Water and electrolyte absorption in the gastrointestinal tract

ABSTRACT

Absorption of water and electrolytes in the gastrointestinal tract are dynamic processes which are closely coupled to each other. The luminal surface of the small intestine is so organized that the surface area available for contact with the intestinal contents is greatly amplified. Water is absorbed passively throughout the GI tract in response to osmotic pressure gradients set up by absorption of solutes, mainly that of sodium chloride, glucose, and amino acids. Sodium may be actively or passively absorbed. In the jejunuma Na+, K+, and Cl- are absorbed mainly by passive solvent drag or convection. In the ileum active absorption of sodium occurs. The soduim absorbed into the cell is actively pumped into the intercellular space by the basolateral membrane Na+ pump. The human colon is responsible for the final modification of the 500-1000 mls of fluid that enter it daily. This fluid is isotonic whith plasma in Na+ concentration. Approximately 80 - 90 percent of the water and more than 90 percent of the Na+ and Cl- that enter the colon daily are absorbed. Several pathological conditions and mechanisms that may result in net fluid and electrolyte secretion have also been discussed (Summary)