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When you enter the body through the brain, you come across the first component of the digestive system—the hypothalamus—a section of the brain the size of an almond. The hypothalamus controls the temperature of the body, hunger, thirst, fatigue, and circadian cycles—A daily cycle of biological activity based on a 24-hour period and influenced by regular variations in the environment, such as the alternation of night and day displayed by many organisms.
As you continue down the body the next component of the digestive system you arrive at is the mouth. The main digestive components in the mouth are the teeth and the salivary glands. The teeth are a mechanical digester—they break food into smaller pieces by mastication (chewing). There are three main types of teeth in the mouth: incisors, canine, and molars. Incisors—the first eight teeth, four on top and four on bottom—cut food when you chew. The next four teeth—two on top and two on the bottom—are the canines. The canine teeth are used to rip and tear food apart. Other animals with canines use theirs to hold onto prey. The molars—the last twenty teeth (in an adult) or eight teeth (in a young child)—are used for chewing and grinding food. The different types of teeth add to 32 for an adult and 20 for a young child.
Along with mechanical digestion in the mouth there is chemical digestion—amylase in saliva turns starch (a polysaccharide (a carbohydrate that consists of many sugar molecules stuck together)) into simple sugars—taking place. When food enters the mouth—is ingested—it is mixed with saliva that is secreted by several sets of glands. Besides the many insignificant glands that secrete saliva, there are three major pairs of salivary glands: the parotid, the submandibular, and the sublingual glands.
The parotid glands, the largest of the pairs, are located at the side of the face, below and in front of each ear. The submandibular glands, which are rounded in shape, lie near the inner side of the lower jawbone, in front of the sternomastoid muscle (the prominent muscle of the jaw). The sublingual glands lie directly under the mucous membrane covering the floor of the mouth beneath the tongue. Saliva—made by the minute glands throughout the mouth and the three main glands—dissolves some of the chewed food and acts as a lubricant, allowing food to squeeze through tight passages in the digestive system. Saliva also contains a starch-digesting enzyme called amylase (ptyalin), which initiates the process of enzymatic hydrolysis (Hightower, M.D. Nicholas Carr. “Salivary Glands.” Encyclopedia Britannica Online. Encyclopedia Britannica, n.d. Web. 30 Sept. 2014.) Enzymatic hydrolization splits starch (a polysaccharide containing many sugar molecules bound in a continuous chain) into simple sugars.
The esophagus—the passageway from the mouth to the stomach—is about 25 centimeters (10 inches) long. The esophagus contains 4 layers—the mucosa, submucosa, muscularis, and tunica adventitia. The mucosa is made up of multiple layers of tissue containing numerous mucous glands. The submucosa is a thick thread-like layer connecting the mucosa to the muscularis. The muscularis is made of an inner layer, in which the fibers are circular, and an outer layer of fibers that run lengthwise in the esophagus. The outer layer of the esophagus, the tunica adventitia, is composed of loose fibrous tissue that connects the esophagus with other internal structures.
Except during the act of swallowing, the esophagus is normally empty, and its channel, is essentially closed by the longitudinal folds of the mucosal and submucosal layers. The upper 3rd of the esophagus is composed of striated (voluntary) muscle. The middle 3rd is a mixture of striated and smooth (involuntary) muscle, and the lower 3rd consists only of smooth muscle. The bolus—chewed food now mixed with saliva and descending down the esophagus—is moved by peristalsis. Peristalsis is the contraction of muscles in the esophagus that cause the bolus to move down the throat. The trachea connects near the middle of the esophagus. When you swallow your brain sends a message to a flap of muscle called the epiglottis. The epiglottis closes over the trachea when swallowing to keep food out of the trachea.
When the bolus reaches the end of the esophageal tube the last thing that is in the bolus’ way of getting to the stomach is the cardiac sphincter. The cardiac sphincter is a muscle that controls what goes in the stomach and what gets out of the stomach. Now past the cardiac sphincter, the bolus enters the stomach. The bolus will digest in the stomach for 3-4 hours. While the food is in the stomach mechanical digestion is taking place. Peristalsis—the squeezing of muscles around the stomach—stirs up the bolus while chemicals digest the bolus. Chemical digestion in the stomach is caused by gastric juice made of the pepsin enzyme and HCl (Hydrochloric Acid/Muriatic acid). A thin layer of mucus lines the stomach to keep the hydrochloric acid—made of hydrogen, chlorine, and water—from burning through your stomach causing stomach ulcers. Pepsin in the stomach is used to break down proteins. While pepsin digests proteins hydrochloric acid also digests proteins and other things found in bolus.
The stomach is the shape of a kidney bean. There are 3 sections of the stomach: the fundus, body, and pyloric end. The bottom curve of the stomach is called the greater curvature because it is larger than the lesser curvature and likewise. At the end of the stomach is the pyloric sphincter. The pyloric sphincter like the cardiac sphincter is a muscle that controls what goes into the stomach and into the small intestine. After the bolus passes through the pyloric sphincter it is called chyme.
