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==Introduction==

==Introduction==

The small intestine extends from the pylorus of the [[Monogastric Stomach - Anatomía & Fisiología|stomach]] to the [[Ciego - Anatomía & Fisiología|ciego]]. The small intestine recieves chyme from the [[Monogastric Stomach - Anatomía & Fisiología|stomach]].  It is the main site of chemical degradation and absorption of chyme. Fats are exclusively broken down in this part of the alimentary tract. Carbohydrates and proteins that are not degraded in the small intestine are available for microbial fermentation in the [[Intestino Grueso - Anatomía & Fisiología|intestino grueso]]. The small intestine produces enzymes for digestion of protein, carbohydrate and fat and absorbs the products of their digestion. Enzymes are produced by glands in the intestinal wall and the [[Pancreas - Anatomía & Fisiología|pancreas]]. The [[Gall Bladder - Anatomía & Fisiología|gall bladder]] produces bile which emulsifies fats for digestion. Absorption is facilitated by ridges in the small intestine and by the presence of villi and microvilli.

The small intestine extends from the pylorus of the [[Estómago Monogástrico - Anatomía & Fisiología|stomach]] to the [[Ciego - Anatomía & Fisiología|ciego]]. The small intestine recieves chyme from the [[Monogastric Stomach - Anatomía & Fisiología|stomach]].  It is the main site of chemical degradation and absorption of chyme. Fats are exclusively broken down in this part of the alimentary tract. Carbohydrates and proteins that are not degraded in the small intestine are available for microbial fermentation in the [[Intestino Grueso - Anatomía & Fisiología|intestino grueso]]. The small intestine produces enzymes for digestion of protein, carbohydrate and fat and absorbs the products of their digestion. Enzymes are produced by glands in the intestinal wall and the [[Páncreas - Anatomía & Fisiología|páncreas]]. The [[Vesícula Biliar - Anatomía & Fisiología|vesícula biliar]] produces bile which emulsifies fats for digestion. Absorption is facilitated by ridges in the small intestine and by the presence of villi and microvilli.

The small intestine consists of three parts. Each part differs in anatomy, but all have the same basic structure and function:

The small intestine consists of three parts. Each part differs in anatomy, but all have the same basic structure and function:

====Carbohydrate Digestion and Absorption====   

====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 - Anatomía & Fisiología|liver]] and [[Muscles - Anatomía & Fisiología|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 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 [[Hígado - Anatomía & Fisiología|hígado]] and [[Muscles - Anatomía & Fisiología|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-saccharides).'' α-Amylase is present in [[Salivary Glands - Anatomía & Fisiología|saliva]]. Salivary α-amylase is inactivated when it enters the [[Monogastric Stomach - Anatomía & Fisiología|stomach]] due to it's acidic pH.

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-saccharides).'' α-Amylase is present in [[Salivary Glands - Anatomía & Fisiología|saliva]]. Salivary α-amylase is inactivated when it enters the [[Estómago Monogástrico - Anatomía & Fisiología|stomach]] due to it's acidic pH.

Carbohydrate digestion continues in the lumen of the [[Small Intestine Overview - Anatomy & Physiology|small intestine]] as pancreatic α-amylase enters the [[Duodeno - Anatomía & Fisiología|duodeno]] 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 are; Maltose into two glucose molecules, sucrose into a glucose and fructose molecule and lactose into a glucose and galactose molecule. These monomers can then be absorbed.

Carbohydrate digestion continues in the lumen of the [[Intestino Delgado - Resumen - Anatomy & Physiology|intestino delgado]] as pancreatic α-amylase enters the [[Duodeno - Anatomía & Fisiología|duodeno]] 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 are; Maltose into two glucose molecules, sucrose into a glucose and fructose molecule and 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.

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.

====Triacylglycerol Digestion and Absorption====

====Triacylglycerol Digestion and Absorption====

Triacylglycerols (TAGs) are digested by lipases. TAG digestion begins in the [[Cavidad Oral - Resumen - Anatomía & Fisiología|cavidad oral]], where lingual lipase is secreted in the [[Salivary Glands - Anatomía & Fisiología|saliva]]. It removes a fatty acid from the 3 position on the glycerol molecule producing 1,2-diacylglycerol(1,2 DAG) and a free fatty acid. TAG digestion continues in the small intestine, with pancreatic lipase and bile from the [[Liver - Anatomía & Fisiología|liver]]. Pancreatic lipase is water soluble and the TAG and 1,2-DAG are lipid soluble. Bile creates an interface for the enzyme to digest the lipid molecules. Bile also emulsifies fats; it reduces the size of lipid droplets increasing the surface area available for digestion. Pancreatic lipase removes any further fatty acids from the 3 position and then from the 1 position to produce 2-monoacylglycerol (2-MAG) and a fatty acid. Pancreatic lipase is unable to remove the fatty acid from the 2 position, so an enzyme called '''isomerase''' transfers the fatty acid from the 2 postion to the 1 postion to produce 1-monoacylglycerol (1-MAG). Pancreatic lipase can then remove the fatty acid from the 1 position to produce a fatty acid and glycerol.

