The foundation of human life is the gastrointestinal organ since humans digest food and absorb nutrition by stomach and intestine after birth.

All kinds of food are digested and decomposed to absorbable states. The nutrients are absorbed and are further used. 

Therefore gastrointestinal dysfunction affects not only digestion and absorption, but also function and metabolism of other organs.

Food ingestion and excretion is a potential damaging factor of gastrointestinal disease and dysfunction. 

The stomach and intestine not only endure the physical and chemical stimulation (such as hot, cold, acid and alkali), but also digest and absorb food with 70% of the energy.

At the same time, the epitheliums of gastrointestinal mucosa regenerate in three-four days. Hence any change in gastrointestinal energy and nutritional supply can lead to dysfunction. This is the cause of multiple digestive system diseases and gastrointestinal dysfunction.
 
There are various reasons for damaging gastrointestinal mucosa:
 
1. Alcohol: alcohol can damage stomach mucosa in the following ways.

 
2. Chemical drugs: chemical drugs can erode the mucosa through interfering with the synthesis of prostaglandin and mucin.
 
3. Carcinogen in alimentary canal: gastrointestinal mucosa can be damaged after prolonged contact with carcinogen.

 
4. Cold and hot food: coffee, strong tea: these foods can cause hyperhydrochloria, stimulate gastrointestinal muscles and decrease the adherence of the mucosa and damage it.
 
5. Stress and nervousness: oxygen consumption of the brain decreases the blood supply of the gastrointestinal tract. Deficiency of blood and oxygen causes the gastrointestinal mucosa to necrotize.
 
6. Surgical trauma: the return of bile and the decrease in synthesis of mucin, leads to mucosa trauma.
 
7. Bacteria and toxin: bacteria and toxin not only can directly damage mucosa, but also damage mucosa by excreting protease and toxin.
 
8. Natural aging: decreasing the capability of synthesis and the capability of metabolism and mucus excretion; leading to the decrease of the mucosa barrier protection capability.
 
The wearing away of tissue can cause a sore or lesion called an ulcer. 

The digestive system is lined with a mucous membrane called the peritoneum. 

The peritoneum secretes mucus not only to mix with food and create chyme, but also to protect itself from the strong hydrochloric acid in the gastric juice, which could eat the membrane away. 

However, sometimes the acid prevails, and the mucosal lining of the digestive tract does develop ulcers.