Digestion of food
- The process of digestion is accomplished by mechanical and chemical processes.
Digestion of 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.
- 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 the process called deglutition.
- The bolus further passes down through the oesophagus by successive waves of muscular contractions called peristalsis.
- The salivary juice contains lysozyme and salivary amylase enzymes.
- Lysozyme present in saliva acts as an antibacterial agent that prevents infections.
- Salivary amylase acts on the starch and splits into a disaccharide maltose.
Digestion in stomach
- The mucosa of stomach has gastric glands, which have three major types of cells
- mucus neck cells which secrete mucus
- peptic or chief cells which secrete the proenzyme pepsinogen
- parietal or oxyntic cells which secrete HCl and intrinsic factor.
- The food mixes thoroughly with the acidic gastric juice of the stomach by the churning movements of its muscular wall and is called the
- The proenzyme pepsinogen, on exposure to hydrochloric acid gets converted into the active enzyme pepsin.
- Pepsin converts proteins into proteoses and peptones.
- 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.
Digestion in small intestine
- 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 is activated by an enzyme, enterokinase, secreted by the intestinal mucosa into active trypsin.
- The bile released into the duodenum contains bile pigments (bilirubin and bili-verdin), bile salts, cholesterol and phospholipids.
- Bile helps in emulsification of fats and also activates lipases.
- The intestinal mucosal epithelium has goblet cells which secrete mucus.
- The secretions of the brush border cells of the mucosa along with the secretions of the goblet cells constitute the succus entericus.
- The mucus along with the bicarbonates and Brunner’s glands from the pancreas protects the intestinal mucosa from acid as well as provide an alkaline medium.
- Proteins, proteoses and peptones in the chyme reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice
- Carbohydrates 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
- The enzymes in the succus entericus act on the end products of all the reactions.
- These final steps in digestion occur very close to the mucosal epithelial cells of the 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.
- The functions of large intestine
- 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.
Fig. Goblet cells