14 Human Nutrition - part 04 -Physiology of digestion

 

14 Human Nutrition - part 04 -Physiology of digestion

Physiology of digestion :

  • Digestion process is carried out by both mechanical as well as biochemical methods.
  • Mechanical digestion includes 
  1. Mastication or chewing of food by teeth 
  2. churning in stomach and
  3. peristaltic movements of gastrointestinal tract
  • Chemical digestion is a series of catabolic (breaking down) reactions that hydrolyse the food. 

Digestion in the buccal cavity :

  •  Both mechanical and chemical digestion processes take place in mouth. 
  • Mastication or chewing of food takes place with the help of teeth and tongue. 
  • Teeth crush and grind the food. Tongue manipulates the food. 
  • Crushing of food becomes easier when it gets moistened by saliva. 
  • Mucus in the saliva lubricates the food as well as it helps in binding the food particles into a mass of food called bolus. 
  • The bolus is swallowed by deglutition. 
  • The tongue presses against the palate and pushes the bolus into pharynx.
  • Bolus further passes to the oesophagus.
     
  • The saliva contains 98% water and 2% other constituents like electrolytes (sodium, potassium, calcium, chloride, bicarbonates), digestive enzyme salivary amylase. 
  • The only chemical digestion that takes place in mouth is by the action of salivary amylase. 
  • It helps in conversion of starch into maltose. 
  • About 30% starch gets converted to maltose in mouth.
  • Saliva also contains lysozyme. It acts as an antibacterial agent that prevents infections.
  • The bolus further passes down through the oesophagus by peristalsis. 
  • Sometimes regurgitation or vomiting takes place due to reverse spasmodic peristalsis. 
  • Food from the oesophagus enters the stomach. 
  • The gastrooesophageal sphincter controls the passage of food into the stomach.

Digestion in the stomach : 

  • Both mechanical and chemical digestion takes place in stomach. 
  • The stomach stores the food for 4-5 hours. 
  • The physical digestion happens by churning of food. 
  • Thick muscular wall of stomach helps churning process. 
  • Churning further breaks down the food particles and also helps in thorough mixing of gastric juice with food. 
  • The mucosa layer of stomach has gastric gland. 
  • Each gastric glands has three major types of cells namely
  1. mucus cells
  2. peptic or chief cells and 
  3. parietal or oxyntic cells.


  • Mucus cells secrete mucus. 
  • Peptic cells secrete proenzyme pepsinogen. 
  • Parietal cells secrete HCl and intrinsic factor which is essential for absorption of vitamin B12. 
  • Thus, gastric juice contains mucus, inactive enzyme pepsinogen, HCl and intrinsic factor. 
  • In infants, stomach also secretes rennin. 
  • Mucus protects the inner lining of stomach from HCl present in gastric juice.
  • HCl in gastric juice makes the food acidic and stops the action of salivary amylase.
  • It kills the germs that might be present in the food. 
  • Pepsinogen gets converted into active enzyme pepsin in the acidic medium provided by HCl. 
  • In presence of pepsin, proteins in the food get converted into simpler forms like peptones and proteoses.
  • Rennin found in gastric juice of infants acts on casein, a protein present in milk. 
  • It brings about curdling of milk proteins with the help of calcium. 
  • The coagulated milk protein is further digested with the help of pepsin.
  • Rennin is absent in adults. At the end of gastric digestion, food is converted to a semifluid acidic mass of partially digested food is called chyme.
  • The chyme from stomach is pushed in the small intestine through pyloric sphincter for further digestion.


Digestion in the small intestine : 

  • In the small intestine, intestinal juice, bile juice and pancreatic juice are mixed with food.
  • Peristaltic movements of muscularis layer help in proper mixing of digestive juices with chyme.
  • Bile juice and pancreatic juice are poured in duodenum through hepato-pancreatic duct. 
  • Bile juice is dark green coloured fluid that contains 
  1. bile pigments (bilirubin andbiliverdin)
  2. bile salts (Na- glycocholate and Nataurocholate)
  3. cholesterol and phospholipid.

