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<big><center>[[Alimentary - includes liver and pancreas|'''BACK TO ALIMENTARY''']]</center></big>
<big><center>[[Alimentary System - Pathology|'''BACK TO ALIMENTARY''']]</center></big>
==Introduction to the Intestines==
=== Basic Anatomy and Physiology ===
The intestines are a portion of the alimentary tract extending from the pylorus to the anus. In mammals, the intestines are divided into two main segments: the small intestine and the large intestine. The small intestine, consisting of the duodenum, jejunum and ileum, acts as the main site of absorption for digestive products in the gastrointestinal tract. The large intestine, consisting of the caecum and colon, functions to extract water, electrolytes and nutrients as well as to concentrate and propulse faecal material to the rectum for defaecation.
Functional differences between the small and large intestine are reflected in their anatomic and histologic organisation (Figure 1). The small intestinal mucosa in many species is highly folded and made up of numerous villi that greatly increase the absorptive surface area. Enterocytes, the main absorptive cells, are the most numerous and characteristic cells of the small intestinal epithelium. Others include goblet cells, paneth cells, neuroendocrine cells, stem cells, and intraepithelial lymphocytes. Another important feature of the small intestine is the presence of Peyer's patches within the lamina propria. They contribute to immune function and generation of immune responses within the mucosa.
In contrast to the small intestine, the large intestinal mucosa has no villi and consists of 2 main cell types: absorptive cells and mucus secreting goblet cells. A distinguishing histologic and anatomic characteristic is the thick muscularis mucosa layer that aids in propulsion of faeces to the rectum. Another important feature is the presence of commensal bacteria which plays different roles depending upon the type of animal (Dyce et. al, 2002; Young & Heath, 2000).
=== Defense Mechanisms ===
'''Secretions'''
*Mucus - inhibits contact and protects the mucosal surface
*Digestive enzymes - nonspecifically target bacteria and viruses
*Bile - kills some bacteria and viruses
'''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.
'''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).
'''Movement'''
*Continuous peristalsis discourages persistence of toxins and aids in their elimination from the gut.
'''Cell-mediated and humoural defences'''
*The lamina propria contains macrophages, B and T lymphocytes, plasma cells, and mast cells.
*Lymphoid aggregates known as Peyer's patches within the 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).
=== Comparative ===
The natural diets of different animal species largely govern the structure and function of the gastrointestinal tract.
Broadly, animals can be classified as carnivores, herbivores or omnivores according to diet. Digestion and absorption of nutrients in carnivorous animals such as the dog and cat takes place largely in the small intestine. Because only a small amount of fermentation occurs in the large intestine of carnivores, they have a minimally developed caecum in comparison to nonruminating herbivores such as the horse which relies heavily on the end products of microbial digestion from the large intestine. The pig, an omnivorous animal has both a relatively long small intestine and an expanded portion of the colon that aids in fermentation of the fibrous material in the diet. Ruminants are an exception as fermentation largely occurs in the stomach (Reece, 2005).
'''References''' <br /br>
Dyce KM, Sack WO, Wensing, CJG: Textbook of Veterinary Anatomy, 3rd ed, p. 129. Saunders, London, England, 2002.
McGavin DM & Zachary, JF: Pathologic Basis of Veterinary Disease, 4th ed, pp. 301-393. Elsevier, St. Louis, Missouri, 2007.
Reece, WO: Functional Anatomy and Physiology of Domestic Animals, 3rd ed., pp. 312-368. Lippincott Williams & Wilkins, London, England, 2005.
Young B, Heath, JW: Wheater's Functional Histology: A Text and Colour Atlas, 4th ed, pp. 249-274. Churchill Livingstone, London, England, 2000.
==Contents==
===[[Intestines Developmental - Pathology|Developmental Anomalies]]===
===[[Physical Disturbances]]===
===[[Intestines Vascular Disturbances - Pathology|Vascular Disturbances]]===
===[[Intestines Proliferative - Pathology|Proliferative Pathology]]===
===[[Intestines Inflammation - Pathology|Inflammatory Pathology]]===
===[[Diarrhoea]]===
===[[Parasites]]===
===[[Diseases due to pathogens]]===
===[[Diseases in animal species]]===
===[[Clinical Pathology - Pathology|Clinical Pathology]]===
'''Intestine'''
'''Intestine'''