<P> Segmentation also occurs during and shortly after a meal within short lengths in segmented or random patterns along the intestine . This process is carried out by the longitudinal muscles relaxing while circular muscles contract at alternating sections thereby mixing the food . This mixing allows food and digestive enzymes to maintain a uniform composition, as well as to ensure contact with the epithelium for proper absorption . </P> <P> Every day, seven liters of fluid are secreted by the digestive system . This fluid is composed of four primary components: ions, digestive enzymes, mucus, and bile . About half of these fluids are secreted by the salivary glands, pancreas, and liver, which compose the accessory organs and glands of the digestive system . The rest of the fluid is secreted by the GI epithelial cells . </P> <P> The largest component of secreted fluids is ions and water, which are first secreted and then reabsorbed along the tract . The ions secreted primarily consist of H, K, Cl, HCO and Na . Water follows the movement of these ions . The GI tract accomplishes this ion pumping using a system of proteins that are capable of active transport, facilitated diffusion and open channel ion movement . The arrangement of these proteins on the apical and basolateral sides of the epithelium determines the net movement of ions and water in the tract . </P> <P> H and Cl are secreted by the parietal cells into the lumen of the stomach creating acidic conditions with a low pH of 1 . H is pumped into the stomach by exchanging it with K . This process also requires ATP as a source of energy; however, Cl then follows the positive charge in the H through an open apical channel protein . </P>

Absorption is a major function of the gi tract. how is absorption accomplished in the gi tract