The exocrine glands present in mucus layer secrete about 1.5 liter fluid mixed with salt and mucus daily. This fluid has no any type of enzymes in it and it is called intestinal juice. It has 98.4% of water in which most of the chemical reaction takes place that is responsible for digestion process. In fact, there is no any type of secretion of enzymes takes place in intestinal juice however digestive enzymes synthesizes in small intestine. These enzymes act inside or outside of epithelium cells present in the layer of lumen instead of secreting in lumen.     

       Bile and pancreatic juice make the chemical surface of the duodenum and small intestine alkaline. This alkaline surface stops the function of pepsin and helps in proper functioning of intestinal enzymes. These enzymes go on breaking carbohydrates, proteins and lipid (fat). As a result of it, carbohydrates are broken into disaccharides and monosaccharide; proteins are broken into peptide and amino acids and Lipid is broken into absorbable unite glycerol and free fatty acids. In this way, lipid is digested completely, but carbohydrates and proteins are digested in the next phase of digestion. 

       There are three types of enzymes found in the microvilli of the epithelium cells of small intestine, called brush border. They complete the digestion of carbohydrates and proteins.

Enterokinase- This enzyme converts inactive trypsinogen present in pancreatic juice into trypsin, which divides peptides along with few other enzymes into amino acid.  

Disaccharidases- Three types of enzymes- sucrose, lactase and maltase convert the remaining disaccharidases into normal monosaccharide and complete the digestion of carbohydrates. 

  • Sucrase- It converts sugarcane (sucrose) into glucose and fructose. 
  • Lactase- It converts milk sugar and lactose into glucose and galactose. After reaching in the liver, the galactose is being converted into glucose. 
  • Maltase- It converts maltose into glucose.

Aminopeptidases or Erepsin- This enzyme acts on polypeptides produces during the digestion of protein and converts it into amino acids. In this way, the digestions of carbohydrates, lipids and proteins are completed and its product is reached in blood circulation and lymphatic system after absorbed by small intestine.