Changes

==Introduction==

==Introduction==

[[Image:smalllargeintestine.jpg|thumb|right|250px|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]]

==Function==

==Function==

'''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. 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.

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.

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

Chyme is transported slowly in the large intestine. 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'''

'''Defense Against Pathogens:'''

''Secretions''

''Secretions''

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

Mucus inhibits contact and protects the mucosal surface. 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. Epithelial cells have a very high turnover rate, thus preventing pathogens with a longer life cycle from successfully colonising.

 

"Tight junctions" between epithelial cells prevent entry of macromolecules and pathogens. 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.   

 

''Cell-mediated and humoural defences''

Continuous peristalsis discourages persistence of toxins and aids in their elimination from the gut.   

*The lamina propria contains macrophages, B and T lymphocytes, plasma cells, and [[Mast Cells|mast cells]].

 

*Lymphoid aggregates known as [[Peyer's Patches - Anatomy & Physiology|Peyer's patches]] within the [[Small Intestine Overview - Anatomy & Physiology|small intestine]] aid in immunity.

''Cell-mediated and humoural defenses''

*Secretory [[IgA]] and [[IgM]] provide humoural immunity and help prevent attachment of pathogens to the intestinal epithelium.

 

*Lysozyme from Paneth cells inhibits bacterial growth (McGavin & Zachary, 2007).

The lamina propria contains macrophages, B and T lymphocytes, plasma cells, and [[Mast Cells|mast cells]]. Lymphoid aggregates known as [[Peyer's Patches - Anatomy & Physiology|Peyer's patches]] within the [[Small Intestine Overview - Anatomy & Physiology|small intestine]] aid in immunity. Secretory [[IgA]] and [[IgM]] provide humoural immunity and help prevent attachment of pathogens to the intestinal epithelium. Lysozyme from Paneth cells inhibits bacterial growth (McGavin & Zachary, 2007).

==Vasculature==

==Vasculature==

*The '''cranial mesenteric artery''' supplies the [[Caecum - Anatomy & Physiology|caecum]], ascending and part of the transverse [[Colon - Anatomy & Physiology|colon]]

The '''cranial mesenteric artery''' supplies the [[Caecum - Anatomy & Physiology|caecum]], ascending, and part of the transverse [[Colon - Anatomy & Physiology|colon]]. The '''caudal mersenteric artery''' supplies the rest of the transverse [[Colon - Anatomy & Physiology|colon]], the descending [[Colon - Anatomy & Physiology|colon]] and the [[Rectum - Anatomy & Physiology|rectum]].

*The '''caudal mersenteric artery''' supplies the rest of the transverse [[Colon - Anatomy & Physiology|colon]], the descending [[Colon - Anatomy & Physiology|colon]] and the [[Rectum - Anatomy & Physiology|rectum]].

==Innervation==

==Innervation==

*Similarly to the [[Small Intestine Overview - Anatomy & Physiology|small intestine]], there are pacemaker cells that generate an action potential.

Similarly to the [[Small Intestine Overview - Anatomy & Physiology|small intestine]], there are pacemaker cells that generate an action potential. Cells are able to function as a syncytium due to gap junctions, allowing the action potential to spread.

*Cells are able to function as a syncytium due to gap junctions, allowing the action potential to spread.

Contractions are generated in the forward (peristaltic) and backward (antiperistaltic) directions. Antiperistaltic contractions move ingesta into the caecum in some species. The formation of action potentials is under a much stronger neural influence than in the [[Monogastric Stomach - Anatomy & Physiology|stomach]] and [[Small Intestine Overview - Anatomy & Physiology|small intestine]]. The large intestine receives '''sympathetic''' and '''parasympathetic''' innervation. The sympathetic have coeliac, cranial mesenteric and caudal mesenteric ganglia. As the sympathetic fibres leave the ganglia, they surround their respective artery. Parasympathetic innervation increases the frequency of action potentials and thus stimulates peristalsis. Sympathetic innervation has the opposite effect. Neurones interact with the myenteric plexus to affect contractility, and with the submucosal plexus to affect secretions. Motility of the large intestine increases during meals, possibly as a result of gastrin and cholecystokinin secretion.

**Contractions are generated in the forward (peristaltic) and backward (antiperistaltic) directions. Antiperistaltic contractions move ingesta into the caecum in some species.

*The formation of action potentials is under a much stronger neural influence than in the [[Monogastric Stomach - Anatomy & Physiology|stomach]] and [[Small Intestine Overview - Anatomy & Physiology|small intestine]].  

*The large intestine recieves '''sympathetic''' and '''parasympathetic''' innervation.

**The sympathetic have coeliac, cranial mesenteric and caudal mesenteric ganglia.

**As the sympathetic fibres leave the ganglia, they surround their respective artery.

*Parasympathetic innervation increases the frequency of action potentials and thus stimulates peristalsis.

*Sympathetic innervation has the opposite effect.

*Neurones interact with the myenteric plexus to affect contractility, and with the submucosal plexus to affect secretions.

*Motility of the large intestine increases during meals, possibly as a result of gastrin and cholecystokinin secretion.

==Lymphatics==

==Lymphatics==

==[[Large Intestines - Histology|Histology]]==

 

 

 

 

==Histology==

 

 

There are numerous scattered lymph nodules in the large intestine. The number of lymph nodules increases compared to the [[Small Intestine Overview - Anatomy & Physiology|small intestine]]. The '''submucosa''' is also much reduced in thickness.

 

==Species Differences==

==Species Differences==

===Carnivore===

===Carnivore===

*Digestion is nearing completion by the time chyme enters the large intestine.

Digestion is nearing completion by the time chyme enters the large intestine. The majority of digestion occurs in the stomach (vomiting undigested bones etc) and small intestine and is reliant on enzymes. The dog and cat posses two [[Anus - Anatomy & Physiology#Species Differences|anal sacs]].

*The dog and cat posses two [[Anus - Anatomy & Physiology#Species Differnces|anal sacs]].

===Ruminant===

===Ruminant===

*Digestion is nearing completion by the time chyme enters the large intestine.

Digestion is nearing completion by the time chyme enters the large intestine. In ruminants, which are foregut herbivores, the majority of microbial fermentation occurs in the forestomach which consists of the [[Rumen - Anatomy & Physiology|rumen]], [[Reticulum - Anatomy & Physiology|reticulum]] and [[Omasum - Anatomy & Physiology|omasum]].

===[[Equine Alimentary System  - Anatomy & Physiology|Equine]]===

===[[Alimentary System  - Horse Anatomy|Equine]]===

*Most digestion occurs in the large intestine.

Most digestion occurs in the large intestine. Horses are [[Hindgut Fermenters - Anatomy & Physiology|hindgut fermenting]] herbivores and as a result have a very large [[Alimentary System - Horse Anatomy#Large Intestine|caecum]] where microbial cellulases digest their fibre rich diet. Taenia are present which slow transit and thus aid the digestive process.

*Taenia present.

===Porcine===

===Porcine===

*Taenia present

Taenia are present.

 

 

 

 

 

 

[[Category:Large Intestine - Anatomy & Physiology]]

[[Category:Large Intestine - Anatomy & Physiology]]

[[Category:To Do - A&P]]

[[Category:A&P Done]]