Fundamentals of Human Nutrition/Absorption
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Most digested molecules of food, as well as water and minerals, are absorbed through the small intestine. The mucosa of the small intestine contains many folds that are covered with tiny fingerlike projections called villi. In turn, the villi are covered with microscopic projections called microvilli. These structures create a vast surface area through which nutrients can be absorbed. Specialized cells allow absorbed materials to cross the mucosa into the blood, where they are carried off in the bloodstream to other parts of the body for storage or further chemical change. This part of the process varies with different types of nutrients
After traveling through the esophagus and esophageal sphincter, bolus enters the stomach. The stomach is known to be the “temporary storage unit for food (Wiley 2013).” While in the stomach, bolus is mixed with secretions from the stomach that are highly acidic. Once mixed, the bolus becomes chyme. Chyme is a mixture of incompletely digested food and stomach secretions. Some absorption does occur in the stomach, however, absorption mainly occurs in the small intestine. The stomach wall contains two layers of muscle, and in the lining of the stomach there are gastric pits with gastric glands that secrete gastric juice. Gastric juice contains water, mucus, hydrochloric acid, and pepsinogen. Gastric juice is stimulated and secreted by a hormone called Gastrin. Gastrin is secreted once food has entered the stomach and is signaled by the stretching of local nerves(Wiley 2013). Pepsinogen is also produced by the gastric glands, is a part of gastric juice, and is an enzyme that kills bacteria present in food. Pepsinogen is activated to form pepsin through the stomach acids, which breaks proteins into shorter chains of amino acids, therefore, assisting in digestion (Wiley 2013). Once the chyme has moved through the stomach it passes through the pyloric sphincter then enters the Small Intestine. Food remains in the stomach for roughly 4 to 5 hours before it is completely emptied. The pyloric sphincter helps to regulate the rate food empties from the stomach. When one eats a high-fat meal, chyme may stay in the stomach for a longer period of time because the gastrointestinal motility is slowed down by the release of certain hormones. Other aspects that could slow the emptying of the stomach are exercise, sadness, or fear (Wiley 2013). (Postlethwaite)
The contents of the stomach move to the small intestines for further digestion and absorption. The small intestine consists of multiple parts with the first one being the duodenum, a c-shaped hollow viscus (Luijkx & Jones, 2005). When the contents of the stomach first enter the duodenum it is very acidic. To neutralize the acidic contents, a combination of bile and alkaline juices secreted from the pancreas enter the duodenum in preparation for more digestion. Stomach and duodenum dysfunctions are very common and can lead to heartburn, indigestion and upper abdominal pain. Overall, the absorption of minerals, vitamins, and other nutrients begins at this first part of the small intestines (Stomach and Duodenum, 2015).
The second part of the small intestines consists of the jejunum and is around 3 to 6 feet long. The ligament of Treitz marks the distinction between the jejunum and the ileum. Villi cover the mucus membrane on the inner surface of the Jejunum and are used for absorption. In comparison to the duodenum and ileum the villi are longer (Belsley, 2015). After the contents of the stomach are broken down in the duodenum, it moves here where the inner walls of the jejunum absorbs the nutrients. There are many circular folds in this part of the small intestine, which increases the surface area for maximum absorption. The jejunum assists in further digesting the contents of the stomach by absorbing nutrients and water that can be used by the body. It is the proximal two-fifths of the small intestine, has a feathery appearance and is located I the left upper abdomen (Jones, 2005).
The last part of the small intestine consists of the ileum. During peristalsis, the muscular walls of the ileum mix and push food towards the large intestines. Located within the ileum are villi that increase the surface area for absorption. The nutrients absorbed here are transferred to the blood stream and liver. Water, some vitamins and fiber remain undigested and are broken down more towards the large intestine (What is the role of Ileum?, 2006).
