All of the actions that take place in the gastrointestinal system above the small intestine is done to prepare chime for absorption into the body. About ninety to ninety-five percent of all nutrition that is absorbed into the body through the small intestine. Prior to reaching the small intestine chyme has been mechanically broken down by chewing and mechanical grinding that takes place in the stomach. Enzymes within the food itself and in saliva started the process of chemical breakdown. These enzymes can effectively make up 70% of all digestion depending on how much the food was cooked and chewed. Stomach acids and pepsin denature proteins which begin to break the down and allow them to be completely digested within the small intestine .
When the chyme enters the small intestine it mixes with pancreatic enzymes and bile. These chemicals break it down further into small elements of proteins, fats, and carbohydrates. These small elements are capable of being absorbed .

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Absorption in the small intestine is possible due to a gradient of electrochemical properties across the lumen’s epithelial cell boundary. Without this gradient, nutrients would not be able to pass from the small intestine into the bloodstream. To remain functional, every cell in the body must maintain low intracellular concentrations of sodium. In polarized cells such as those in the epithelium, this low concentration is maintained through sodium pumps in the basolateral membrane. Sodium pumps remove 3 sodium ions from the cell while taking in 2 potassium ions in exchange. Therefore, sodium pumps create a gradient of electrochemical charge as well as sodium concentration throughout the basolateral membrane .

Water moves into cells through osmosis and is dependent on the absorption of solutes, especially sodium. Sodium is absorbed into cells in a few different way but typically in conjunction with glucose and amino acids, therefore efficient absorption of sodium relies on the absorption of these organic elements. Sodium that has been absorbed is quickly exported from the cell by their sodium pumps. Water disperses due to the osmotic gradient created by sodium. Water and sodium, then pass into capillary blood flow .

The brush border releases hydrolases which break down polysaccharides and disaccharides into monosaccharides which are small enough for absorption. Glucose is transported from the intestine across the epithelium and into the bloodstream. Both glucose and galactose are transported via the sodium-dependent hexose transporter or SGLUT-1. The molecule transports both sodium ions and glucose together. First, the transporter faces toward the lumen where it can bind with sodium ion. After the sodium binds to the molecule, it changes to open the pocket that binds with glucose. When this binding occurs both pockets close and moves into the cell. The sodium disconnects into the cytoplasm which causing the binding with glucose to become unstable. The glucose then disconnects into the cell as well which forces the molecule to return to its original position .

As the chyme reaches the end of the small intestine, almost all nutrients that are valuable in the body has been removed from the chyme and absorbed into the body. The elements of value are water, the electrolytes which are potassium, sodium, and chloride, proteins, carbohydrates and fats in the broken down forms of glucose, amino acids, and fatty acids, vitamins, minerals, phytochemicals, and antioxidants .

It is due to the mechanical and chemical breakdown of the nutrients that have taken place prior to reaching the small intestine, the addition of hyrolases which are released in the small intestine, and the addition of sodium pumps in the cells that line the lumen which make this location best suited for absorption throughout the gastrointestinal system.

    References
  • Barron, J. (2010, March 8). Physiology Of The Small Intestine, Part 1. Retrieved from Baseline of Health: https://jonbarron.org/article/physiology-small-intestine-part-1
  • Bowen, R. (2006, July 10). The Small Intestine. Retrieved from Colorado State University: http://www.vivo.colostate.edu/hbooks/pathphys/digestion/smallgut/index.html
  • Laster, L., & Ingelfinger, F. (1961). Intestinal Absorption — Aspects of Structure, Function and Disease of the Small-Intestine Mucosa. New England Journal of Medicine, 1192-1200.