Digestive system The digestive system consists of the

Digestive system

The digestive system consists of the digestive tract –oral cavity , esophagus , stomach , small and intestine , rectum and anus –and its associated glands – salivary glands , liver and pancreas. Its function is to obtain the molecules necessary for the growth and energy needs of the body.

Digestion begins in the mouth where food ground by the teeth ; saliva also initiate carbohydrate digestion , digestion continue in the stomach and small intestine. Water absorption occur in the large intestine , causes the undigested contents to become semisolid.

General structure of GIT It is a hollow tube composed of lumen whose diameter varies surrounded by a wall made uo of four layers : the mucosa , submucosa , muscularis and serosa.

The mucosa comprises form 1. An epithelial lining : A: To provide a selectively permeable barrier B: To promote the absorption of the products of digestion C: To produce hormones 2. And the muscularis mucosa , usually formed from thin inner circular and outer longitudinal layer of smooth muscle , The mucosa is frequently called the mucous membrane.

• The submucosa is composed of dense connective tissue with many blood and lymphatic vessels and a submucosal plexus (Meissnner's nerve plexus). It may contain glands and lymphoid tissue. • The abundant lymphatic nodules in the lamina properia and submucosa layer is to protect the epithelial lining , this is because of the majority of the epithelium of digestive tract is simple , thin epithelium. The lamina propria is rich in macrophage and lymphoid cells some of which produce antibodies (mainly Ig. A).

The muscularis contains two layers of smooth muscles inner circular and outer longitudinal layer , it also contain the Myenteric (Auerbach ) nerve plexus which lies between these layers. The contraction of the muscularis layer , generated and coordinated by nerve plexuses , propel and mix the food in the tract. These plexus is formed by nerve cell aggregate (small autonomic ganglandia) , the number of these ganglandia increase in region of greatest motility.

The serosa is a thin layer of loose connective tissue , rich in blood and lymphatics , adipose tissue , and a simple squamous epithelium (mesothelium ).

Hirschsprung disease

The oral cavity : This cavity is lined with keratinized or non keratinized stratified squamous epithelium. The keratin layer found in gum and hard palate , the lamina propria (LP) has several papillae , and rest on bone In the lips , a transition from the nonkeratinized to keratinized epithelial. can be observed.

Tongue The tongue is a mass of striated muscle covered by mucous membrane , the muscle fibers cross one another in three planes. Because the connective tissue of the LP penetrate the spaces between muscle fibers , the mucous membrane is strongly adherent to the muscle fibers. The mucous membrane is smooth on the ventral surface. The tongue dorsal surface is irregularly covered anteriorly by a great number of small eminence called Papillae. The post one third of the dorsal surface is separated from the anterior two third by a V-shaped boundary. Behind this boundary is lingual lymphoid tonsils.

Papillae : P are elevations of the oral epithelium and LP there are 4 types : 1 -Filiform p : 2 -Fungiform P : 3 -3 - Foliate P : 4 -Circumvallate P :

Tongue

• There are scattered mucous glands in the oral cavity act in the same way as the serous glands. • There are four qualities in human taste perception : saltiness , sourness , sweetness and bitterness.

Taste buds are onion shaped structures each one has 50 -100 cells , rest on basal lamina , in its apical portion the taste cells project microvilli that poke through an opening called the taste pore , each T bud is composed from taste cells and supportive cells with basal undifferentiated cells which replace all the cells of Taste bud.

pharynx Is a transitional space between the oral cavity and the respiratory and digestive systems , The pharynx is lined by nonkeratinized stratified squamous epithelial in the region continuous with the esophagus and by ciliated pseudostratified in the region continuous with the nasal cavity. The pharynx contains the tonsils , the mucosa also has many small mucous glands , dense connective tissue , the skeletal muscles located outside this layer.

Esophagus Histology: Clinical application: Gastroesophgeal reflux disease

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