Process of Digestion in Mouth, Stomach , Small and Large Intestine in Human body. MCQ

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Process of Digestion: In Mouth and Oesophagus

The process of digestion is accomplished by mechanical and chemical processes.
The buccal cavity performs the mastication. It is the process of biting and grinding the food with the help of teeth and saliva.
Mastication is followed by deglutition. It is the process of swallowing the masticated food (bolus).
The swallowed food is pushed down to the alimentary canal with the help of peristalsis.
The gastro-oesophageal sphincter controls the passage of food into the stomach.
The saliva secreted into the oral cavity contains electrolytes 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.

Process of Digestion: In Stomach

The stomach stores food for 4-5 hours.
The food mixes thoroughly with the acidic gastric juice in 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 enzymes of the stomach.
Pepsin converts proteins into proteases 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 pepsin.
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.

Process of Digestion: In Small Intestine

Various types of movements are generated by the muscularis layer of the small intestine.
These movements help in a thorough mixing of the food with various secretions in the intestine and thereby facilitate digestion.
The bile, pancreatic juice and 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, procarboxypeptidase, 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 biliverdin), 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 along with 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 along with the bicarbonates from the pancreas protects the intestinal mucosa from acid as well as provide an alkaline medium (pH 7.8) for enzymatic activities.
Submucosal glands (Brunner’s glands) also help in this.
Proteins, proteases and peptones (partially hydrolysed proteins) in the chyme reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice as given below:

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

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

Nucleases in the pancreatic juice act on nucleic acids to form nucleotides and 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 near the mucosal epithelial cells of the intestine

Process of Digestion: In Large 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 the 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 the ileocaecal valve, which prevents the backflow of the faecal matter.
It is temporarily stored in the rectum until defecation.