Digestion and absorption
The process of digestion involves intake of the food (Ingestion), breakdown of the foot into micromolecules (Digestion)absorption of these molecules into the blood stream (Absorption) the absorbed substances becoming components of cells (Assimilation) and their elimination of the undigested substances (Egestion) a digestive system includes the alimentary canal and associated digestive gland.
STRUCTURE OF THE ALIMENTARY CANAL
The alimentary canal is continuous muscular digestive tract that begins with an anterior opening the mouth and opens out posteriorly through the anus. The alimentary canal consists of mouth, buccal cavity, rectum and anus. The mouth is concerned with the reception of food and leads to the buccal cavity or oral cavity.
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Mechanical digestion is initiated in the buccal cavity by chewing with the help of teeth and tongue. Chemical digestion is through salivary enzymes secreted by the salivary glands. Each tooth is embedded in a socket in the jaw bone ; this type of attachment is called thecodont. Human beings and many mammals form two sets of teeth during their lifetime, a set of 20 temporary milk teeth which gets replaced by a set of 32 permanent teeth. This type of dentition is called diphyodont. The permanent teeth are of four different types namely Incisors (I) chisel like cutting teeth, Canines (C) dagger shaped tearing teeth, Premolars (PM) for grinding and Molars (M) for grinding and crushing. Arrangement of teeth in each half of the upper and lower jaw, in the order of I, C, PM and M can be represented by a dental formula, in human the dental formula is 2123/2123.
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Mineral salts like calcium and magnesium are deposited on the teeth and form a hard layer of Calculus called plague. If the plague formed on teeth is not removed regularly, it would spread down the tooth into the narrow gap between the gums and enamel and causes inflammation, called gingivitis. which leads to redness and bleeding of the gums and to bad smell. The hard chewing surface of the teeth is made of enamel and helps mastication of food.
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Tongue is a freely movable muscular organ attached at the posterior end by the frenulum to the floor of the buccal cavity and is free in the front. It act as a Universal toothbrush and helps in Intake of food chew and Mix food with saliva swallow food and also to speak. The upper surface of the tongue has small projection called papillae with taste buds. The oral cavity leads into a short common passage for food and air called pharynx. The oseophagus and the trachea open into the pharynx. Food passes into the oesophagus through a wide opening gullet at back of the pharynx. A cartilaginous fap called epiglottis prevent the entry of food into the glottis during swallowing. Two masses of lymphoid tissue called tonsils are also located at the sides of the pharynx.
Oesophagus a thin long muscular tube concerned with conduction of the food to a 'J' shaped stomach passing through the neck, thorax and diaphragm. A copy of sphincter (Gastro oesphageal sphincter) regulates the opening of oesophagus into the stomach. If the cardiac sphicnter does not contract properly during the churning action of the stomach the gastric juice with acid may flow back into the oesophagus and cause heartburn result in GERD (Gastero Oseophagus Reflex Disorder).
The stomach functions as the temporary storage organ for food and is located in the upper left portion of the abdominal cavity. It consists of three parts - a cardiac portion into which the Oseophagus opens; a fundic portion and a pyloric portion that opens into the deodenum. The opening of the stomach into the deodenum is guarded by the pyloric sphincter. It periodically allows partially digested food to enter the duodenum and also prevents regurgitation of food. The inner wall of stomach has many Folds called gastric rugae which unfolds to accommodate a large meal.
The small intestine assist in the final digestion and absorption of food. It is the longest part of the alimentary canal and has three regions a 'U' shaped duodenum (25 cm long), coiled middle portion jejunum (2.4 M long), and highly coiled ileum (3.5 M long). The wall of the duodenum has Brunner's glands which secrete mucus and enzymes is the longest part of the Small intestine and opens into the caecum of the Large intestine. The ileal mucosa numerous vascular projections called villi which are involved in process of absorption and the cells lining the villi produce numerous microscopic projection called microvilla giving a brush border appearance that increase the surface area enormously. Along with villi, the ileal mucosa also contain mucus secreting goblet cells and lymphoid tissue known as Peyer's patches which produce Lymphocytes. The wall of the small intestine bears crypts between the base of villi called crypts of leiberkuhn. The large intestine consist of caecum, colon and rectum. The caecum is a small blind pouch like structure that opens into the colon and heat possesses a narrow finger like tubular projection called vermiform appendix. Both caecum and vermiform appendix are large in herbivorous animal and act as an important site for cellulose digestion with the help of symbiotic bacteria. The colon is divided into four regions - an ascending, a transverse, a descending part and a sigmoid colon. The colon is lined by dilations called haustra. The 'S' shaped sigmoid colon opens into the rectum. Rectum is concerned with temporary storage of faeces. The the anus is guarded by two anal sphincter muscles. The anal mucosa is folded into several vertical folds and contains arteries and veins called columns. Anal column may get enlarged and causes piles or haemorrhoids.
HISTOLOGY OF GUT
The wall of the alimentary canal from oesophagus to rectum consists of four layers namely serosa, muscularis, sub-mucosa and mucosa. The serosa is the outermost layer and is made up of thin squamous epithelium with some connective tissues. Muscularis is made up of smooth circular and longitudinal muscle fibres with the network of nerve cells and parasympathetic nerve fibres which controls peristalsis. The sub-mucosal layer is formed of loose connective tissue containing nerves, blood, lymph vessels and the sympathetic nerve fibres that control the secretion of intestinal juice. The innermost layer lining the lumen of the alimentary canal is the mucosa which secretes mucous.
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