Fundamentals of Human Nutrition/Absorption
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).Max absorption occurs here!
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
Within the study of nutrition, the word diffusion is regularly used when discussing the absorption of micronutrients, such as specific ions, and their net change across membranes from areas of high concentration to areas of low concentration. Since these particles move randomly (Philbert 2), diffusion requires no energy or proteins to take place, as items that “diffuse” through a membrane are small enough to pass through the lipid bilayer of cells. Nutrients within the gastrointestinal tract are of high concentration and through absorption and diffusion move into the bloodstream, an area that contains these nutrients in a lower concentration, to be utilized by the human body.
Within the study of nutrition, facilitated diffusion is very similar to normal diffusion in the fact that it too is a natural movement of molecules and large ions across a lipid bilayer membrane and down a concentration gradient. Facilitated diffusion, like normal diffusion, requires no energy and is passive; however, the ions and molecules are so large that they must be transported by transmembrane integral proteins (Pratt 264). These proteins “facilitate” the transport of macronutrients and large ions as they passively move down a concentration gradient.
Uses energy and protein pumps to move materials in and out of the plasma membrane (Singer). Great example is the sodium/potassium pump. Active transport which moves molecules against an electrochemical gradient requires energy (Karp, 2008). Active transport uses a transport protein, but uses energy in the form of ATP to move nutrients against the gradient, from areas of low concentration to areas of high concentration. Energy is needed to overcome the influences of diffusion and osmosis. The mechanism draws its energy from the absorbance of light, the hydrolysis of ATP, or other mechanisms that work together to move particles through their concentration gradients (Karp, 2008).
Endocytosis and Exocytosis
Endocytosis and Exocytosis occurs when molecules are too big to cross through a membrane using passive or active transport. The membrane folds back on itself to form a cavity or pouch where it surrounds and engulfs the molecules. Then, the membrane disassembles and the contents are released on the other side. Endocytosis occurs when molecules are entering a cell membrane, while exocytosis is when molecules exit a cell membrane.
Karp, Gerald. (2008) Cell and Molecular Biology (5th ed.). Hoboken, NJ: Wiley
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).
The circulatory system includes arteries, capillaries, veins, blood vessels and the heart. The heart is a major organ involved in the system, and its main function is to pump blood through the body. Messages tell the heart when to pump blood and whether it should pump a lot or a little depending on what is needed (Kids Health, 2015). The blood circulatory system can be very complicated and there are many parts and organ systems involved.The blood circulatory system consists of vessels in which the flow of blood is continuous with the heart, which acts as the pump. As blood moves through the vascular system, it transports materials as needed by picking them up and delivering them where necessary.
The route blood takes throughout the body is as follows: Blood in the right side of the heart leaves by passing through what is known as the pulmonary artery. This blood then loses carbon dioxide and in exchange gains oxygen in the lungs. It then moves through the pulmonary vein to the left side of the heart. Blood can also leave the left side of the heart though the aorta, which is the main artery propelling the blood through the body. From the aorta, blood can either go to the lower body, or to the head and upper body. The blood leaving the aorta that is in route to the lower body can then either make its way to the digestive tract followed by the liver, or to the legs, pelvis, and kidneys. After this, blood returns to the right side of the heart. Lymph, a colorless fluid that is derived from body tissue (this includes the digestive system), enters the bloodstream and is then brought up to the right side of the heart (Whitney & Rolfes, 2013).
Different organ systems play different roles in the vascular system. Body tissues derive nutrients from the blood and deposit waste such as carbon dioxide back into the blood. The lungs exchange carbon dioxide and oxygen, while the digestive system supplies nutrients. Finally, the kidneys act as a filter for wastes in the blood, (not including carbon dioxide) and excrete them in the urine. (Whitney & Rolfes, 2013)
Systemic circulation is what provides functional blood to all of the different body tissues in need. It carries oxygenated blood that is found in the left ventricle, out of the aorta, through arteries, to the capillaries found in tissue. The blood circulates in the capillaries, where certain exchanges occur with cells, and then collects into veins. The deoxygenated blood returns through these veins to the right side, or right atrium, of the heart. (Systemic Circulation, 2015). In short, blood travels this route: Heart → arteries → capillaries → veins → heart. (Whitney & Rolfes, 2013)
Whitney & Rolfes (2013). Understanding Nutrition, 14th edition. Cengage Learning.
Systemic Circulation (2015). Retrieved from: http://www.ncbi.nlm.nih.gov/pubmedhealth/PMHT0023062/
Kids Health (2015). Retrieved from: http://kidshealth.org/parent/general/body_basics/heart.html
Within the human body, the Lymphatic System is yet another organ system responsible for the absorption of nutrients. It consists of a collection of loosely organized ducts and vessels that move fluids in the direction of the heart. This system allows for the absorption of nutrients through a one-way passage for fluids from the tissue spaces to enter the blood (Whitney 82). The lymphatic system is dissimilar to the vascular system in that there is no pump mechanism; rather, lymph, a clear yellow tinted fluid lacking in red blood cells and platelets, moves around in the interstitial fluid and gathers into small vessels. Lymph vessels known as lacteals absorb fatty acid macronutrients within the gastrointestinal tract. While the majority of other micronutrients and macronutrients are assimilated into the body through the small intestine to be transported directly to the liver by the vascular system, only large fats and fat-soluble vitamins are transported by the lymphatic system. The nutrient rich fatty lymph, known as chyle is then transported and collected in the thoracic duct, the chief vessel of the lymphatic system, located dorsal to the heart. From the thoracic duct, the fluid is transported to the subclavian vein, where the lymph and its nutrients merge into the vascular system via bloodstream. The vascular system then transports the fatty acids and fat-soluble vitamins to where they are needed within the body.
