For instance, if your finger comes into contact with a thorn on a rose bush, a sensory neuron transmits a signal from your finger up through the spinal cord and into the brain. Another neuron in the brain sends a signal that travels back to the muscles in your hand and stimulates muscles to contract and you jerk your finger away.

All of this happens within a tenth of a second. All nerve impulses travel by the movement of charged sodium, potassium, calcium, and chloride atoms. These are some of the essential minerals in our diets—essential because they are absolutely required for central nervous system function.

Nerves communicate with each other via chemicals built from amino acids called neurotransmitters. Eating adequate protein from a variety of sources will ensure the body gets all of the different amino acids that are so important for central nervous system function.

Every day the brain uses over 20 percent of the energy obtained from nutrients. Its main fuel is glucose and only in extreme starvation will it use anything else ketones.

For acute mental alertness and clear thinking, glucose must be systematically delivered to your brain. This does not mean that sucking down a can of sugary soda before your next exam is a good thing. Just as too much glucose is bad for other organs, such as the kidneys and pancreas, it also produces negative effects on the brain.

Excessive glucose levels in the blood can cause a loss of cognitive function and chronically high blood glucose levels can damage brain cells. The good news is that much research is directed toward determining the best diets and foods that slow cognitive decline and maximize brain health.

Gu, Y. doi: The protective effects of blueberries on the brain are linked to their high content of anthocyanins, which are potent antioxidants and reduce inflammation. A small study published in the April issue of the Journal of Agricultural and Food Chemistry found that elderly people who consumed blueberry juice every day for twelve weeks had improved learning and memorization skills in comparison to other subjects given a placebo drink.

Krikorian, R. et al. More clinical trials are evaluating the effects of blueberries and other foods that benefit the brain and preserve its function as we age.

The muscular system allows the body to move voluntarily, but it also controls involuntary movements of other organ systems such as the heartbeat in the circulatory system and peristaltic waves in the digestive system. It consists of over six hundred skeletal muscles, as well as the heart muscle, the smooth muscles that surround your entire alimentary canal, and all your arterial blood vessels.

Muscle contraction relies on energy delivery to the muscle. Each movement uses up cellular energy and without an adequate energy supply, muscle function suffers.

Muscle, like the liver, can store the energy from glucose in the large polymeric molecule glycogen. But unlike the liver, muscles use up all of their own stored energy and do not export it to other organs in the body.

When muscle energy stores are diminished, muscle contraction weakens. However, muscle is not as susceptible to low levels of blood glucose as the brain because it will readily use alternate fuels, such as fatty acids and protein to produce cellular energy.

Fatty acids are transported from fat-storing cells to the muscle to rectify the nutrient deficit. However, fatty acids take more time to convert to energy than glucose, thus decreasing performance levels.

It is important not to assume that carbohydrate loading works for everyone. Without accompanied endurance training you will not increase the amount of stored glucose. If you plan on running a five-mile race for fun with your friend and decide to eat a large amount of carbohydrates in the form of a big spaghetti dinner the night before, the excess carbohydrates will be stored as fat.

In fact, throughout the Tour de France—a twenty-two-day, twenty-four-hundred-mile race—the average cyclist consumes greater than 60 grams of carbohydrates per hour. This organ system is responsible for regulating appetite, nutrient absorption, nutrient storage, and nutrient usage, in addition to other functions, such as reproduction.

The glands in the endocrine system are the pituitary, thyroid, parathyroid, adrenals, thymus, pineal, pancreas, ovaries, and testes. The glands secrete hormones , which are biological molecules that regulate cellular processes in other target tissues, so they require transportation by the circulatory system.

Adequate nutrition is critical for the functioning of all the glands in the endocrine system. A protein deficiency impairs gonadal-hormone release, preventing reproduction.

Athletic teenage girls with very little body fat often do not menstruate. Children who are malnourished usually do not produce enough growth hormone and fail to reach normal height for their age group.

Probably the most popularized connection between nutrition and the functions of the endocrine system is that unhealthy dietary patterns are linked to obesity and the development of Type 2 diabetes.

The Centers for Disease Control and Prevention CDC estimates that twenty-six million Americans have Type 2 diabetes as of This is 8. The maps in Note 3. You can see that those counties with the highest incidence of obesity also have the highest incidence of Type 2 diabetes.

