally like neutrophils, secrete anti-microbial substances
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What are Brunner's glands of the intestinal crypts?
produce mucus-rich alkaline secretions to protect mucosa from acid chyme and to facilitate digestion
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What is the function of B cells in the intestinal crypts?
produce secretory IgA
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Water permeability of the SI __________ from proximal to distal.
decreases
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Intraluminal pH of the SI ________ from proximal to distal due to...
increases; a gradual increase in bicarb and extensive ileal Cl- absorption
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What is the migrating motor complex (MMC)?
interdigestive phase under motilin influence
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What is the clinical significance of the large extra-intestinal secretory volumes (fluid absorbed from the GI)?
if obstruction occurs and these volumes are lost, hypovolemic shock occurs fairly quickly
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What is the purpose of the high bacterial counts in the large intestine? (3)
hind-gut fermentation: bacteria produce VFAs, which supply 30% of horses kcal per day; bacteria also produce essential AAs, FAs, and vitamins (vit K); immune function
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What are the types of bacteria within the large intestine? (5)
anaerobes, cellulytic bacteria, enterobacteria (facultative anaerobes), protozoans, archae
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What characteristic lets you know you are in the ventral large colon? What is the purpose of these structures?
ventral colon haustra (sacculations): mixing movements
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What are the neurohormonal controls of mucosal secretion/absorption in the large intestine? (3)
- RAAS: electrolyte homeostasis, K+ excretion
- ADH and ANP: balance Na+/ water content (osmolality), aquaporin expression (movement of water)
- Enteric nervous system: sympathetic (proabsorption), parasympathetic (prosecretion)
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How does SNS tone influence the ENS?
pro-absorption
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How does PNS tone influence the ENS?
pro-secretion
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What is the highest VFA absorption to the lowest (based on size)?
acetate>>> propionate>> butyrate
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What anatomic sphincters exist to assist with aboral flow through the equine GI tract?
- lower esophageal sphincter (true)
- pyloric sphincter (true)
- ileocecal valve (functional)
- cecocolic orifice (functional)
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The ileal lumen is slightly smaller than the jejunal lumen to allow for...
macronutrient absorption across jejunal brush border.
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What is the purpose of the pelvic flexure narrowing from the left ventral colon?
allows haustral mixing of ingesta for effective bacterial fermentation
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What is an example of a drug that should be avoided in colicky horses and why?
xylazine because it decreases intestinal motility
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Describe the process of protein digestion in the small intestine.
- 1. proteins digested to AA by pancreatic proteases and brush border enzymes
- 2. AAs are absorbed by active transport into the absorptive cells and move to the opposite side (transcytosis)
- 3. AAs leave the villus epithelial cell by facilitated diffusion and enter the capillary via intercellular clefts
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Describe the process of absorption of monosaccharides and amino acids in the small intestine.
- 1. initial luminal digestion (polysaccharides to disaccharides; proteins to polypeptides and peptides)
- 2. extensive brush border digestion and absorption
- 3. cotransport with Na+ and H+ ito enterocytes and then to portal vein by transcellular movement
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Describe the absorption of monoglycerides and FFAs from the intestinal lumen.
- 1. luminal digestion of Tags via lipases
- 2. re-esterification in enterocytes into chylomicrons
- 3. transport via lacteal to cisterna chyli, thoracic duct, then cranial vena cava
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Electrolytes are absorptions from the SI lumen by __(2)__.
transcellular and paracellular pathways
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What are the sources of bicarb in the horse's body? (5)
saliva, stomach (goblet cells), pancreas, bile, +/- Brunner's glands
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