Physiology

On average, the luminal fluid load of the gut composed of oral intake and endogenous secretions is approximately 9 L per day.

Small intestine is very efficient in absorption and greater than 98% of fluid load (8.8 L per day) is absorbed. This efficiency is due to the presence of mucosal permeability to passive flux of water and electrolytes. Colon absorbs 1.8 L of fluid per day. Absorption in the colon occurs against high electrochemical gradients to make solid stool. Less than 0.2 L is excreted as stool.

Net intestinal secretion or absorption is the result of interaction of the effects of many different factors, including peptides, active amines, hormones, and neurotransmitters, that circulate in the blood or are released locally from mucosal epithelial cells, lamina propria cells, or enteric neurons. These factors can be classified as secretagogues or pro-absorptive agents, depending on the overall effect of the agent on intestinal cells.

Water transport appears to be largely a passive process that occurs through both paracellular and transcellular routes in the intestine, coupled with solute movement.

A variety of transport proteins are required for intestinal transport of water and electrolytes. Ion transporters can be grouped into three major classes: pumps, channels, and carriers. Pumps are energy driven and capable of transporting ions against large electrochemical gradients like Na+K+- ATPase.
Channels are ion selective and conductive like the Cystic fibrosis transmembrane regulator (CFTR).
Carrier transport proteins facilitate ion and nutrient transport across cell membranes.
Because this is not an active process, transport rate and direction are determined by the existing electrochemical gradients for the substrate.