Small intestinal fluid is secreted by the small intestinal glands in the mucosa of the small intestine. It is weakly alkaline in nature. In adults, it is secreted in an amount of about 1 to 3 L per day. small intestinal fluid is secreted and absorbed at the same time, and the exchange of this fluid provides a medium for the absorption of nutrients in the small intestine. In addition to water and electrolytes, small intestinal fluid also contains mucus, immune proteins and two enzymes: enterokinase and small intestinal amylase. In the past, it was thought that the small intestinal fluid also contained various other digestive enzymes, but it has been shown that the other digestive enzymes are not secretions of the small intestinal glands, but are present in the epithelial cells of the small intestinal mucosa. They are several peptidases that break down polypeptides into amino acids and several monosaccharidases that break down disaccharides into monosaccharides. After nutrients are absorbed into the epithelial cells, these digestive enzymes continue to digest the nutrients. As the epithelial cells at the tip of the villi are shed, these digestive enzymes enter the small intestinal fluid. The low pH in the small intestine prevents the solubilization of certain primary bile acids and reduces the formation of lipid microclusters, which are necessary for the absorption of fatty acids and monoglycerides. Patients with gastrinoma may have malabsorption of vitamin B12 independent of endocannabinoids. Although gastric secretion of endocannabinoids is normal, low pH in the small intestine interferes with endocannabinoid facilitation of vitamin B12 absorption in the distal jejunum, which is restored when the luminal pH is adjusted to 7. Low pH in the small intestine is commonly seen in gastrinomas. Related tests for low pH in the small intestine: Gastric fluid pH (pH): Normal gastric fluid pH is 0.9 to 1.8, if pH < 0.9 is too acidic, pH 3.5 to 7.0 is low acid, and pH > 7.0 is no acid. Various drugs affecting gastric acid secretion, such as omeprazole and cimetidine, as well as drugs affecting the acidity of gastric juice, such as sodium bicarbonate, were discontinued 3 days prior to gastric fluid collection. Basal gastric acid secretion measurement (BAO): Basal gastric acid secretion (BAO) refers to the amount of gastric acid secretion in the absence of food or drug stimulation for a continuous period of time. Gastrin: Gastrin (G) is an important gastrointestinal hormone, mainly secreted by G cells, which are mainly distributed in the gastric sinus and metabolized in the kidneys, and serum gastrin is increased in renal insufficiency. Gastrin secretion is mainly affected by the expansion of the gastric sinus by food, and its secretion is reflexively promoted by vagal excitation or increased by the chemical stimulation of food and its decomposing food, while increased gastric acid and sympathetic excitation inhibit gastrin secretion.