The colon is the longest part of the large intestine and is located in the abdominal cavity. It is divided into four sections, the ascending, transverse, descending, and sigmoid colon. After passing through the small intestines, water, fiber, and some vitamins mix with mucus and bacteria to form feces. The feces will move through the colon and the lining of the colon will absorb some of the vitamins, minerals and water. Feces will continue moving down the colon until it reaches the walls of the sigmoid colon where they will contract and cause the feces to move into the rectum (Large Intestine, 2015)
The colon is the last stage of the digestion process, were remaining materials in foods are absorbed. The main material that is absorbed in the colon is water, while the colon also absorbs sodium ions and chloride ions (R. Bowen, 1998). Unlike the small intestine, the colon does not play a large role in the absorption of nutrients from food, but acts more as the final division between what the body wants to save and expel as waste (Sandle, 1998).
The structural components of the colon are as follows:
The cecum- The beginning of the large intestine that hold food material before it passes through the colon1. The ascending colon- a smaller tube structure that thins out the food material1. The right colic flexure- The right turn that the colon makes to wind itself throughout the gut1. The transverse colon- This is the largest part of the colon. It moves more than any other part and has a slightly concave formation1. The descending colon- The part of the colon that heads down towards the rectum1. The sigmoid colon (the left colic flexure)- The final left turn in the colon1. The anus- The end of the colon where excretion of fecal material occurs1. All structures found from the Canadian Cancer Society
The methods of absorption differ for each of the materials the colon absorbs. For the sodium ions, the ions are transported by the lumen, the space inside the colon, across the inner layer of the colon, called the epithelium, by active sodium pumps located in the membranes of the epithelial cells (R. Bowen, 1998).
The chloride ions are absorbed through the process of exchange. The colon secretes bicarbonate ions into the lumen which facilitates chloride absorption through the epithelial cells (R. Bowen, 1998).
Finally water is absorbed through the regular osmosis processes that the body uses in the small intestine. The water is then diffused into the blood from the lumen directly (R. Bowen, 1998). Once all the materials are absorbed from the digested food, all waste materials are expelled through fecal matter. The composition of most feces are 75% water and 25% solids, being most bacteria and undigested organic matter (R. Bowen, 1998).
The absorption processes in the colon are a vital step in maintaining body regularity. The water and other materials absorbed here make sure that nothing ingested goes to waste. The human body is a highly complex system of processes that require many specialized inputs so achieve efficiency, and the absorption processes in the colon make sure the body gets what it needs.
Anatomy and physiology of the colon and rectum - Canadian Cancer. (n.d.). Retrieved July 27, 2015, from http://www.cancer.ca/en/cancer-information/cancer-type/colorectal/anatomy-and-physiology/?region=bc Bowen, R. (1998). Absorption, Secretion and Formation of Feces in the Large Intestine. Retrieved July 27, 2015, from http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/largegut/absorb.html Bowen, R. (1995). Absorption of Water and Electrolytes. Retrieved July 27, 2015, from http://www.vivo.colostate.edu/hbooks/pathphys/digestion/smallgut/absorb_water.html Sandle, G. (1998). Salt and water absorption in the human colon: A modern appraisal. Retrieved July 27, 2015, from http://gut.bmj.com/content/43/2/294.full
The process in which materials are moved across a membrane caused by the presence of a concentration gradient. Materials move from high concentrations to low concentrations (Singer).
Uses carrier proteins to move materials across a concentration gradient (Singer).
Uses energy and protein pumps to move materials in and out of the plasma membrane (Singer). Great example is the sodium/potassium pump
3.3.3 The circulatory system
When glucose is absorbed, it enters the bloodstream. The concentration of glucose in the bloodstream is regulated by the liver and hormones that are secreted by the pancreas. It is pumped by the heart (Whitney 82).
Materials that are absorbed through the small intestine enter the lymph. One way route for fluid from the tissue spaces to enter the blood (Whitney 84).
Belsley, Dr. S. (2015). Your small intestine and digestion. Retrieved from http://www.laparoscopic.md/digestion/intestine Jones, Dr. J. (2005). Jejunum. Retieved from http://radiopaedia.org/articles/jejunum
Large Intestine. (2015). Retrieved from http://www.gesa.org.au/content.asp?id=100
Luijkx, Dr. T., & Jones, Dr. J. (2005). Duodenum. Retrieved from http://radiopaedia.org/articles/duodenum
Stomach and Duodenum. (2015). Retrieved from http://www.ddc.musc.edu/public/organs/stomach.html
What is the role of Ileum?. (2006). Retrieved from http://www.innovateus.net/health/what-role-ileum