Science Biojournal- Entry 16

Stage 6: Small intestine

After being processed in the stomach, food is passed to the small intestine via the Pyloric sphincter. This is where the majority of digestion and absorption takes place as chyme enters the duodenum. Here it is further mixed with three different liquids:

1. bile, which emulsifies fats to allow absorption, neutralizes the chyme, and is used to excrete waste products such as bilin and bile acids (which has other uses as well). It is not an enzyme, however. The bile juice is stored in a small organ called the gall bladder.
2. pancreatic juice made by the pancreas.
3. intestinal enzymes of the alkaline mucosal membranes. The enzymes include: maltase, lactase and sucrase, to process sugars; trypsin and chymotrypsin are also added in the small intestine.

Most nutrient absorption takes place in the small intestine. As the acid level changes in the small intestines, more enzymes are activated to split apart the molecular structure of the various nutrients so they may be absorbed into the circulatory or lymphatic systems. Nutrients pass through the small intestine's wall, which contains small, finger-like structures called villi, each of which is covered with even smaller hair-like structures called microvilli. The blood, which has absorbed nutrients, is carried away from the small intestine via the hepatic portal vein and goes to the liver for filtering, removal of toxins, and nutrient processing.

The small intestine and remainder of the digestive tract then undergoes peristalsis to transport food from the stomach to the rectum and allow food to be mixed with the digestive juices and absorbed. The circular muscles and longitudinal muscles are antagonistic muscles, with one contracting as the other relaxes. As the circular muscles contract, the lumen becomes narrower and longer and the food is squeezed and pushed forward. When the longitudinal muscles contract, the circular muscles relax and the gut dilates to become wider and shorter to allow food to enter. In the stomach there is another phase that is called Mucus which promotes easy movement of food by wetting the food. It also nullifies the effect of HCl on the stomach by wetting the walls of the stomach as HCl has the capacity to digest the stomach.