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عدد المساهمات : 9680
تاريخ التسجيل : 06/08/2009
 | موضوع: Small intestine physiology  الثلاثاء ديسمبر 07, 2010 6:04 am | |
| I. Motility in the Small Intestine 2 forms: Mixing and Propulsive
A. Mixing or Segmentation Contractions Most important and common contraction in the stomach
• Distention (the chyme against the wall is the stimulus)
• Breaks up chyme to increase surface area
• Mixes chyme with secretions and presents digestive materials to mucosa to be absorbed
• Rate related to electrical slow waves initiated by interstitial cells of Cajal (pacemaker cells) in longitudinal layer:
• duodenum 12-14/min
• ileum 8-9/min
• Myenteric plexus will occur in absence of extrinsic or parasympathetic nerves
B. Propulsive (Peristaltic) Contractions
– Distention (stretch) initiates
– Usually weak
» die out after few cm
– Parasympathetic and myenteric nerves
– Influenced by nervous signals, hormonal factors influence the relative degree of depolarization or hyperpolarization of sm. muscle cell.
C. Law of the Intestine
– chyme moves analward with the rate of contractions is faster at duodenum than at the terminal ileum.
II. Secretions of Small Intestine
the Small intestine does NOT secrete enzymes, there are enzymes adhered to the microvilli brush border, but they are not secreted into the lumen.
• 1-2 liters daily
• Stimulus: distension or irritation by hypertonic or acidic chyme
• Isotonic; mostly water, some mucous; enzyme poor
• Brunner’s Glands
– lst few cm of duodenum
– Secrete alkaline mucus
• Crypts of Lieberkuhn
– mucous from goblet cells and serous secretion from epithelial cells secrete an extracellular fluid, 1800 ml daily
– watery vehicle for absorption of substances, actions of the enzymes of the pancreas and brush border enzyme
Small Intestine - Enzymes
• Located on brush border
• Peptidases break peptides into a.a., sucrase, maltase, iosmaltase, lactase these 4 break disaccharides into glucose, galactose, and fructose intestinal lipase, enterokinase-the brush border enzyme which activates trypsinogen to tyrpsin
• Disease like Celica Sprue which damages the brush border and you get malabsorption from the loss of enzymes and nutrients can’t be adequately digested/absorbed.
• Also, the genetic disorder” Lactate Deficient: lactose goes to the colon, where it’s fermented and you get gas and diarrhea.
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