Structure of the stomach
It is a highly muscular, J-shaped organ.
It is located in the upper left portion of the abdominal cavity.
The stomach has three parts:

• Cardiac portion: The esophagus opens here.
• Fundic region
  
• Pyloric portion: It opens in the first part of small intestine.

The opening of the stomach into the duodenum is guarded by the pyloric sphincter. 
The innermost layer lining the lumen is the mucosa. This layer forms irregular folds called rugae in the stomach.
Mucosa form gastric glands in the stomach.
Function of the stomach
The gastric glands present in its walls secrete gastric juice containing hydrochloric acid (HCl) and enzymes, like pepsinogen. 
HCl activates pepsinogen into pepsin and kills bacteria.
Proteins are broken into smaller fragments called peptones by the enzyme called pepsin.
It eventually leads to the formation of chyme, which is moved to the intestine through pylorus.