DIGESTION AND ABSORPTION PART-2:PROCESS OF DIGESTION

           In the previous post, we have discussed the anatomy of digestive system in brief. In this post we are going to discuss the process of digestion. For the previous post click here.

Process of Digestion :

•It is completed in mechanical and chemical method. 

•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 such as Sodiam ion, Potassiam ion, Chloride ion, Bicarbonate ion etc. 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.

•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 (Cassle's Intrinsic factor)

(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.

•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 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 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 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.

1st step in convertion of macromolecules in simple observable form:

•Proteins, proteoses and peptones (partially hydrolysed proteins) in the chyme reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice named Trypsin/Chymotrypsin and Carboxypeptidase and got converted into dipeptides.

•Carbohydrates (Polysaccharides/starch)     in the chyme are hydrolysed by pancreatic amylase into disaccharides.

•Fats are broken down by lipases with the help of bile into di-and monoglycerides.

•Nucleases in the pancreatic juice acts on nucleic acids to form nucleotides and nucleosides.

Final step in forming absorbable form:

•The enzymes in the succus entericus act on the end products of the previous  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 biomacromolecules 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.

Excretion of Waste Products:

•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.

Influence of Hormone in Digestive System:

•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.