Watch the National Health video to see the relationship between the rise in obesity and the rise in Type 2 diabetes.

What is the causal relationship between overnutrition and Type 2 diabetes? The prevailing theory is that the overconsumption of high-fat and high-sugar foods causes changes in muscle, fat, and liver cells that leads to a diminished response from the pancreatic hormone insulin.

When cells are resistant to insulin they do not take up enough glucose and fatty acids and so glucose and fatty acids remain at high concentrations in the blood. The continuously high amounts of glucose and fatty acids in the blood impair the release of insulin from the pancreas, further exacerbating the situation.

The chronic elevation of glucose and fatty acids in the blood also causes damage to other tissues over time, so people who have Type 2 diabetes are at increased risk for cardiovascular disease, kidney disease, nerve damage, and eye disease.

Do your part to slow the rising tide of obesity and Type 2 diabetes in this country. It provides information on education resources, projects, and programs, and spotlights news on diabetes and obesity. LEAN: Leading Employees to Activity and Nutrition. The program provides free web-based resources with the mission of designing worksites that prevent obesity.

The immune system is comprised of several types of white blood cells that circulate in the blood and lymph. Their jobs are to seek, recruit, attack, and destroy foreign invaders, such as bacteria and viruses. Other less realized components of the immune system are the skin which acts as a barricade , mucus which traps and entangles microorganisms , and even the bacteria in the large intestine which prevent the colonization of bad bacteria in the gut.

Immune system functions are completely dependent on dietary nutrients. In fact, malnutrition is the leading cause of immune-system deficiency worldwide.

When immune system functions are inadequate there is a marked increase in the chance of getting an infection. Changing Flows Balance Nutrient Absorption and Bacterial Growth along the Gut Agnese Codutti, Jonas Cremer, and Karen Alim Phys. Article References Citing Articles 2 Supplemental Material Article References Citing Articles 2 Supplemental Material PDF HTML Export Citation.

Abstract Small intestine motility and its ensuing flow of luminal content impact both nutrient absorption and bacterial growth.

Research Areas. Physical Systems. Physics of Living Systems Nonlinear Dynamics Fluid Dynamics Interdisciplinary Physics General Physics. Optimizing Flow Speed is Essential for the Gut Published 23 September Fluid dynamics simulations suggest that the varying flow speed inside the small intestine maximizes nutrient absorption while minimizing excess bacteria.

See more in Physics. alim tum. Issue Vol. Authorization Required. Log In. Other Options Buy Article » Find an Institution with the Article ».

Figure 1 Gut motility determines flows. Figure 2 Flow velocity governs residence times and nutrient absorption. Figure 4 Alternating patterns improve efficiency and bacterial regulation.

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Mechanical digestion is a purely physical process that does not change the chemical nature of the food. Instead, it makes the food smaller to increase both surface area and mobility.

It includes mastication , or chewing, as well as tongue movements that help break food into smaller bits and mix food with saliva. Although there may be a tendency to think that mechanical digestion is limited to the first steps of the digestive process, it occurs after the food leaves the mouth, as well.

Segmentation , which occurs mainly in the small intestine, consists of localized contractions of circular muscle of the muscularis layer of the alimentary canal. These contractions isolate small sections of the intestine, moving their contents back and forth while continuously subdividing, breaking up, and mixing the contents.

By moving food back and forth in the intestinal lumen, segmentation mixes food with digestive juices and facilitates absorption. In chemical digestion , starting in the mouth, digestive secretions break down complex food molecules into their chemical building blocks for example, proteins into separate amino acids.

These secretions vary in composition, but typically contain water, various enzymes, acids, and salts. The process is completed in the small intestine. Food that has been broken down is of no value to the body unless it enters the bloodstream and its nutrients are put to work.

This occurs through the process of absorption , which takes place primarily within the small intestine. There, most nutrients are absorbed from the lumen of the alimentary canal into the bloodstream through the epithelial cells that make up the mucosa.

Lipids are absorbed into lacteals and are transported via the lymphatic vessels to the bloodstream the subclavian veins near the heart. The details of these processes will be discussed later.

