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Small Intestine - Anatomy & Physiology (view source)
Revision as of 08:14, 16 July 2008
, 08:14, 16 July 2008→Function
==Function==
==Function==
*To mix content and transport chyme.
*'''To mix content and transport chyme.'''
*Contraction of the two muscle layers facilitates mixing and transportation.
*Contraction of the two muscle layers facilitates mixing and transportation.
*There are two types of muscle contraction:
*There are two types of muscle contraction:
***Transports chyme through the small intestine but contractions can be weak to allow time for absorption
***Transports chyme through the small intestine but contractions can be weak to allow time for absorption
*When a peristaltic ‘’wave’’ of contraction reaches the end of the [[Ileum - Anatomy & Physiology|ileum]], another starts in the [[Duodenum - Anatomy & Physiology|duodenum]]. This is known as a ''migrating myoelectric complex''.
*When a peristaltic ‘’wave’’ of contraction reaches the end of the [[Ileum - Anatomy & Physiology|ileum]], another starts in the [[Duodenum - Anatomy & Physiology|duodenum]]. This is known as a ''migrating myoelectric complex''.
*'''To secrete enzymes for the digestion of chyme and absorb the products of digestion.'''
*In digestion, large, complex molecules are broken down into their constituents. They are then absorbed and used by the body for energy or used as building blocks for new complex molecules. The three main molecules that undergo digestion are carbohydrates, triacyglycerols and proteins.
*'''Carbohydrate Digestion and Absorption'''
*The main soluble carbohdrates found in food are starch, found mainly in plants, and glycogen, found mainly in animal meat.
**There are two types of starch, ''amylose'' which has α1-4 glycosidic links and, ''amylopectin'' which has α1-4 glycosidic links and α1-6 glycosidic links making it branched (branches every glucose 25 residues).
**''Glycogen'' is synthesised in the liver and muscle and is similar to amylopectin as it has both α1-4 glycosidic links and α1-6 glycosidic links. However it is more highly branched with shorter branches (branches every 12-18 glucose residues).
*The first stage of carbohydrate digestion begins with α-amylase, which is an endoglycosidase. This means it breaks bonds in the middle of the polymer to produce di-, tri- and oligo-saccarides.
*This begins in the [[Oral Cavity- Anatomy & Physiology|oral cavity]] where [[Oral Cavity - Salivary Glands - Anatomy & Physiology|saliva]] is produced that contains α-amylase. Salivary α-amylase is inactivated when it enters the stomach due to the acidic pH.
*Carbohydrate digestion continues in the lumen of the [[Small Intestine - Anatomy & Physiology|small intestine]] as pancreatic α-amylase enters the [[Duodenum - Anatomy & Physiology|duodenum]] in the pancreatic duct. This is the site of the majority of carbohydrate digestion.
*The second stage is the digestion of di-, tri-, and oligo-saccharides to monosaccharides.
**This is done by di-, tri-, and oligo-saccharidases which have a glycocalyx to trap their substrate. They are bound to enterocytes.
***The main dissacharides that are broken down:
***Maltose into two glucose molecules.
***Sucrose into a glucose and fructose molecule.
***Lactose into a glucose and galactose molecule.
*These monomers can then be absorbed.
**Absorption of glucose and galactose is coupled to sodium absorption and occurs through a symport called SGLT-1.
**Sodium potassium pumps in the enterocyte plasma membrane pump sodium out of the cell so that there is a higher concentration in the intestinal lumen than in the enterocyte.
**There is a net negative charge on the cell.
**Sodium diffuses down it's concentration and electrochemical gradient back into the enterocyte through the symport. This releases some energy.
**The energy release is used to transport glucose and galactose up their concentration gradients into the enterocyte.
**Glucose and galactose can then diffuse into the blood (portal vein) by carrier mediated diffusion via a GLUT-5 transporter.
==Regulation & Control==
==Regulation & Control==