Physiology and functions of Digesion
Most of the food is ingested in forms which are unavailable to the organism, since they cannot be absorbed from the gastrointestinal tract until they have been reduced to smaller molecules. The break down of the naturally occurring food stuffs into assimable forms is the work of digestion.
Digestion is defined as the break down of large insoluble food molecules into small water soluble molecules so that they can be absorbed into the plasma.
The chemical changes occurring in digestion are accomplished with the help of digestive enzymes which catalyze hydrolysis of proteins into amino acids, arch to monosaccharides and fats to fatty acids and glycerol. Thus digestion is a form of catabolism that is often divided into two processes : mechanical and chemical digestion. Mechanical digestion refers to the physical breakdown of large pieces of food into smaller pieces which can subsequently be accessed by digestive enzymes. In chemical digestion enzymes breakdown food into small molecules which the body can use. During the course of various digestive reactions, some of the minerals and vitamins present in the food are also changed into more assimable forms, so that they can be utilized by the cells for energy production.
Kinds of Digestion:
In animals, two kinds of digestive modes have been observed:
(1) Intracellular digestion: When digestion of food is carried out in food vacuoles inside the cell, it is known as intracellular digestion. This is characterstic of some lower animals like Protozoa, Porifera and Coelenterata.
(2) Extracellular Digestion: When digestion is completed in the lumen of alimentary canal, it is known as extracellular digestion. The extracellular digestion is an adaptation to deal with larger food particles which are gradually reduced in size. It is characterstic of higher animals like crustaceans, insects, cephalopods, tunicates and all vertebrates.
General Organization of the Alimentary Canal (in Mammals)
The alimentary canal of higher animals is a long muscular tube begining from the mouth and ending at the anus. A typical vertebrate gastro-intestinal tract is made up of the following histological layers from outside inward:
(a) Serosa. The serosa is the visceral layer of coelomic epithelium that covers all the visceral organs including the alimenlary canal. It is made up of flattened mesothelial cells and serves to prevent friction between organs.
(b) Muscle layer. Junt below the serosa lies the muscle cont which is composed of two layers of amooth muscles, the inner layer of circular muscles and the outer layer of longitudinal muscle fibres. The muscles provide the peristaltie movement for the panange of food.
(c) Submucosa. The submucosa in the thickest layer composed of dense elnatic connective tissue fibres with scattered fatty tissue. Fine capillaries of blood and lymph and nerve fibres traverse the entire submucosa.Intestinal glands are unually nunk deep into the submucosa and their secretion is controlled by the autonomic nerve fibres..
(d) Mucosa. The mucosa is the innermost layer of the gut. It is endodermal in origin and consists of three sublayers: the innermost layer is a simple columnar epithelial coat, often invaginated to form mucin or enzyme secreting glands; the middle layer, known as lamina propria, is a thin layer of connective tissue supporting the epithelial layer over basement membrane and contains small nodules of lymphatic tissue, capillaries and nerve fibres; the outermost layer or muscularis mucosa is a thin layer of smooth muscle fibres and elastic connective tissue arranged in two rings of longitudinal and circular fibres
Functional regions of the alimentary canal.
The alimentary canal of animals show striking adaptation to the nature of food and their feeding habite According to C.M. Yonge the alimentary canal of animals has the following important functional regions :
1. The region of reception. This region includes the mouth and mouth cavity (oral or buccal cavity). This region is primarily meant for food selection and mechanical breakdown of the food. The selection of food is made by taste, smell and texture. The mouth cavity contains teeth and tongue and salivary glands which secrete enzymes in a few mammals and the lubricating fluid, the mucous. The buccal cavity merges behind into a short chamber, the pharynx which leads posteriorly into the oesophagus through a wide aperture, the gullet.
2. The region of conduction : This region includes the oesophagus which is a long narrow, elastic and muscular tube. It runs straight down through the neck dorsal and parallel to the trachea. It pierces the diaphragm, enters the abdomen and joins the stomach.
3. The region of internal trituration and digestion: This region includes the stomach which is the broadest part of the alimentary canal. It is situated on the left side in the anterior part of the abdominal cavity. It is divided into two region: the larger anterior part is the cardiac stomach into which the csophagus opens and the smaller posterior part is the pyloric stomach. The stomach wall has umerous gastric glands. The stomach performs 3 main functions: storage of food, mechanical churning of food and its partial digestion.
4. The region of final digestion and absorption: This region includes the small intestine which follows the stomach. It is a long, narrow and much convoluted tube divided into three parts: duodenum, jejunum and ileum.
The duodenum is the smallest, first part into which the pyloric stomach opens. It forms a U-shaped loop enclosing the pancreas. A bile duct and a Pancreatic duct open into its distal limb. The lining of duodenum has numerous intestinal glands (crypts of Librekuhn) and characteristic Brunner's glad. The duodenum is followed by the jejunum behind which lies the is no clear distinction between the two but can be differentiated storical The jejunum is about 2.5 metres long, greatly coiled and suspended in t abdominal envity by folds of mesenteries. The internal lining is provided with numerous finger-like processes called villi which increase the absorptive area The ileum is the last part of the small intestine. It is about 3m long and contains villi similar to jejunum. It is also suspendend in the abdominal cairty by mesentery. Digestion of food in completed and its end products are absorbed in the small intestine. In herbivorous mammals a long thin-walled tube called caecum is presenta the junction of ileum and colon in which cellulose of plant material is digested bacterial action.
5. The region of faeces formation. This region includes the large intestine. It is the last region of the alimentary canal where absorption of water from the undigested food takes place and subsequently the undigested food materials are twisted together with mucus into faeces.
The large intestine is divided into two regions: colon and rectum. The len is continued into colon via the ileocaecal value. The colon is wide, about 45 cm long and has constricted wall. The rectum is a narrow terminal part, about 75 cm long, faeces is stored in it. The rectum opens to the outside through anus.
Functions of Digestive System:
The different functions of digestive system are:
1. Ingestion of food
2. Secretion of digestive juices and enzymes.not tou
3. Mastication of food
4. Mixing of food and digestive enzymes
5. Digestion of food.
6. Absorption of end vitamins,products of digestion, water, mineral, etc
7. Maintenance of water balance and blood sugar level in the body
Reference
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