Briefly describe the absorption ways and mechanisms of nutrients.

The substances absorbed by the small intestine are not only those ingested by the mouth, but also water, inorganic salts and some organic components secreted by various digestive glands into the digestive tube, most of which will be reabsorbed in the small intestine. For example, the total amount of various digestive juices secreted into the digestive tube can reach 6-7L every day, and the water intake from the mouth exceeds 1 L every day, while the water lost from the feces is only about 1.50mL every day. Therefore, the amount of liquid reabsorbed into the body every day can exceed 8L. If such a large amount of water is not reabsorbed, it will seriously affect the relative stability of the internal environment and endanger life. This is the seriousness of a large amount of fluid loss in a short time during acute vomiting and diarrhea. Under normal circumstances, the small intestine also absorbs hundreds of grams of sugar, 100g or more of fat, 50- 100g of amino acids and 50- 100g of ions every day. In fact, the absorption capacity of the small intestine far exceeds this figure, so the absorption of the small intestine has a great reserve.