![]() Learning Objectives Describe the role played by the small intestine in the absorption of nutrients Key Takeaways Digested food is able to pass into the blood vessels in the wall of the small intestine through the process of diffusion. Conversely when glucose level drops too low glucagon is secreted and restoresĪbsorption in the Small Intestine The absorption of nutrients occurs partially by diffusion through the wall of the small intestine. When blood glucose concentration rises too high, insulin is secreted which decreases the level. Then during succeeding hours, liver releases glucose back into blood to maintain a narrow range of blood glucose level. When after a meal blood glucose rises and insulin is secreted, two thirds of the glucose absorbed from gut is stored in the liver in the form of glycogen. Glucose absorption : Regulation of blood glucose level In a normal person the blood glucose level is narrowly controlled by following mechanisms: Liver acts as a blood glucose buffer system. Once into the cell, other transport proteins and enzymes cause facilitated diffusion of glucose through basal and lateral membranes of the cell into interstitial fluid and from there into the blood. Actually this protein drags glucose along with sodium ion from the lumen into the cell. The transport protein that helps in this case, has a peculiarity. ![]() As a result of low sodium inside the cells, sodium ions are transported from intestinal lumen by facilitated diffusion (diffusion with the help of transport protein). This leads to low sodium concentration inside the cell. In the first stage sodium ion from inside the cells are transported to interstitial fluid. The process of transport of glucose from intestinal lumen into the absorptive cell has two stages. Glucose is absorbed in small intestine by absorptive cells. Almost 80 percent of these monosaccharides are glucose. The carbohydrate foods we eat, are digested into monosaccharides (glucose, fructose, galactose). How Is Glucose Absorbed From The Gastrointestinal Tract? How Are Blood Glucose Levels Maintained?Ībsorption of glucose The transport of nutrients from intestinal lumen into blood stream is called absorption. Just after a meal, there will be abundant glucose in the lumen of the intestine, favoring absorption. Because transport of Na+ and glucose is coupled, we need to add the free energy inherent in Na+ transport to the free energy inherent in glucose transport to get the overall free energy for the process. Glucose is transported across the apical plasma membrane of the intestine by the sodium-glucose cotransporter (purple). Absorption Absorption is the means whereby nutrients such as glucose are taken into the body to nourish cells. Epithelial cells are able to transport substances in one direction across the epithelium because different sets of transporters are localized in either the apical or basolateral membranes. The basolateral plasma membrane faces the ECF. In the drawing, the apical plasma membrane is drawn as a wavy line, because intestinal epithelial cells have a high degree of apical plasma membrane folding to increase the surface area available for membrane transport (these apical plasma membrane folds are known as microvilli). ![]() The apical plasma membrane faces the lumen. Not only do tight junctions limit the flow of substances between cells, they also define compartments in the plasma membrane. For a substance to cross the epithelium, it must be transported across the cell's plasma membranes by membrane transporters. The epithelium forms a barrier because cells are linked by tight junctions, which prevent many substances from diffusing between adjacent cells. In the small intestine, for instance, the simple columnar epithelium forms a barrier that separates the lumen from the internal environment of the body (note that the internal environment in which body cells exist is the extracellular fluid or ECF). Epithelia form linings throughout the body. ![]()
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