The process of digestion is accomplished(साध्य) by mechanical and chemical processes.
Buccal cavity
Mechanical processesThe teeth and the tongue with the help of saliva masticate and mix up the food thoroughlyपूर्णपणे.
Mucus in saliva helps in lubricating and adheringचिकट the masticated food particles into a bolusअन्नाचा गोल,गोळा.
Pharynx & Oesophagus
The bolus is then conveyedपाठवणे, सुपूर्द into the pharynx and then into the oesophagus by swallowing or deglutitionगिळणे.
The bolus further passes down through the oesophagus by successive waves of muscular contractions called peristalsis.
The gastro-oesophageal sphincter controls the passage of food into the stomach.
Chemical processes
The saliva secreted into the oral cavity contains electrolytes (Na+, K+, CI-, HCO+) and enzymes - salivary amylase and lysozyme.
The chemical process of digestion is initiated in the oral cavity by the hydrolytic action of the carbohydrate splitting enzyme, the salivary amylase.
About 30 per cent of starch is hydrolyzed here by this enzyme (optimum pH 6.8) into a disaccharide - maltose.
Lysozyme present in saliva acts as an antibacterial agent that prevents infections.
Stomach
Mechanical processes
The stomach stores the food for 4-5 hours. The food mixes thoroughly with the acidic gastric juice of the stomach by the churning movements of its muscular wall and is called the chyme.
The mucosa of the stomach has gastric glands. Gastric glands have three major types of cells namely -
(i) mucus neck cells which secrete mucus;
(ii) peptic or chief cells which secrete the proenzyme pepsinogen; and
(iii) parietal or oxyntic cells which secrete HCl and intrinsic factor (factor essential for absorption of vitamin B 12).
Chemical processes
Pepsin -
The proenzyme pepsinogen, on exposure to hydrochloric acid gets converted into the active enzyme pepsin, the proteolytic enzyme of the stomach.
Pepsin converts proteins into proteoses and peptones (peptides).
Mucus and bicarbonates
The mucus and bicarbonates present in the gastric juice play an important role in lubrication and protection of the mucosal epithelium from excoriation by the highly concentrated hydrochloric acid.
Hydrochloric acid
HCl provides the acidic pH (pH 1.8) optimal for pepsins.
Rennin
Rennin is a proteolytic enzyme found in gastric juice of infants which helps in the digestion of milk proteins.
Lipases
Small amounts of lipases are also secreted by gastric glands.
Small intestine
Various types of movements are generated by the muscularis layer of the small intestine.
These movements help in a thorough mixing up of the food with various secretions in the intestine and thereby facilitate digestion.
The pancreatic juice, bile and the intestinal juice are the secretions released into the small intestine.
Pancreatic juice
Pancreatic juice and bile are released through the hepato-pancreatic duct.
The pancreatic juice contains inactive enzymes - trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases and nucleases.
Trypsinogen is activated by an enzyme, enterokinase, secreted by the intestinal mucosa into active trypsin, which in turn activates the other enzymes in the pancreatic juice.
Bile
The bile released into the duodenum contains bile pigments (bilirubin and bili-verdin). bile salts, cholesterol and phospholipids but no enzymes.
Bile helps in emulsification of fats, i.e. .. breaking down of fats into very small micelles.
Bile also activates lipases.
The intestinal mucosal epithelium has goblet cells which secrete mucus.
Intestinal juice
The secretions of the brush border cells of the mucosa alongwith the secretions of the goblet cells constitute the intestinal juice or succus entericus.
Succus entericus juice contains a variety of enzymes like disaccharidases (e.g .., maltase), dipeptidases, lipases, nucleosidases, etc.
The mucus along with the bicarbonates from the pancreas protects the intestinal mucosa from acid as well as provide an alkaline medium (pH 7.8) for enzymatic activities. Sub-mucosal glands (Brunner's glands) also help in this.
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