Absorption of fats is more detailed than the absorption of other nutrients. Absorption of fats first starts with turning fats into water soluble molecules. Bile acids emulsify fats in order for digestive enzymes, such as lipase, to hydrolyze them and release free fatty acids and monoglycerides. Fats are then absorbed into the lipid bilayer of the enterocyte membrane through simple diffusion. In order for molecules to be transported in the bloodstream, cholesterol, triglycerides and other larger lipids are transported using lipoproteins. A large lipoprotein, called chylomicron, transports particles through the lymph vessels to be transported to the blood stream (Goodman, 2010). Smaller free fatty acids are transported directly into the bloodstream through the hepatic portal vein to the liver. The upper parts of the small intestine is the most abundant amount of the absorption of fat, but it is also absorbed in the ileum.
Blood and Sugar Absorption
Blood plays an important role in the transport of glucose molecules from the mucosal cells of the ileum to the portal vein. Sugars are affected by the amount of Na+ concentration in the intestinal lumen, if there is a high concentration of Na+ then sugar intake in the epithelial cells is inhibited. Glucose and Na+ share the same carrier molecule, which leads to Na+ and glucose moving in and out of the cell together when intracellular Na+ is low (Ganong, 1999). The Na+ is transported to the lateral intercellular spaces while the glucose diffuses into the bloodstream. Galactose also moves with glucose, however other sugar molecules have their own carrier, or are absorbed by simple diffusion (Ganong, 1999).
Vitamins are divided between fat soluble and water soluble. Fat soluble vitamins, such as vitamins A, D, E, and K, are first absorbed into the lymph and then the blood, many requiring protein carriers. These vitamins are stored in cells associated with fats and are less readily excreted. Water soluble vitamins, like B vitamins and vitamin C, are absorbed directly into the blood stream and travel freely and are readily excreted in urine.
Ganong, William F. (1999). Review of Medical Physiology (12th ed.). New York, NY: Appleton & Lange Goodman, Barbara E. (2010). Insights into digestion and absorption of major nutrients in humans. American Physiological Society. 34 (2), 44-53.
3.3.4 The method of Absorption
Once food has entered the digestive system, the body will take action toward absorbing its nutrients, approximately four to five hours after being masticated and ingested into the body. Most absorption occurs in the small intestine, which is carpeted with projections termed villi, which are composed of even smaller structures called microvilli. The villi are constantly in motion in an effort to capture any and all nutrients that come in their path, thus breaking them down even further with the help of digestive enzymes, such as pepsin and pepsinogen. Pepsin is the first enzyme in the breakdown of proteins, which is then finished off by accessory enzymes, which includes pepsinogen. Different classes of macromolecules, vitamins, and minerals are all absorbed via unique processes. Once a nutrient has been trapped by these structures, it will enter either the bloodstream or lymphatic system. Smaller molecules as well as water-soluble vitamins are usually transported directly to the bloodstream and then delivered to the liver. Larger molecules and fat-soluble molecules cluster into strcutures called chylomicrons and then enter the lymphatic system. The chylomicrons bypass the liver at first and enter the bloodstream at a point close to the heart. Since fats are insoluble in water, the formation of the chylomicron allows for the transport of these hydrophobic molecules through the bloodstream, which contains water, and then to their appropriate destination.
3.3.5 Optimizing absorption
It is an urban myth that the combination of one type of food with another when consuming a meal will inhibit the absorption of specific nutrients. On the contrary, it has been found that combining one food with another in a meal can actually optimize the absorption of nutrients in the different foods. By combining particular foods together, an individual can enhance their body’s ability to use each nutrient in the most sufficient manner. Studies have shown that overall absorption of nutrients is a result of the overall composition of a meal in contrast to it being entirely determined by an individual food item. For example, vitamin D is necessary for the absorption of calcium. Without it, an individual is prone to diseases such as osteomalacia (bone weakness due to a lack of vitamin D) or osteoporosis (bone weakness due to lack of calcium). So, the presence of both vitamin D and calcium in dairy products such as milk or yogurt will optimize the absorption of these nutrients. Without the presence of vitamin D, the calcium would pass through the body unrecognized and unabsorbed. Furthermore, another example could be that the vitamin C from some orange slices can aid in the absorption of the iron in a spinach salad. The bioavailability of nutrients can be enhanced in broader methods as well. For example, consuming a meal that contains both fat and fat-soluble vitamins will increase the absorption of those vitamins.
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