Small intestinal epithelium is characterised by:
A large absorptive and secretory surface area created by villous processes and by microvilli on the luminal brush border;
Intercellular tight junctions, joining the lateral borders of adjacent enterocytes;
A basolateral membrane in close contact with the vasculature of the villi;
Functional polarity, with asymmetrical distribution of proteins involved in solute transport between the apical (luminal) and basolateral (serosal) surfaces of the cell;
Digestive enzymes are located on the apical surface and the ATPase sodium pump on the basolateral surface;
A characteristic electrochemical potential profile.
Electrolyte transport proteins
Exchangers and co-transporters
Na+ solute co-transporters H+ solute co-transporters Na+/H+ exchangers Cl- /HCO3- exchangers Na + /K + /2Cl- co-transporter
The epithelial cell types are:
Enterocytes, with absorptive and secretory functions; Goblet cells, which secrete mucus; Paneth cells, which produce antibacterial substances; Enteroendocrine cells;
Owing to the nature of distribution of cells, there is functional heterogeneity in electrolyte, fluid and nutrient transport along the crypt-villus axis.
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