In defecation , the final step in digestion, undigested materials are removed from the body as feces. Age-related changes in the digestive system begin in the mouth and can affect virtually every aspect of the digestive system.

Swallowing can be difficult, and ingested food moves slowly through the alimentary canal because of reduced strength and tone of muscular tissue. Neurosensory feedback is also dampened, slowing the transmission of messages that stimulate the release of enzymes and hormones. Pathologies that affect the digestive organs—such as hiatal hernia, gastritis, and peptic ulcer disease—can occur at greater frequencies as you age.

Problems in the small intestine may include duodenal ulcers, maldigestion, and malabsorption. Problems in the large intestine include hemorrhoids, diverticular disease, and constipation. Conditions that affect the function of accessory organs—and their abilities to deliver pancreatic enzymes and bile to the small intestine—include jaundice, acute pancreatitis, cirrhosis, and gallstones.

In some cases, a single organ is in charge of a digestive process. For example, ingestion occurs only in the mouth and defecation only in the anus.

However, most digestive processes involve the interaction of several organs and occur gradually as food moves through the alimentary canal Figure 2. Figure 2.

The digestive processes are ingestion, propulsion, mechanical digestion, chemical digestion, absorption, and defecation. Some chemical digestion occurs in the mouth. Some absorption can occur in the mouth and stomach, for example, alcohol and aspirin. Neural and endocrine regulatory mechanisms work to maintain the optimal conditions in the lumen needed for digestion and absorption.

These regulatory mechanisms, which stimulate digestive activity through mechanical and chemical activity, are controlled both extrinsically and intrinsically. The walls of the alimentary canal contain a variety of sensors that help regulate digestive functions.

These include mechanoreceptors, chemoreceptors, and osmoreceptors, which are capable of detecting mechanical, chemical, and osmotic stimuli, respectively. Stimulation of these receptors provokes an appropriate reflex that furthers the process of digestion.

This may entail sending a message that activates the glands that secrete digestive juices into the lumen, or it may mean the stimulation of muscles within the alimentary canal, thereby activating peristalsis and segmentation that move food along the intestinal tract.

The walls of the entire alimentary canal are embedded with nerve plexuses that interact with the central nervous system and other nerve plexuses—either within the same digestive organ or in different ones.

These interactions prompt several types of reflexes. Extrinsic nerve plexuses orchestrate long reflexes, which involve the central and autonomic nervous systems and work in response to stimuli from outside the digestive system. Short reflexes, on the other hand, are orchestrated by intrinsic nerve plexuses within the alimentary canal wall.

These two plexuses and their connections were introduced earlier as the enteric nervous system. Short reflexes regulate activities in one area of the digestive tract and may coordinate local peristaltic movements and stimulate digestive secretions. For example, the sight, smell, and taste of food initiate long reflexes that begin with a sensory neuron delivering a signal to the medulla oblongata.

The response to the signal is to stimulate cells in the stomach to begin secreting digestive juices in preparation for incoming food. In contrast, food that distends the stomach initiates short reflexes that cause cells in the stomach wall to increase their secretion of digestive juices.

A variety of hormones are involved in the digestive process. The main digestive hormone of the stomach is gastrin, which is secreted in response to the presence of food. Gastrin stimulates the secretion of gastric acid by the parietal cells of the stomach mucosa.

Other GI hormones are produced and act upon the gut and its accessory organs. Hormones produced by the duodenum include secretin, which stimulates a watery secretion of bicarbonate by the pancreas; cholecystokinin CCK , which stimulates the secretion of pancreatic enzymes and bile from the liver and release of bile from the gallbladder; and gastric inhibitory peptide, which inhibits gastric secretion and slows gastric emptying and motility.

A reduced LES pressure, or tone, reduces its ability to tightly constrict and increases the likelihood that you will regurgitate or burp. Some foods are known to affect tone; for example, foods high in sugars and starches, both carbohydrates, increase the likelihood of regurgitation, while dietary fiber, also a carbohydrate, decreases the frequency of regurgitation and heartburn.

Although people sometimes say that there is a relationship between dietary fats and heartburn, one has yet to be found in a comprehensive study such as the National Health and Nutrition Examination Survey.

