When the food enters into stomach G-cells secrete gastrin hormones which stimulate the secretion of gastric juice by gastric glands.
Secretion of gastric juice is controlled by nerve, hormones and chemical substances.

Some drinking substances also stimulates the secretion of gastric juice such a soup, alcohol, caffeine, histamine. These drinking substance and gastric juice stimulate the desire of appetite. So these substances are called appetiser juice.

Digestion by Rennin (Chymosin) :
- Rennin is active in the childhood stage of mammals only. It converts milk into curd like substance (clot the milk)
and then digests it. In adult stages, it is inactive .
- Rennin, acts on milk protein casein. Casein is a soluble protein .
- In presence of Rennin, casein gets converted into insoluble Ca-paracaseinate. This process is termed as Curdling of milk. After becoming insoluble, milk can remain in the stomach for a longer time. Rennin is absent in adult human (curdling of milk is done by HCI, pepsin and chymotrypsin in human).

Digestion by pepsin
Inactive pepsinogen on getting proper pH converts into active pepsin.
Protease :- The enzyme which breakes the peptide bond. These protease are of two types-
Exopeptidase :- The peptidase enzyme which breaks the outer and marginal bond of polypeptide called exopeptidase. In this process amino acid and polypeptides are formed.
Endopeptidase :- The peptidase enzyme which breaks the inner peptide bond of large polypeptide and forms the small polypeptides such as pepton, proteoses and peptides.
# Pepsin is an endopeptidase. It breaks proteins into smaller molecules.
Protiens ------> Peptones + Proteoses + Peptides
# In stomach, endopeptidases are found so only digestion of proteins can take place properly in the stomach.

Digestion by Gastric Lipase -
# It converts fats into fatty acids and monoglyceride. It is secreted in a less amount so less digestion of fats takes
place here.
# This lipase acts on emulsified fat and convert it into fatty acid and glycerol. 1% emulsified fat is present in food.
# Peristalsis continues during the process of digestion so the gastric -juice mixes properly with the food. Due to
peristalsis the food is converted into a paste. This form of food which is thick, acidic and semidigested in the stomach is called chyme.
# After short intervals, the pyloric sphincter keeps on opening and closing so the chyme is fed into
installments.

DIGESTION OF FOOD IN SMALL INTESTINE -
# In small intestine mechanical and chemical digestion occurs.

Mechanical digestion :
# This process of digestion mainly occurs by the help of segmentation. It is a mixing of kind with digestive juice
and bring food particle in to contact of mucosa.
# It starts with contraction of circular muscle this action constrict the intestine in small segment and further each
segment constrict from middle and divide each constricted segment again, finally first contracted fibre relax and
each small segment unite with adjoining small segment, so large segment form again.

Chemical digestion :
When food leaves the stomach through its pyloric end and enters the duodenum it is called chyme (acidic).
The intestinal mucosal epithelium has goblet cells which secrete mucus.
The secretions of the brush border cells of the mucosa alongwith the secretions of the goblet cells constitute the intestinal juice or succus entericus. This juice contains a variety of enzymes like enterokinase, aminopeptidase, disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc. The mucus alongwith the bicarbonates from the pancreas protects the intestinal mucosa from acid as well as provide an alkaline medium (pH 7 .8) for enzymatic activities. Sub-mucosal glands (Brunner's glands) also help in this.

No significant digestive activity occurs in the large intestine. The functions of large intestine are :
(i) absorption of some water. minerals and certain drugs
(ii) secretion of mucus which helps in adhering the waste (undigested) particles together and lubricating it
for an easy passage.
(iii) Escherichia coli (bacterium) lives in the colon which feeds on undigested matter. This bacterium, in turn
produces Vitamin - B12 , B1, B2 and K that are absorbed by the wall of the colon.

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. It is temporarily stored in the rectum defaecation.