16.2 DIGESTION OF FOOD
16.2 DIGESTION OF FOOD:
- The process of digestion is accomplished by mechanical and chemical processes.
1. Digestion in buccal (oral) cavity:
- The buccal cavity performs two major functions, mastication of food and facilitation of swallowing.
- The teeth and the tongue with the help of saliva masticate and mix up the food thoroughly.
- Mucus in saliva helps in lubricating and adhering the masticated food particles into a bolus.
- The bolus is then conveyed into the pharynx and then into the oesophagus by swallowing or deglutition.
- 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.
- The saliva secreted into the oral cavity contains electrolytes (Na+, K+, Cl–, HCO3–) 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 hydrolysed here by this enzyme (optimum pH 6.8) into a disaccharide – maltose.
- Lysozyme present in saliva acts as an antibacterial agent that prevents infections.
2. Digestion in the stomach
- The mucosa of 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 (Castle's intrinsic factor essential for absorption of vitamin B12).
- 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 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).
- 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.
- HCl provides the acidic pH (pH 1.8) optimal for pepsins.
- Rennin is a proteolytic enzyme found in gastric juice of infants which helps in the digestion of milk proteins.
- Small amounts of lipases are also secreted by gastric glands.
3. Digestion in the 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 bile, pancreatic juice and the intestinal juice are the secretions released into the small intestine.
- 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.
- The mucus alongwith 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.
- Proteins, proteoses and peptones (partially hydrolysed proteins) in the chyme reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice as given below:

- 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 the fats into very small micelles.
- Bile also activates lipases.
The intestinal juice or succus entericus:
- The intestinal mucosal epithelium has goblet cells which secrete mucus.
- The secretions of the brush border cells of the mucosa alongwith the secretions of the goblet cells constitute the intestinal juice or succus entericus.
- This juice contains a variety of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc.

- The enzymes in the succus entericus act on the end products of the above reactions to form the respective simple absorbable forms.
- These final steps in digestion occur very close to the mucosal epithelial cells of the intestine.
- The breakdown of bio-macromolecules mentioned above occurs in the duodenum region of the small intestine.
- The simple substances thus formed are absorbed in the jejunum and ileum regions of the small intestine.
- The undigested and unabsorbed substances are passed on to the large intestine.
- No significant digestive activity occurs in the large intestine.
- The functions of large intestine are:
- (i) absorption of some water, minerals and certain drugs;
- (ii) secretion of mucus which helps in adhering the waste (undigested) particles together and lubricating it for an easy passage.
- The undigested, unabsorbed substances called faeces enters into the caecum of the large intestine through ileo-caecal valve, which prevents the back flow of the faecal matter.
- It is temporarily stored in the rectum till defaecation.
Rgulation of digestion:
- The activities of the gastro-intestinal tract are under neural and hormonal control for proper coordination of different parts.
- The sight, smell and/or the presence of food in the oral cavity can stimulate the secretion of saliva.
- Gastric and intestinal secretions are also, similarly, stimulated by neural signals.
- The muscular activities of different parts of the alimentary canal can also be moderated by neural mechanisms, both local and through CNS.
- Hormonal control of the secretion of digestive juices is carried out by local hormones produced by the gastric and intestinal mucosa.

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