Approximately 9L of water enter the intestines, of which only about 100-200mL of water are excreted daily. The remainder are absorbed by the duodenum and jejunum (4L), the ileum (3.5L) and the colon (1.4L). Water can travel through the paracellular or trancellular routes. In the paracellular route, water travels through tight junctions, which are not permeable to macromolecules, but are permeable to water and electrolytes. In the trancellular route, water travels through the basolateral and apical membranes of cells. Water transport can happen through leaky or tight epithelia. In leaky epithelia (usually found in the small bowel), water is absorbed with solutes, which in turn drags in more solutes. In tight epithelia (usually found in the colon), basolateral membranes are highly permeable (to WHAT???), but apical membranes and tight junctions are impermeable.
- Highly selective sodium channels driven by an electrochemical potential, usually in the colon.
- Coupled with sugar or amino acids through a combined electrochemical potential, usually in the villi of the small bowel.
- Coupled with chlorine and potassium in a tri-transporter through a combined chemical potential difference, also in the villi of the small bowel
- By a sodium-hydrogen exchange through a combined chemical potential difference, usually in the villi of the small bowel.
Chlorine and bicarbonate
Chlorine and bicarbonate transport can be either paracellular or transcellular. In paracellular transport (usually in the colon), an electrical potential created by sodium transport is exploited. In transcellular transport, chlorine and bicarbonate are transported by one of three mechanisms:
- By secondary active transport systems
- Coupled with sodium and potassium in a tri-transporter in the small bowel
- By chlorine-bicarbonate exchange in the small bowel or colon
Potassium is predominantly absorbed paracellularly. Water absorption leads to higher concentrations of potassium in the lumen than in the extracellular fluid. A colonic active apical membrane (hydrogen-potassium)-ATPase enzyme, up-regulated by hypokalemia and acidosis pumps potassium into and hydrogen out of the lumen.