The bile, pancreatic juice and intestinal juice are the secretions released into the Small intestine. Movements generated by the muscular layer of the small intestine helps in the thorough mixing of the food with various secretions in the intestine and thereby facilitate digestion. The pancreatic juice contains enzymes such as trypsinogen, chymotrypsinogen, carboxypeptidases, pancreatic amalyses, pancreatic lipases and nucleases. Trypsinogen is activated by an enzyme, enterokinase, secreted by the intestinal mucosa into active trypsin, which in turn activates the enzyme chymotrypsinogen in the pancreatic juice. The bile contains bile pigments '(bilirubin and biliverdin) as the break down products of hemoglobin of dead RBCs, bile salts, cholesterol and phospholipids but has no enzymes., Bile helps in emulsification of fats. Bile salts reduce the surface tension of fat droplets and break them into small globules. Bile also activates lipases to digest lipids. Proteins and partially digested proteins in the chyme on reaching the intestine are acted upon by the proteolytic enzymes of pancreatic juice. Trypsin hydrolyses proteins into polypeptides and peptones while chymotrypsin hydrolyses peptide bonds associated with specific amino acids. The pancreatic amylase converts glycogen and starch into maltose. Lipase acts on emulsified fat (triglycerides) and hydrolyses them into free fatty acid and monoglycerides. Monoglycerides are further hydrolysed to fatty acid and glycerol. Nucleases in the pancreatic juice break the nucleic acid into nucleotides and nucleosides. The secretions of the Brunner's gland along with the secretions of the intestinal glands constitute the intestinal juice or succus entericus. The enzymes in the intestinal juice such as maltase, lactase, sucrase (invertase), dipeptidases, lipases, nucleosidases act on the breakdown products of bile and pancreatic digestion.

The mucus along with the bicarbonate ions from the pancreas provides an alkaline medium (pH 7.8) for the enzymatic action. As a result of digestion, all macromolecules of food are converted into their corresponding monomeric units.

(i) Indigestion: It is a digestive disorder in which the food is not properly digested leading to a feeling of fullness of stomach. It may be due to inadequate enzyme secretion, anxiety, food poisoning, overeating and spicy food.
(ii) Constipation: In this condition, the faeces are retained within the rectum because of irregular bowel movement due to poor intake of fibre in the diet and lack of physical activities.
(iii) Vomiting: It is reverse peristalsis. Harmful substances and contaminated food from stomach are ejected through the mouth. This action is controlled by the vomit centre located in the medulla oblongata. A feeling of nausea precedes vomiting.
(iv) Jaundice: It is the condition in which liver is affected and the defective liver fails to break down haemoglobin and to remove bile pigments from the blood. Deposition of these pigments changes the colour of eye and skin yellow. Sometimes, jaundice is caused due to hepatitis viral infections.
(v) Liver cirrhosis: Chronic disease 'of liver results in degeneration and destruction of liver cells resulting in abnormal blood vessel and bile duct leading to the formation of fibrosis.
(vi) Gall Stones: Any alteration in the composition of the bile can cause the formation of stones in the gallbladder. The stones are mostly formed of crystallized cholesterol in the bile. The gallstone causes obstruction in the cystic duct, hepatic duct and also hepato-pancreatic duct causing pain, jaundice and pancreatitis.
(vii) Appendicitis: It is the inflammation of the vermiform appendix, leading to severe abdominal pain. The treatment involves the removal of appendix by surgery. If treatment is delayed the appendix may rupture and results in infection of the abdomen, called peritonitis.
(viii) Hiatus hernia (Diaphragmatic hernia): It is a structural abnormality in which superior part of the stomach protrudes slightly above the diaphragm. In some people, injury or other damage may weaken muscle tissue, by applying too much pressure (repeatedly) on the muscles around the stomach while coughing, vomiting, and straining during bowel movement and lifting a heavy object. Heart bum is also common in those with a hiatus hernia. In this condition, stomach contents travel back into the oesophagus or even into oral cavity and causes pain in the centre of the chest due to the eroding nature of acidity.

Digestion in the stomach:
1. Food remains in the stomach for 4 to 5 hours, the rhythmic peristaltic movement churns and mixes the food with gastric juice and make it into a creamy liquid called chyme.
2. The gastric secretion is partly controlled by autonomic reflexes.
3. The secretion of gastric juice begins when the food is in the mouth.
4. The gastric juice contains HCI and proenzymes. The proenzyme pepsinogen, on exposure to HCI gets converted into the active enzyme pepsin which converts proteins into proteoses and peptones (peptides).
5. The HCI provides an acidic medium (PH1.8) which is optimum for pepsin, kills bacteria and other harmful organisms and avoids putrefaction.

6. The mucus and bicarbonates present in the gastric juice play an important role in lubrication and protection of the mucosal epithelium from the eroding nature of the highly acidic HCI.
7. Another proteolytic enzyme found in gastric juice of infants is rennin helps in the digestion of milk protein, caseinogen to casein in the presence of calcium ions. This enzyme secretion gradually reduces with aging.

(a) Stomach
(i) The acidic medium of gastric juice (1.8 pH) mainly aids in the digestion of proteins
(ii) The digestion of proteins starts in the stomach.
(iii) After a meal gastric artery supplies more oxygenated blood for the stomach for the digestion of proteins.
(iv) HCI secreted by oxyntic cells activates the pepsinogen and rennin for the digestion of proteins.
(v) Inactive pepsinogen and inactive rennin are the precursors of active enzymes found in gastric juice secreted by parietal cells
Enzymatic reactions
(i) Proteins + pepsinogen + HCl → pepsin
(ii) Pepsin proteins → polypeptides + tri, dipeptides + amino acids
(iii) Rennin + HCl + milk protein casein → curd
(b) Duodenum
(i) Pancreatic juice containing inactive enzymes like chymotrypsinogen and trypsinogen and active carboxyl peptidase involves in the digestion of proteins
(ii) The catalytic enzyme converts in active enzyme chymotrypsinogen into active chymotrypsin and inactive, trypsinogen into active trypsin
Enzymatic reactions:
(i) Chymotrypsin + proteins → polypeptides + tri, dipeptides and amino acids
(ii) Trypsin + protein →polypeptides + tri, di peptides and amino acids
(iii) Carboxyl peptidase + polypeptides + tri, di peptides → free amino acids
(c) Small intestine
(i) Intestinal juice or succus entericus contains an enzyme peptidase
Enzymatic reaction
Dipeptides, Tripeptides + Peptidase → Free amino acids

(i) The mechanical stimulation of smell, sight and taste is responded.
(ii) This stimulation results in the secretion of saliva.
(iii) Grinding, chewing, cutting and tearing of food in the mouth refer to mechanical functions by teeth. It is called as mastication.
(iv) Water, electrolytes (Na⁺, K⁺, Cl^-, HCO₃^-), salivary amylase (ptyalin).
(v) An antibacterial enzyme lysozome and a lubricating agent mucus (glycoprotein) are the constituents of saliva.
(vi) 30% of starch is hydrolysed by ptyalin.
(vii) Cooked starch in ingested food is converted into a disaccharide (maltose)