The gastrointestinal tract starts with the mouth, which leads to the gullet via the stomach, small intestine, large intestine, and ends at the anus. In due course of the journey, the ingested food is broken down by both physical and chemical means to release nutrients which are absorbed into the blood stream. The ingested food is physically broken down in the mouth by chewing so as to reduce its size for increased surface area over which enzymatic reaction will take place. Enzymatic reaction is known as digestion and this is defined as the chemical breakdown of the ingested complex food molecules by the action of biological enzymes, into simplest form that can be absorbed into the blood stream and assimilated into living cells. In human beings, various components of the ingested food are digested and absorbed at various sections of the alimentary canal (Sullivan & Cooley, 2009).
According to Sullivan & Cooley (2009), the first digestion process starts at the mouth in which cooked starch is reduced into simpler sugars by the action of ptyalin. Digestion of the remaining cooked starch is completed by amylase in the small intestine where duodenum and pancreas take part. Absorption of simpler sugars as the products of digested starch, take place in the small intestine as well. The digestion of proteins starts in the stomach in which protein molecules are reduced into amino acids by the action of hydrochloric acid and the protein digestive enzymes called proteases. Digestion of the remaining proteins is completed at the small intestines by the action of trypsin, and the amino acids are absorbed into the blood stream. Digestion of lipids starts at the small intestines in which fats and oils are emulsified and digested by the enzyme called lipase into fatty acids. Vitamins and minerals are generated and absorbed in the large intestines. The discussion will focus on the digestion of ingested hamburger, French fries, and root beer in the 55 year old man's alimentary canal starting from the mouth up to the large intestines.
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Hamburger is defined as a sandwich comprised of cooked ground meat, normally placed in a chopped bread roll. This meal is usually served with bacon, lettuce, onion, tomato, and bacon. Therefore the meal of hamburger consists of proteins, fats, carbohydrates and vitamins. French fries are defined as elongated pieces of deep-fried potato and therefore majorly contain carbohydrates. For the French fries to test well, common salt should be added. Root beer is a beverage that is made from a combination of cherry tree bark, vanilla, sarsaparilla root, licorice root, sassafras root bark, molasses, anise, and nutmeg among other ingredients. Root beer contains some alcohol which is as a result of fermentation. The 55 year old chewed the hamburger and the French fries before he swallowed them.
Chewing ensured reduction of large pieces into small sized pieces to increase the surface area for an accelerated enzymatic action. In due course of chewing, the hamburger and French fries are mixed with saliva which was secreted by the salivary glands. Saliva was secreted after the tongue had sensed the food in the mouth, and it was comprised of water, ptyalin enzyme, and mucus. Both hamburger and French fries contained carbohydrates in the form of starch. Some portion of moistened starch was digested by the ptyalin into simple sugars such as glucose, while the remaining portion was digested later in the small intestines.
When the chewed food boluses were fully moistened and lubricated, they were pushed by the pharynx, which is at the back of the mouth, into the esophagus and continued the journey to the stomach by a process called peristalsis. The food boluses passed over the epiglottis which is a flap of cartilage covering the windpipe to avoid entrance of food particles into the windpipe. When food boluses reached at the end of esophagus, the sphincter muscle relaxed to release the food into the stomach. Once the boluses reached at the stomach, the gastric glands in the mucus membrane of the stomach were stimulated by the gastrointestinal hormone to secrete gastric juice. The major components in the gastric juice included hydrochloric acid, pepsin, rennin, mucus and water. Hydrochloric acid was very important because it activated the pepsin enzymes and kills all the germs that were ingested accidentally together with food.
The mucus protected the stomach from the effect of hydrochloric acid, while water played its role as a solvent for the entire mixture. Digestion of proteins present in the ingested hamburger started at the stomach and the remaining portion was digested last at the small intestines. The muscular walls of the stomach mixed the content thoroughly through a process called churning to form a semi liquid mass of partly digested food. Pepsin digests the hamburger proteins into amino acids, and the entire solution was released into the small intestine as the pyloric sphincter muscles at the bottom of the stomach relaxed.
The small intestine is a muscular tube that is long and coiled, and it is divided into three distinguishable parts namely, duodenum, jejunum, and ileum. Once the semisolid food arrived at the duodenum as the first part of the small intestine, pancreatic juice, and bile were secreted. The pancreatic juice was composed of bicarbonate solution, water and those enzymes designated to digest carbohydrates, fats, and proteins. The bicarbonate solution was alkaline and it ensures that the acidic content from the stomach is neutralized to favor the action of digestive enzymes. The bile was released from the gallbladder and it took part in emulsifying the fats and oils present, and in turn the emulsified lipids were digested by the lipase enzyme into fatty acids.
