16.2 Digestion of food

The process of digestion is accomplished by mechanical and chemical processes. 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– , 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 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.

Starch----(Salivary Amylase/pH 6.8)---> Maltose

The mucosa of stomach has gastric glands. Gastric glands have three major types of cells namely –

  • mucus neck cells which secrete mucus;
  • 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 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. 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:

Proteins, peptones, Proteoses ---------------(Carboxypeptidase , Trypsin/Chymotrypsin)-------> Dipeptides

 

Carbohydrates in the chyme are hydrolysed by pancreatic amylase into disaccharides.

Polysaccharides (starch) -----(Amylase)------> Disaccharides

 

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

Fats ----(lipases)--> Diglycerides-------> Monoglycerides

 

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

Nucleic acids ------(Nucleases)-------> Nucleotides-----------------> Nucleosides

 

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.

Dipeptides -------(Dipeptidases)----------> Amino acids

Maltose -------(Maltase)-----------> Glucose + Glucose

Lactose -------(Lactase)------------> Glucose + Galactose

Sucrose -------(Sucrase)------------>  Glucose + Fructose

Nucleotides -------(Nucleotidases)------------>   Nucleosides -------(Nucleosidases)------------>   Sugars + Bases

Di andMonoglycerides -------(Lipases)------------>   Fatty acids + Glycerol

 

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. No significant digestive activity occurs in the large intestine.

The functions of large intestine are:

  • absorption of some water, minerals and certain drugs;
  • 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. 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 the local hormones produced by the gastric and intestinal mucosa.

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