Digestion and absorption

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

These include:

ATPase pumps

Exchangers and co-transporters

Na+ solute co-transporters H+ solute co-transporters Na+/H+ exchangers Cl- /HCO3- exchangers Na + /K + /2Cl- co-transporter

Ion channels

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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