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The Digestion of Larry the Liver Sandwich
Larry, the Liver Sandwich, consists of lettuce, tomato, ketchup, butter, bread, onions, mayonnaise, and of course liver! Unfortunately, Larry is deliciously eaten by a human, and must be digested by the body so that the body can use the substances in the sandwich. The main nutrients found in the materials making up the sandwich are amino acids, found in the tomato and liver, carbohydrates, found in the onions and bread, water, found in the vegetables, lipids/fats, found in the butter and mayonnaise, and vitamins, found in all the vegetables. Dietary fiber is also found in some of the substances such as onions, lettuce, and bread. From the moment a human bites into the sandwich, the digestion process begins.
The food undergoes a form of mechanical digestion, in the mouth. The teeth of the human bite, and break down the food into smaller pieces. This is a form of mechanical digestion. Salivary glands in the mouth produce secretions, through ducts (tiny tubes), that are mixed with the food. The salivary glands in the mouth are called parotid gland, (lies at the sides of the face) sublingual gland (lies beneath the tongue), and submandibular gland (lies beneath the posterior floor of the oral cavity). The saliva contains water and an enzyme called salivary amylase that breaks down the ester bonds between the sugar molecules of large starches into smaller starches or maltose. Parts of the bread, onion and other bits having the presence of starch, would be broken down into smaller starches or maltose. The saliva also dissolves solid food to make it susceptible to the action of later intestinal secretions, and lubricates the mouth and esophagus for the passage of solids. Because the food in the mouth does not remain in the mouth for a long period of time, the food particles are not fully digested.
The tongue, a muscle that rolls food into a swallowable sized ball called a bolus, pushes it to the back of the mouth. When food is swallowed, the soft palate, located in the posterior part of the roof of the mouth, closes off the nasopharynx and the epiglottis covers the glottis, forcing the bolus to pass down the esophagus. The epiglottis prevents the food from entering the trachea. The bolus is moved down the esophagus from the muscle contractions called peristalsis. At the base of the esophagus, the bolus comes across a constriction; the cardiac sphincter closes and opens before the bolus can enter the stomach. The cardiac sphincter prevents the acidic contents of the stomach from backing up into the esophagus. After the partial chemical digestion, occurring in the mouth, and after the food has become bolus and has moved down the esophagus, the food go through mechanical digestion again in the stomach. The stomach has three layers of muscle that churn the food, causing the release of gastrin. Gastrin is a hormone that travels from the walls of the stomach into the blood stream. Gastrin affects the stomach as it moves through the body, causing the release of gastric juice. Gastric juice contains water, HCl, and pepsinogen. The HCl reacts with the pepsinogen to produce pepsin, an enzyme that digests proteins. The HCl kills bacteria and also provides the pepsin with an optimum pH of .5 to function in. Because the stomach has acidic juices, it contains a thick layer of mucous to protect the walls of the stomach. Under the influence of the acidic environment of the stomach, the bolus turns into a thick, soupy liquid called chyme. The chyme then moves through the pyloric sphincter, which repeatedly opens and closes, allowing chyme to enter the duodenum, the first section of the small intestine, in small squirts only. The most extensive part of digestion occurs in the small intestine; here most food products are further hydrolyzed and absorbed. The duodenum is specialized by the presence of chemoreceptors, chemical sensitive nerve endings that are able to detect the different biochemicals in the food material. The presence of the acid chyme triggers the release of secretin from the duodenum. Secretin is a hormone that moves through the blood (it is endocrine because it moves through the blood) to the pancreas where it causes the release of pancreatic juice. Pancreatic juice moves through pancreatic duct, and consists of sodium bicarbonate, water, and the four enzymes pancreatic amylase, trypsin, nuclease and lipase. The sodium bicarbonate turns the acid chyme basic with a pH of approximately 8.5. The sodium bicarbonate has a great function in that the materials present in the small intestine are able to operate at its optimum pH. The water functions in hydrolysis, the trypsin breaks protein into peptides, the pancreatic amylase, which has similar function as the salivary amylase different location, nuclease breaks down DNA into nucleotides, and the lipase breaks fats into fatty acids and glycerol; these processes using specific enzymes are a form of chemical digestion. The presence of lipids in the acid chyme causes the release of a hormone called CCK, cholecystokinin. The CCK affects the gall bladder and the pancreas, causing the release of bile and pancreatic juice. The bile is made in the liver, and stored in the gall bladder and emulsifies fats to create greater surface area for the lipase to turn lipids into fatty acids and glycerol. It is composed of cholesterol, bile salts and pigments from the breakdown of hemoglobin called biliruben and biliverdin.
