Human internal organs include digestive system, respiratory system, urinary system and reproductive system. From top to bottom, they mainly include thyroid, trachea, upper aorta, superior vena cava, heart, lung, esophagus, diaphragm, liver, inferior vena cava, lower aorta, gallbladder, spleen, stomach, kidney, pancreas, small intestine, large intestine, ureter, appendix, bladder, rectum and so on.

The organizational structure of an organ is compatible with its functions. It is generally easier to notice some intuitive organs with concentrated tissues. Such as eyes, ears, nose, tongue and other sensory organs, heart, liver, lung, stomach, kidney and other internal organs. Many organs are easily overlooked and are not considered as organs. Such as any skeletal muscle, skin, etc.

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The energy needed by the human body in various physiological activities and the raw materials needed by the body in growth, development and tissue renewal are all supplied by food, and the nutrients contained in food must be digested and absorbed by the digestive system before they can be used by the human body. Food enters the mouth, after chewing with teeth and stirring with tongue, it is mixed with saliva and flows into the stomach along the esophagus through glossopharyngeal action. Stay in the stomach for 3 ~ 5 hours, fully mix with gastric juice for preliminary digestion, and push chyme into the small intestine through the movement of the stomach.

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