Triacylglycerols (TAGs) are digested by lipases. TAG digestion begins in the [[Cavidad Oral - Resumen - Anatomía & Fisiología|cavidad oral]], where lingual lipase is secreted in the [[Salivary Glands - Anatomía & Fisiología|saliva]]. It removes a fatty acid from the 3 position on the glycerol molecule producing 1,2-diacylglycerol(1,2 DAG) and a free fatty acid. TAG digestion continues in the small intestine, with pancreatic lipase and bile from the [[Hígado - Anatomía & Fisiología|hígado]]. Pancreatic lipase is water soluble and the TAG and 1,2-DAG are lipid soluble. Bile creates an interface for the enzyme to digest the lipid molecules. Bile also emulsifies fats; it reduces the size of lipid droplets increasing the surface area available for digestion. Pancreatic lipase removes any further fatty acids from the 3 position and then from the 1 position to produce 2-monoacylglycerol (2-MAG) and a fatty acid. Pancreatic lipase is unable to remove the fatty acid from the 2 position, so an enzyme called '''isomerase''' transfers the fatty acid from the 2 postion to the 1 postion to produce 1-monoacylglycerol (1-MAG). Pancreatic lipase can then remove the fatty acid from the 1 position to produce a fatty acid and glycerol.

''NB: Pancreatic lipase works quickly, whilst isomerase works slowly. Thus, 2-MAG often accumulates and is absorbed (70% of digested TAG are absorbed as 2-MAG). A small proportion is absorbed as 1-MAG (6%).''

''NB: Pancreatic lipase works quickly, whilst isomerase works slowly. Thus, 2-MAG often accumulates and is absorbed (70% of digested TAG are absorbed as 2-MAG). A small proportion is absorbed as 1-MAG (6%).''

====Protein Digestion and Absorption====

====Protein Digestion and Absorption====

Protein digestion begins in the [[Monogastric Stomach - Anatomy & Physiology|stomach]] where pepsin is secreted as a zymogen, pepsinogen. Pepsin is an endopeptidase and produces smaller polypeptides. Pepsin prefers to break peptide bonds of larger polypeptides, where there is a large hydrophobic amino acid on the N-terminal side. Protein digestion continues in the small intestine. There are three endopeptidases in the small intestine; trypsin; chymotrypsin; and elastase. They are all secreted as zymogens; inactive precursors.

Protein digestion begins in the [[Estómago Monogástrico - Anatomy & Physiology|stomach]] where pepsin is secreted as a zymogen, pepsinogen. Pepsin is an endopeptidase and produces smaller polypeptides. Pepsin prefers to break peptide bonds of larger polypeptides, where there is a large hydrophobic amino acid on the N-terminal side. Protein digestion continues in the small intestine. There are three endopeptidases in the small intestine; trypsin; chymotrypsin; and elastase. They are all secreted as zymogens; inactive precursors.

Trypsin is secreted as trypsinogen, chymotrypsin is secreted as chymotrypsinogen and elastase is secreted as proelastase. Trypsinogen is initially activated by enterokinase (activation involves the cleavage of 6 amino acids). Trypsinogen can then activate itself, and also chymotrypsin and elastase. The short polypeptides produced from their digestion are further digested by exopeptidases which remove amino acids from the end of the polypeptide chain.  

Trypsin is secreted as trypsinogen, chymotrypsin is secreted as chymotrypsinogen and elastase is secreted as proelastase. Trypsinogen is initially activated by enterokinase (activation involves the cleavage of 6 amino acids). Trypsinogen can then activate itself, and also chymotrypsin and elastase. The short polypeptides produced from their digestion are further digested by exopeptidases which remove amino acids from the end of the polypeptide chain.  

==Regulation & Control==

==Regulation & Control==

The functions of the small (and large) intestine are regulated by three mechanisms: Endocrine hormones, paracrine hormones and neural transmitters. Unlike the [[Monogastric Stomach - Anatomía & Fisiología|stomach]], control is mainly local, with superimposed co-ordination through the extrinsic ANS. All signals affect the small intestine via sensory neurones. Neurones interact with two plexuses in the intestinal wall. Neurones in the ''myenteric plexus'' produce changes in muscle ''contractility''. Neurones in the ''submucosal plexus'' produce changes in ''secretion'' and ''blood flow'' to the small intestine. Plexuses are connected via interneurones.

The functions of the small (and large) intestine are regulated by three mechanisms: Endocrine hormones, paracrine hormones and neural transmitters. Unlike the [[Estómago Monogástrico - Anatomía & Fisiología|stomach]], control is mainly local, with superimposed co-ordination through the extrinsic ANS. All signals affect the small intestine via sensory neurones. Neurones interact with two plexuses in the intestinal wall. Neurones in the ''myenteric plexus'' produce changes in muscle ''contractility''. Neurones in the ''submucosal plexus'' produce changes in ''secretion'' and ''blood flow'' to the small intestine. Plexuses are connected via interneurones.

Excitatory neurotransmitters of the parasympathetic nervous system include: substance P, acetylcholine (via muscarinic M1 and M2 receptors) and serotonin (5-HT).

Excitatory neurotransmitters of the parasympathetic nervous system include: substance P, acetylcholine (via muscarinic M1 and M2 receptors) and serotonin (5-HT).