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Intestinal juice is secreted by the small intestine. It contains water, pancreatic juice, bile, and a number of enzymes such as maltase peptidases, nuclease, sucrase, and lactase. Maltase breaks down maltose into glucose. The hydrolysis of disaccharides by enzymes such as maltase, completes the digestion of carbohydrates. The enzyme peptidases, breaks any remaining peptide bonds in the proteins to release amino acids. Sucrase breaks down sucrose into glucose and fructose, while lactase breaks down lactose into glucose and galactose. The function of the intestinal juice is to complete the process begun by the pancreatic juice. The mechanical pressure of food partly digested in the intestine stimulates the flow of intestinal juice. Similar to the esophagus, the food goes through mechanical digestion of peristalsis in the small intestine as well.
Within the jejunum, also part of the small intestine, parts of the food is digested. The last part of the small intestine is the ilium. It contains fingerlike projections called villi in which absorption takes place. The outer layer of the villi is of columnar epithelium and blood vessels (arteriole and venule) and a small lymphatic vessel called a lacteal. The fatty acids and glycerol are absorbed through the lacteal tube and moved into the lymphatic system, and the glucose and amino acids are absorbed into the circulatory system. The fluids with absorbed materials move through the hepatic portal vein to the liver, where it then flows through the hepatic vein to the heart and other body parts.
Materials that cannot be digested and absorbed by the small intestine move into the large intestine, which consists of caecum, the colon, the rectum, and the anal canal. The indigestible materials pass through the ilium to the large intestine through the ileo-caecal sphincter. The caecum, which is also known as a blind pouch, is the first part of the large intestine and has a small projection called the appendix. The large intestine is made up of the ascending colon, transverse colon, and descending colon. The large intestine mainly absorbs water, salt, and vitamins, and store indigestible materials to be excreted. The large intestine also has a sigmoid colon that leads to the rectum. The rectum stores feces, indigestible food that has been converted by bacteria, and opens at the anus where defecation occurs. The dietary fiber gained from the onions and lettuce, help with the process of defecation. Defecation is the releasing of the feces. It is released through the anal sphincter.
When food is brought into the body and digested, the blood glucose level increases. The pancreas secretes the hormones called insulin and glucagon from cells called islets of Langerhans, to regulate the body¡¯s use of glucose. Insulin is secreted into the blood when blood glucose levels are high (after meals). This causes the liver and muscle to store glucose as glycogen energy reserve, stimulate the liver, fat and muscle cells to take up and metabolize glucose, and aid in the build up of fats and proteins, inhibiting its use of energy source. Glucagons is secreted into the blood in response to low blood sugar levels (between meals), to promote the break down of nutrients and increase blood glucose levels. This is an example of negative feedback occurring, where a change in the body is reversed in such a way to return the body to its original condition, to regain homeostasis. The pancreas is an endocrine gland in that it is hormone producing and moves things through the blood, however, the pancreas is also an exocrine gland in that it produces digestive juices that move through ducts and not the blood.
As one can see, for Larry the Liver Sandwich to be digested, a long series of processes must occur. Larry is mechanically and chemically broken down, absorbed, and the non-digestible food material becomes wastes, which are excreted through the anal sphincter. The human body will use the nutrients absorbed from Larry the Liver Sandwich to supply the body with the necessary nutrients needed to help it function properly in its daily life.
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