GLOSSARY

Water reabsorption in the gut is the absorption of about 98–99 percent of the roughly 8–9 litres of fluid that enter the intestine each day; it is passive and follows the absorption of solutes (especially sodium) by osmosis.

The Key Principle

Water always follows solutes: absorption of sodium, glucose, amino acids, and chloride creates an osmotic gradient, and water is absorbed by the transcellular route (aquaporins) and the paracellular route.

Sites of Reabsorption

• Small intestine: about 6–7 litres (about 80 percent); the jejunum is more permeable (rapid iso-osmotic absorption).
• Colon: about 1.5–2 litres, with a maximum capacity of about 4–5 litres per day; sodium is absorbed through ENaC channels (influenced by aldosterone).
• Lost in stool: about 100–200 millilitres.

Influencing Factors