Gastrointestinal Tract, a Sensory Organ Sensing the External Environment in the Body

Gastrointestinal (GI) tract seems to be the organ in body; however, its lumen is external environment because opening from mouth and anus. Moreover, it cannot thought of place where there are more than 1,000 species 100-trillion numbers microorganisms as an internal environment. Many multicellular animals including humans’ separates between by special tissues called epithelia. Barrier functions epithelia prevent invading luminal antigens, harmful substances, microorganisms, etc, so that all other cells excepting epithelial can leave expose them. However, on ground function nutrient ingestion, structure epithelium lining GI strengthen not much consists only one-layer simple columnar epithelium, different epidermis consisting a strong barrier stratified squamous Therefore, must have host-defense making up for weak simultaneously conduct contradictory ingestion prevention invasion substances microorganisms.