The small intestine is made of 3 parts: the duodenum, jejunum, and ileum. The 1st 22 centimeters of the small intestine is the duodenum. The digestive enzymes are added in the duodenum. The digestive juices added to the small intestine are: pancreatic juice—amylase (turns starches into sugar), trypsin (breaks down protein like pepsin), and lipase (breaks down fat)—and bile. The pancreatic juice—amylase, trypsin, and lipase—is made in the pancreas. The pancreatic juice assists in the digestion and absorption of nutrients in the small intestine. The enzymes in pancreatic juice help break down the carbohydrates, proteins, and lipids (organic compounds that are fatty acids or their derivatives and are insoluble in water but soluble in organic solvents) in the chyme.
The pancreas is made of 3 sections: the head, body, and tail. While the pancreas adds digestive juices to the small intestine no food actually goes into the pancreas. The pancreas is a digestive helper (an organ that contributes to digestion but never has food inside of it). Bile is made in the liver. The liver is a digestive helper like the pancreas. After the liver makes bile it sends the bile to store in the gallbladder via the hepatic ducts. Once in the gallbladder the bile sits until needed. In the case that the bile is made of too much cholesterol and sits too long, it can cause gallstones. Gallstones are the excessive buildup of cholesterol in the gallbladder causing a stone-like ball to form. When passing a gallstone you may experience a lot of pain.
Now, back to the more important stuff. When needed, the gallbladder sends bile into the cystic duct—tube connecting gallbladder to the bile duct. As the bile goes down the bile duct it dumps into the ampulla—tube connecting the pancreas to the small intestine. When the digestive enzymes reach the small intestine they emulsify—mix together—with the chyme to make an almost black liquid. Along with pancreatic juice there is intestinal juice. Intestinal juice contains erepsin, lipase (breaks down fat), lactase (catalyzes the hydrolysis of lactose to glucose and galactose), enterokinase, amylase (turns starches into sugar), and mucus.
In the small intestine there is also mechanical digestion taking place. While the pepsin, trypsin, and lipase in the pancreatic juice and the erepsin, lipase, lactase, enterokinase, and amylase in the intestinal juice break down the chyme chemically, the bile—a liquid mechanical digester—breaks down the “chunks” of fat in chyme into smaller pieces. Peristalsis is another form of mechanical digestion in the small intestine. Although peristalsis is usually used to move food through the digestive system, the small intestine, like the stomach also uses peristalsis to squeeze the food.
The next 200 centimeters of the small intestine is the jejunum. The jejunum is the part of the small intestine that the pancreatic juice—chemical digestion in the small intestine—and the bile—mechanical digester in the small intestine—digest the chyme. The pepsin, trypsin, and lipase in the pancreatic juice are chemical digesters. Peristalsis is again present in a digestive organ. Along the way are tiny structures called villi. Villi increase the surface area of the small intestine by 600 times and absorb nutrients from the chyme. There are so many villi on the interior of the small intestine that without the villi the surface area of the small intestine would be 600 times smaller.
The last 350 centimeters of the small intestine is the ileum. In the ileum, villi absorb the nutrients from the chyme. The chyme has now reached the end of the small intestine and now comes to the ileocecal sphincter. The ileocecal sphincter permits what goes into the large intestine. The large intestine is made of 8 parts: the cecum, appendix, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and the anus. The first part of the large intestine is the cecum. When food goes through the ileocecal sphincter it reaches the cecum. The cecum is a bowl-like structure in the large intestine. Connected to the cecum is the appendix. The appendix is a worm like tube that stores a little bit of every type of bacteria in your large intestine in case the bacteria is killed in the intestine.
The appendix sometimes becomes swollen and when this happens it causes a lot of pain. If the appendix becomes swollen most people get an appendectomy. If left for too long the appendix can rupture and all of the bacteria stored in the appendix is secreted into the body cavity and at that point you will have a large chance of dying.
The next parts of the large intestine are the ascending colon, transverse colon, descending colon and the sigmoid colon. The ascending colon absorbs and recycles water and vitamins B and K. The transverse colon also absorbs and recycles water and vitamins. The descending colon stores feces—poop—before it goes into the rectum. The sigmoid colon squeezes feces into the rectum. The rectum is said to be the last part of the large intestine. The rectum serves as a final storage unit for feces. Between the rectum and the anus is the anal canal. The anal canal is different from the rectum only because the lining is different. As the feces passes through he anal canal via peristalsis it comes to the final roadblock from being outside the body: the anal sphincter. The anal sphincter keeps feces inside your body until you are ready to defecate—expulsion of feces. When you are ready to defecate the sphincter relaxes and out goes the feces through your anus.
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