!Small Intestine - Overview
!!Anatomy
The small intestine is the longest part of the ((Digestive - System Overview|gastrointestinal tract)) (gut) between the ((Digestive - Stomach Overview|stomach)) and the ((Digestive - Large Intestine Overview|large intestine)) and includes the ((Digestive - Duodenum|duodenum)), ((Digestive - Small Intestine - Jejunum, Ileum|jejunum and ileum)). It´s length varies between 3 and 9 meters in adults and is even dependent on the tonus of the ((Digestive - Intestine Smooth Muscle|muscle layer)), while it´s diameter is between 2,5-3 cm. Food from the stomach is allowed into the duodenum through the ((Digestive - Stomach - Mucosa (Lesser Curvature and Pylorus)|pylorus)) by a muscle called the pyloric sphincter.

The small intestine is where most __chemical digestion__ takes place. Most of the digestive enzymes that act in the small intestine are secreted by the ((Digestive - Pancreas Overview|pancreas)) and enter the small intestine via the ((Digestive - Pancreas Ducts|pancreatic duct)). Enzymes enter the small intestine in response to the hormone __cholecystokinin__, which is produced in the small intestine in response to the presence of nutrients. The hormone __secretin__ also causes bicarbonate to be released into the small intestine from the pancreas in order to neutralize the potentially harmful acid coming from the stomach.

The three major classes of nutrients that undergo digestion are __proteins, lipids (fats) and carbohydrates__, which are broken down by pancreatic enzymes:

Proteins are broken down by trypsin and chymotrypsin, fats by lipase together with secretes from ((Digestive - Liver - Parenchyma|liver)) and ((Digestive - Gallbladder and Bile Ducts|gall bladder)) containing bile salts. Some carbohydrates are broken down by pancreatic amylase, others need to be carried on into the large intestine to be treated by bacteria. The nutrients are then ready for __absorption__ into ((Circulatory - Blood|blood and lymph fluid)), which happens in the ((Digestive - Duodenum Mucosa (Villi)|villi ))and microvilli, finger-like structures that enlarge the surface of the small intestine, especially in the ((Verdauung - Dünndarm - Jejunum, Ileum|ileum)).

{img fileId="243" thumb="box"}graphic: wikipedia/Mariana Ruiz Villarreal

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{img fileId="204" thumb="box"}graphic source: wikipedia

!!Brain
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!!!Relay:
*__Brain Stem__ ( + / - ) for secretory & absorptive ((Digestive - Small Intestine - Jejunum, Ileum|mucosa)) and
*__Midbrain__ (+/-) for ((Digestive - Intestine Smooth Muscle|smooth muscle))
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*__Cerebellum__ (+/-) for ((Digestive - Visceral Fascia - Peritoneum|Peritoneum))/((Digestive - Visceral Fascia - Greater Omentum (Epiploon)|greater omentum))
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*__Cerebral Medulla__ (-/+) for ((Digestive - Intestinal Lymphatic Tissue|lymphatic tissue, and Peyer´s plaques)) in the ((Digestive - Small Intestine - Jejunum, Ileum|Ileum))
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*__Cerebral Cortex__ (-/+) for ((Digestive - Duodenum - Bulbus Mucosa |duodenum bulbus mucosa))
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