Changes

==Introduction==

==Introduction==

[[Image:smalllargeintestine.jpg|thumb|right|150px|Illustration of the jejunum, caecum, transverse and ascending colon - © RVC 2008]]

[[Image:smalllargeintestine.jpg|thumb|right|250px|Illustration of the jejunum, caecum, transverse and ascending colon - © RVC 2008]]

 

The large intestine extends from the [[Ileum - Anatomy & Physiology|ileum]] of the [[Small Intestine Overview - Anatomy & Physiology|small intestine]] to the [[Anus - Anatomy & Physiology|anus]]. Water, electrolytes and nutrients are absorbed which concentrates the ingesta into faeces. Faeces are stored prior to defeacation. There is no secretion of enzymes and any digestion that takes place is carried out by microbes. All species have a large microbial population living in the large intestine, which is of particular importance to the [[Hindgut Fermenters - Anatomy & Physiology|hindgut fermenters]]. For this reason, hindgut fermenters have a more complex large intestine with highly specialised regions for fermentation.

The large intestine extends from the [[Ileum - Anatomy & Physiology|ileum]] of the [[Small Intestine Overview - Anatomy & Physiology|small intestine]] to the [[Anus - Anatomy & Physiology|anus]]. Water, electrolytes and nutrients are absorbed which concentrates the ingesta into faeces. Faeces are stored prior to defeacation. There is no secretion of enzymes and any digestion that takes place is carried out by microbes. All species have a large microbial population living in the large intestine, which is of particular importance to the [[Hindgut Fermenters - Anatomy & Physiology|hindgut fermenters]]. For this reason, hindgut fermenters have a more complex large intestine with highly specialised regions for fermentation.

===Development===

===Development===

The caecum, ascending and part of the transverse colon have already been considered in the [[Small Intestine Overview - Anatomy & Physiology|development of the small intestine]]. The hindgut forms the portion of the transverse [[Colon - Anatomy & Physiology|colon]] that lies to the left of the midline, the descending [[Colon - Anatomy & Physiology|colon]] and [[Avian Vent and Cloaca - Anatomy & Physiology#The Cloaca|cloaca]]. The anal membrane breaks down to allow communication with the exterior.     

 

The caecum, ascending, and part of the transverse colon have already been considered in the [[Small Intestine Overview - Anatomy & Physiology|development of the small intestine]]. The hindgut forms the portion of the transverse [[Colon - Anatomy & Physiology|colon]] that lies to the left of the midline, the descending [[Colon - Anatomy & Physiology|colon]] and [[Avian Vent and Cloaca - Anatomy & Physiology#The Cloaca|cloaca]]. The anal membrane breaks down to allow communication with the exterior.     

                                                                                            

                                                                                            

The large intestine can be divided into:

The large intestine can be divided into: The [[Caecum - Anatomy & Physiology|caecum]], [[Colon - Anatomy & Physiology|colon]], [[Rectum - Anatomy & Physiology|rectum]] and [[Anus - Anatomy & Physiology|anus]].

'''Water Absorption'''

'''Water Absorption'''

*Chyme entering the large intestine is semi solid. By the time chyme leaves the large intestine as faeces it is solid because most of the remaining water has been absorbed.

Chyme entering the large intestine is semi solid. By the time chyme leaves the large intestine as faeces it is solid because most of the remaining water has been absorbed. Water absorption is achieved by the generation of an osmotic gradient between the gut lumen and the enterocytes. Sodium is actively transported into the enterocyte and is followed passively by chloride ions to increase the osmotic potential of the enterocyte.

**Water absorption is achieved by the generation of an osmotic gradient between the gut lumen and the enterocytes. Sodium is actively transported into the enterocyte and is followed passively by chloride ions to increase the osmotic potential of the enterocyte.

 

'''Vitamin Absorption'''

'''Vitamin Absorption'''

*The large intestine is colonised by a large quantity of bacteria that produce vitamins as a consequence of their own metabolism. The large intestine absorbs these vitamins, in addition to vitamins present in the diet.

The large intestine is colonised by a large quantity of bacteria that produce vitamins as a consequence of their own metabolism. The large intestine absorbs these vitamins, in addition to vitamins present in the diet.

 

'''Transportation'''

'''Transportation'''

*Chyme is transported slowly in the large intestine.

Chyme is transported slowly in the large intestine. Chyme from the small intestine passes into the caecum and the colon in similar amounts.

*Chyme from the small intestine passes into the caecum and the colon in similar amounts.

 

'''Storage and Defecation'''

'''Storage and Defecation'''

*Remaining water, waste and undigested food is stored as faeces prior to defecation. Defecation is the periodical expulsion of faeces into the environment.

Remaining water, waste and undigested food is stored as faeces prior to defecation. Defecation is the periodical expulsion of faeces into the environment.

 

'''Defence Against Pathogens'''

'''Defence Against Pathogens'''

''Secretions''

''Secretions''

*Mucus - inhibits contact and protects the mucosal surface

Mucus inhibits contact and protects the mucosal surface. Digestive enzymes nonspecifically target bacteria and viruses. Bile kills some bacteria and viruses.

*Digestive enzymes - nonspecifically target bacteria and viruses

 

*Bile - kills some bacteria and viruses

''Epithelium''

''Epithelium''

*"Tight junctions" between epithelial cells prevent entry of macromolecules and pathogens into the intestinal tract.

"Tight junctions" between epithelial cells prevent entry of macromolecules and pathogens into the intestinal tract. Epithelial cells have a very high turnover rate, thus preventing pathogens with a longer life cycle from successfully colonising.

*Epithelial cells have a very high turnover rate thus preventing pathogens with a longer life cycle from successfully colonising.

 

''Commensal flora''

''Commensal flora''

*Commensal flora competitively inhibit attachment of pathogens to enterocytes in addition to competing for nutrition and substrates.  Many also produce inhibitory growth substances that are toxic to other bacteria (McGavin and Zachary, 2007).

Commensal flora competitively inhibit attachment of pathogens to enterocytes in addition to competing for nutrition and substrates.  Many also produce inhibitory growth substances that are toxic to other bacteria (McGavin and Zachary, 2007).

''Movement''

''Movement''

*Continuous [http://en.wikipedia.org/wiki/Peristalsis| peristalsis] discourages persistence of toxins and aids in their elimination from the gut.   

*Continuous [http://en.wikipedia.org/wiki/Peristalsis| peristalsis] discourages persistence of toxins and aids in their elimination from the gut.