Duodenum was the first part of the alimentary canal where the digestion of lipids took place. Also in the duodenum the starch that was not digested in the mouth, and the proteins that were not digested in the stomach, underwent further digestion. Absorption of the alcohol part of root beer started at the duodenum. The content in the duodenum continued the journey to the jejunum which was the middle part of the small intestine and this is where most of digestion and absorption of food and alcohol occurred. The ileum is the last part of the small intestine, and this is where absorption of bile salts as well as the remnants of digested food which were not absorbed in the jejunum took place. The wall of ileum was highly folded and had numerous finger-like projections which enhanced maximum absorption of amino acids, fatty acids, glucose, and vitamin B12. Also in the ileum final digestion of proteins and carbohydrates took place. This final part of the small intestine is called the distal ileum.
Absorption from distal ileum
According to Sherwood (2010), absorption is the process by which digested food substances, water, mineral salts, and alcohol get out of the gut into the blood stream. Amino acids, glucose, fatty acids, water, vitamins, alcohol and mineral salts got their ways into the blood stream through the Superior Mesenteric Vein. The superior mesenteric vein is defined as a blood vessel that carried blood from the jejunum and ileum. The superior mesenteric vein combines with the splenic vein just behind the neck of the pancreas to form the hepatic portal vein. To the left of the superior mesenteric vein lies the superior mesenteric artery, whose origin is the abdominal aorta.
Blood entered the hepatic portal vein from where it was drained into the liver. In the liver, excess amino acids were turned into other usable molecules by the process known as de-amination. Through this process, nitrogen from the amino acids was converted into ammonia and this was further converted into urea by the liver in the urea cycle. Urea is later excreted through the kidneys. The alcohol that reached the liver could not be stored, but about 90 percent of it was broken down into water, carbon dioxide and fat. In the liver excess glucose was converted into glycogen by the insulin hormone which is secreted by the liver. Generally the liver played a significant role in eliminating toxic substances that were accidentally absorbed into the blood stream.
After the metabolism in the liver, the de-oxygenated blood moved from the liver via hepatic veins into the inferior vena cava which is also called the posterior vena cava. The posterior vena cava carried the de-oxygenated blood from the lower parts of the body into the heart's right atrium. Blood was pumped from the right atrium into the right ventricle via the tricuspid valve. The blood from the right ventricle of the heart is pumped into the pulmonary artery through the pulmonary valve and trunk. The de-oxygenated blood from the pulmonary artery moved into the lungs where it left carbon dioxide and carried oxygen. The oxygen-rich blood was carried from the lungs via the pulmonary vein to the left atrium.
Blood in the left atrium was pumped into the left ventricle through the bicuspid valve. The more muscular left ventricle pumped the blood into the systemic circulation via the aorta. Blood from the abdominal aorta was carried through the left renal artery into the left kidney. The blood that entered the left kidney was fully of oxygen and glucose for proper functioning of the kidney in due course of urine production. The blood in the renal artery had sufficient pressure for the success of blood filtration process. The left kidney received blood containing urea which was toxic and needed to be removed from the blood (Lesley, Joseph, Tom & Andrew, 2006).
The blood with urea entered the kidney Nephron through the afferent arteriole. The more pressurized blood forced its way through the Glomerulus which is a network of blood capillaries and products such as urea, water, and excess salts were filtered out into the Bowman's capsule by a process called ultra filtration. The Bowman's capsule was a cup-like sac at the head of a nephron's tubular component that performed the initial step in blood filtration to form urine. The urine passed through proximal convoluted tubule which was highly folded to maximize re-absorption of water and salts.
The urine from the proximal convoluted tubule passed through the descending and ascending Loop of Henle to the distal convoluted tubule which was highly folded to maximize re-absorption of water and sodium chloride. The urine out of the distal convoluted tubule was emptied into the collecting duct. The collecting duct emptied the urine into the ureter which carried the urine into the urinary bladder. Once the urinary bladder was full of urine the sphincter muscles at the base of the bladder relaxed due to high pressure exerted by urine, and released urine into the urethra through the penis as the exit from the body (Lesley, Joseph, Tom & Andrew, 2006).
Summary of homeostasis
Homeostasis can be defined as the process by which living organisms biologically regulate the internal environment for a maintained stable condition, through complex dynamic equilibrium adjustments that are controlled by integrated regulation mechanisms. The consumption of hamburger results into absorption of large quantities of amino acids at the distal ileum into the blood stream. The normal amount of amino acids in the blood stream is regulated by the de-amination processes of the liver. De-amination processes results into the production of urea. Urea is filtered in the kidney to result into clean blood. Urea mixes with water and sodium chloride to form urine which gets out of the body through the urethra.
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