The small intestine is composed of the duodenum, the jejunum and the ileum.
The intestinal epithelium is specialized and, like other epithelia, these cells have different structures and proteins on the mucosal side, which is the side bathed by the solution in the gut lumen (where the food is), and the serosal side, which is bathed in extracellular fluid that communicates with the bloodstream.
Figure 1: Small intestine anatomy and structure
The intraluminal pH rapidly changes from highly acidic in the stomach to about pH 6 in the duodenum. It gradually increases in the small intestine from pH 6 to about pH 7.4 in the terminal ileum. The pH drops to 5.7 in the caecum, but again gradually increases, reaching pH 6.7 in the rectum. However, the rectal pH can vary with diet.
Typically, saline solutions used in the lab that are physiologically compatible with the jejunum are composed of Na+ 140 mmol/L, K+ 5 mmol/L, Ca2+ 2.5 mmol/L, Mg2+ 1.5 mmol/L, Cl- 100 mmol/L, HEPES/Tris, as this saline is similar in ionic concentrations to extracellular fluid.