  • Bile does not contain any digestive enzyme.
  • Bile salts neutralise the acidity of chyme and make it alkaline. 
  • It brings about emulsification of fats. 
  • It also activates lipid digesting enzymes or lipases. 
  • Bile pigments impart colour to faecal matter.
  • Pancreatic juice secreted by pancreas contains 
  1. pancreatic amylases
  2. lipases and inactive enzymes trypsinogen and
  3. chymotrypsinogen.
  •  Pancreatic juice also contains nucleases- the enzymes that digests nucleic acids. 
  • The intestinal mucosa secretes digestive enzymes. 
  • The goblet cells of mucosa produce mucus. 
  • Mucus plus intestinal enzymes together constitute intestinal juice or succus entericus. 
  • The intestinal juice contains various enzymes like 
  1. dipeptidases
  2. lipases
  3. disaccharidases etc.


  • Both pancreatic and intestinal lipases initially convert fats into fatty acid and diglycerides.
  • Diglycerides are further converted to monoglycerides by removal of fatty acid from glycerol. 
  • The mucus and bicarbonates present in pancreatic juice protect the intestinal mucosa and provide alkaline medium for enzymatic action. 
  • Sub-mucosal Brunner's glands help in the action of goblet cells. 
  • Most of the digestion gets over in small intestine. 


Action of pancreatic juice :

  • Pancreatic amylase acts on glycogen and starch and convert those to disaccharides. 
  • Lipases hydrolyse fat molecules into fatty acids and monoglycerides. 
  • Inactive trypsinogen present in pancreatic juice is converted to its active form, trypsin. 
  • This conversion is brought about by enterokinase present in intestinal juice.
  • Trypsin converts proteins as well as proteoses and peptones to polypeptides. 
  • It also converts chymotrypsinogen to active chymotrypsin.
  • Chymotrypsin converts polypeptides to dipeptides.
  • Nucleases present in pancreatic juice help in digestion of nucleic acids to pentose sugar and nitrogenous base.




Action of intestinal juice :

The end of digestion of food is marked by Conversion of 

    1.  Proteins into amino acids
    2.  Fats to fatty acids and monoglycerides
    3.  Cucleic acids to sugar and nitrogenous base and
    4.  Carbohydrates to monosaccharides 
    • Food is now called chyle. 
    • Chyle is an alkaline slurry which contains various nutrients ready for absorption.
    • The nutrients are absorbed and undigested remains are transported to large intestine.
    • Remember, mucosa of large intestine produces mucus but no enzymes. 
    • Some carbohydrates and proteins do enter the large intestine. These are digested by the action of bacteria that live in the large intestine.
    • Carbohydrates are fermented by bacterialaction and hydrogen, carbon dioxide and methane gas are produced in colon.
    • Protein digestion in large intestine ends up into production of substances like indole, skatole and H2S. 
    • These are the reason for the odour of faeces. 
    • These bacteria synthesise several vitamins like B vitamins and vitamin K.
    • It is essential that the digestive enzymes and juices are produced in sequential manner and at a proper time. 
    • These secretions are under neurohormonal control. 
    • Sight, smell and even thought of food trigger saliva secretion.
    • Tenth cranial nerve stimulates secretion of gastric juice in stomach. 
    • Even the hormone gastrin brings about the same effect. 
    •  Intestinal mucosa produces hormones like 
    1. secretin
    2. cholecystokinin (CCK) and 
    3. gastric inhibiting peptide (GIP). 
    • Secretin inhibits secretion of gastric juice. It stimulates secretion of bile juice from liver, pancreatic juice and intestinal juice. 
    • CCK brings about similar action and induces satiety that is feeling of fullness or satisfaction. 
    • GIP also inhibits gastric secretion.


    do you know?

    • Pancreatitis is inflammation of the pancreas. 
    • It may occur due to alcoholism and chronic gallstones. 
    • Other reasons include high levels of calcium, fats in blood.
    •  However, in 70% of people with pancreatitis, main reason is alcoholism.

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