Digestive+System+-+Physiology

=__THE DIGESTIVE SYSTEM__ ======== in progress= =__ANATOMY OF THE SYSTEM__=

Full system, labels.
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==__The Alimentary Canal__==

__ Structure of the mouth __ [tongue, teeth,]


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 * [[image:SalivaryGlands.jpg caption="SalivaryGlands.jpg"]] ||
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 * [[image:SalivaryGlands (2).jpg width="336" height="252" align="right" caption="SalivaryGlands (2).jpg"]] ||
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__ Structure of stomach __ [layers of wall, general shape, sphincters]


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__** Functioning of the stomach **__

The stomach is probably the most important processing organ of the digestive system. If the processing in the stomach is not completed for some or other reason, further processing of some nutrient may be problematic or may not happen at all.

As soon as saliva with "a taste of food" enters the stomach, Gastric juice is secreted. The secretion is regulated by neural and hormonal factors Some of these factors include: The presence of food or rising pH caused by the presence of saliva, cause the release of the hormone gastrin

Gastrin causes stomach glands to produce Protein-digesting enzymes, Mucus and Hydrochloric acid

Hydrochloric acid makes the stomach contents very acidic

The Acidic pH a ctivates pepsinogen (the inactive form of the enzyme pepsin) to pepsin for protein digestion

and Provides a hostile environment for microorganisms that may be present in the foods consumed.


 * __ The Functions of the Protein digesting enzymes __**

A.Pepsin—an active protein-digesting enzyme that breaks down the full protein molecules into smaller pieces [peptones and polypeptides] If the protein molecules are not broken down into large chunks, further processing is not possible.

B.Rennin—works on digesting milk protein in infants, not adults

Alcohol and aspirin are absorbed from the stomach into the blood and therefore its effect is felt very quickly after ingestion.


 * __ Processing of the food in the stomach __**

Food must first be well mixed with gastric juices and this is achieved by peristalsis that occurs in the lower stomach

Propulsion of the food and the Grinding action of the various layers of the stomach caused by the alternating contractions of these layers.

Retropulsion is achieved by the muscle movement in the lower regions of the stomach, virtually "throwing the food back where it came from" - the upper regions of the stomach and mixing with gastric juices continues until food is digested into a thick soup called chyme.

Chyme is acidic due to the high HCl content of gastric juice.

__** Propulsion of food into the Duodenum **__

The pylorus meters out chyme into the small intestine -about 3 - 5 ml at a time.

The stomach empties in 1̵ - 4 hours depending on the size of the meal and its content

Food leaves stomach- into coiled duodenum and h as to mix with bile & pancreatic juices

__ The Enterogastric Reflex __

Duodenal distension occurs as chyme accumulates and causes stretch receptors to send impulse to Medulla Oblongata

Gastric peristalsis is inhibited and the Pyloric sphincter tightens;. The food in duodenum is not propelled further and is allowed to m ix with juices & becomes less fluid and the food mass now becomes alkaline

The Pyloric sphincter re-opens – and the process is repeated until all chyme has exited the stomach.

__** Structure of Small and Large Intestines **__ [layers of wall, structure of villus]


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 * [[image:ileocaecaljunction.jpg width="236" align="right" caption="ileocaecaljunction.jpg"]] ||
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 * || [[image:smallintestine.jpg caption="smallintestine.jpg"]] ||
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 * ileocaecaljunction.jpg ||


 * [[image:IntestinalMucosa.jpg width="432" align="right" caption="IntestinalMucosa.jpg"]] ||
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 * || [[image:craniumcommander/EntericPlexus.jpg width="432" height="316" caption="EntericPlexus.jpg"]] ||
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__** Digestion in the SMALL INTESTINE **__

The first part of the Small intestine is called the Duodenum. The Pancreas sends pancreatic juice and Liver sends bile via ducts to the duodenum. These ducts open together into the duodenum at Oddi's sphincter. Pancreatic juice and bile are both alkaline and neutralises the acidity of the chyme and creates the alkaline medium in which the enzymes of the small intestine operates. Bile reduces the fluidity of the digested food mass and it moves along more slowly and can now be driven along by peristalsis. This slow movement allows for food to be adequately mixed and thus digested by the enzymes present.
 * __ BILE __ does not contain any enzymes. Being highly alkaline, it acts as an antiseptic in the small intestine. It emulsifies fats and plays a role in fat absorption. **

Pancreatic juice contains bicarb ions that also contributes to making the environment alkaline. PJ contains enzymes :
 * Amylase which breaks down starches into maltose
 * Lipase that breaks down fats into fatty acids and glycerol
 * Trypsinogen which is an inactive enzyme and becomes activated to active Trypsin in the alkaline environment by Enterokinase, an enzyme produced in the duodenum. Trypsin breaks down the peptides and peptones that exited the stomach as the first phase of protein digestion. These peptones are broken down into dipeptides and tripeptides,

The Glands in the Small intestine, crypts of Lieberkühn, secrete a number of enzymes that further and finally completes digestion
 * Lipase - fat digestion
 * Enterokinase (mentioned above) activates enzymes
 * Maltase - breaks down maltose into glucose
 * Erepsin - breaks down the di- and tripeptides into amino acids
 * Lactase - breaks down Lactose into glucose and galactose
 * Sucrase - breaks down Sucrose into glucose and fructose

All digestable foods are hopefully now digested. The human body is not designed to digest cellulose fibre, which is an important component of our diet to 'scrape' the alimentary canal clean and absorb and trap water to prevent the moving food mass from becoming dehydrated.

__ABSORPTION OF DIGESTED FOOD __

As soon as food is in its simplest form and it is close enough to blood vessels, it can move across into the blood vessel, some quicker than others.



Simplified diagrams to show movement of monomers of the large polymers that we ingest. Note the role of the bile salts in the absorption of the monomers of fats.

__ **The Accessory Glands** __ salivary glands - above with mouth

__ Structure and functions of liver __


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__ The Role of the liver in glucose metabolism under special conditions [diabetes] __


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__ Structure and functions of pancreas __


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 * || [[image:PancreasIsletLangerhans.gif width="295" height="199" align="right" caption="PancreasIsletLangerhans.gif"]] ||
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==__Blood supply to all structures involved__==

__The Hepatic Portal Blood system__


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 * __ PROCESSING AND CONTROL OF BLOOD SUGAR __**


 * || [[image:Glucosemetablism.png width="546" height="417" caption="Glucosemetablism.png"]] ||
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 * || [[image:gastricemptyingraph.jpg width="384" height="322" align="right" caption="gastricemptyingraph.jpg"]] ||
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 * __LEVELS OF INSULIN AND GLUCAGON FOLLOWING A MEAL RICH IN CARBOHYDRATES__**


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 * || [[image:InsulinGlucose.jpg width="313" height="270" align="right" caption="InsulinGlucose.jpg"]] ||
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Blue represents the glucose derived from the meal present in the blood. Insulin is secreted to remove the glucose from the blood. The diagram far right shows a cell of the body as it takes in glucose due to the presence of the insulin. The mitochondrion 'burns' the glucose for energy for the cell. Red represents the amount of insulin secreted over time Green represents the amount of glucagon over the same period.

Other actions of Insulin is depicted below:


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