Question

Describe physiology of digestion of food in human beings.

Answer 1

• The process of digestion is accomplished by mechanical and chemical processes. it starts in the mouth where the buccal cavity with the help of tongue and teeth masticates (chew) the food and thoroughly mix it with the saliva. Mucus in the saliva helps in lubricating and adhering the chewed into a bolus which is conveyed into pharynx and then into esophagus by swallowing or deglutition. The bolus passes further down the esophagus by successive waves of muscular contractions called peristalsis. The chemical digestion of the food is initiated in the oral cavity itself where the starch present in the food is acted upon by the salivary amylase present in the saliva and converts it into maltose.

• The food reaching the stomach is stored there for 4-5 hours and is mixed thoroughly with the acidic gastric juice by the churning movements of the muscular wall and is called chyme. The pro enzyme, pepsinogen present in the gastric juice is changed into active Pepsin by the action of hydrochloric acid. Pepsin is the proteolytic enzyme of the stomach which acts on proteins and converts it into proteoses and peptones. The mucus and bicarbonates prevents the stomach wall from the HCl. Rennin is a proteolytic enzyme found in gastric juice of infants which helps in digestion of milk proteins. Small amounts of lipases are also secreted by gastric juice.
• Various movements of the muscularis layer of the small intestine helps in a thorough mixing of food with the various secretions in the intestine. The bile, the pancreatic juice and the intestinal juice are the various secretions released into small intestine. Pancreatic juice is released through hepato-pancreatic duct into the duodenum. The pancreatic juice contains inactive enzymes- trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases and nucleases. Trypsinogen is activated by the enzyme, enterokinase into active trypsin which in turn activates the other enzymes in the pancreatic juice. Proteins, proteoses and peptones in the chyme reaching the intestine are acted upon by the proteolytic enzymes (trypsin, chymotrypsin, carboxypeptidases) of the pancreatic juice and are converted into dipeptides. Carbohydrates are hydrolysed into disaccharides by the action of pancreatic

amylase. Fats are broken down by lipases with the help of bile into di- and monoglycerides. Nucleic acids are acted upon by the nucleases to form nucleotides and nucleosides.

• Succus entericus act on the end products of the enzymatic activity of the pancreatic enzymes. Dipeptides are converted into amino acids by the dipeptidases found in succus entericus. Nucleotides forms sugars and bases by the action of nucleotidases. Di- and mono glycerides forms fatty acids and glycerol by the action of lipases. Maltose is converted into glucose + glucose by action of maltase. Lactose into glucose and galactose by action of lactase and Sucrose into glucose and fructose by sucrase. This breakdown is completed in the duodenum region and the products formed are simple and are absorbed in the jejunum and ileum region of the small intestine.
• The undigested and unabsorbed food is passed onto large intestine and after the absorption of water, minerals are drugs it is temporarily stored in rectum till defecation.