While acidic or spicy foods can irritate the lining of the esophageal, they are not thought to contribute to regurgitation. Food and beverages that lower pressure include peppermint, spearmint, chocolate, alcohol, and coffee.

Consumption of these foods encourages regurgitation because the sphincter does not close tightly enough after swallowing. A small meal size, limiting consumption of sugars and starches, and avoiding late-night eating are recommended practices to reduce the likelihood of regurgitation and heartburn.

The mucus layer lining the esophagus serves to lubricate a passing bolus of food, but the thicker mucus layer that lines the stomach has a different task.

It provides a continuous barrier that protects the stomach from the corrosive effects of enzymes and acids that would damage unprotected stomach cells. An example is the digestion of protein that begins in the stomach as pepsinogen is converted to the active form pepsin.

Without the protection of the mucus layer, stomach cells exposed to pepsin would be damaged, resulting in sores in the stomach lining or an ulcer. When there is a breakdown in the thick mucus layer protecting the stomach lining from the caustic effects of acid and pepsin, gastric ulcers may result.

Stomach pain and bleeding that comes and goes is a sign that underlying tissue is damaged. Genetics, stress, smoking, and the long-term use of nonsteroid anti-inflammatory drugs like aspirin or ibuprofen are among the factors that contribute to ulcer development. Sometimes a peptic ulcer is caused when the mucous coating of the stomach is damaged by infection by Helicobacter pylori H.

pylori is a bacteria that is transmitted person to person oral-oral route through saliva or vomit as well as through water that is contaminated with feces oral-fecal route. Antibiotics are effective in treating ulcers where a chronic infection with a bacterial infection is the causative factor.

pylori bacteria are spread through close contact and exposure to vomit. Help stop the spread of H. pylori by washing your hands! Treatment of ulcers may include stress-reduction techniques and antacids to counteract stomach secretions and reduce pain. It is a good idea to stop smoking and reduce alcohol consumption as well.

The stomach is a J-shaped pouch positioned between the esophagus and the small intestine. It is grapefruit sized and expands when filled. It churns and mixes food received from the esophagus. When stimulated by the presence of food or drink, the stomach secretes hydrochloric acid, which lowers contents to a pH of less than two, creating an acidic environment.

This activates the enzyme pepsinogen, converting it to pepsin, which begins the digestion of protein. It also denatures or uncoils protein molecules, making it easier for pepsin to work.

How acidic are stomach contents? Consider that vinegar has a pH of two; grapefruit juice, three; black coffee, five; distilled water neutral , seven; and baking soda alkaline , nine.

This highly acidic environment discourages bacterial growth and helps in the prevention of bacterial diseases, such as foodborne illness. Endocrine cells in the stomach produce gastrin, somatostatin, and ghrelin, which are hormones that help regulate stomach function.

Gastrin regulates gastric acid production and stimulates appetite. Conversely, somatostatin counteracts gastrin and reduces its production when a meal is over and eating more food is not imminent. Although ghrelin is sometimes called the hunger hormone, its role goes beyond stimulating appetite.

The ability of your stomach to expand, or its capacity, is related to the amount of food that you routinely eat at one sitting. In most cases, stomach capacity is about thirty-two to forty-six ounces.

People who habitually overeat have larger stomach capacities than they would if they ate smaller portions. While the stomach does not shrink, making a habit of eating smaller amounts tightens stomach muscles and reduces the overall ability to stretch.

As a result, stretching sensors that signal that the stomach is full are activated at a smaller capacity when fewer calories have been consumed. After mixing is complete, the stomach moves food and gastric secretions to the small intestine in a watery solution called chyme. Stomach muscles contract in waves to squirt chyme through the pyloric sphincter, separating the stomach from the small intestine at a rate of one to five milliliters per thirty seconds, or about one to two teaspoons per minute.

It takes two to four hours for a typical meal to pass completely into the small intestine. The type of food or drink affects the rate of passage. Isotonic liquids, which have the same solute concentration as body cells, leave the stomach more quickly than hypertonic liquids or solids, which tend to spend the most time in the stomach.

A hypertonic liquid has a higher solute concentration than body cells or blood, while hypotonic liquid has a lower one. An example of an isotonic liquid is Gatorade or Powerade.

Sweetened, carbonated beverages are hypertonic, and water is hypotonic. Foods that are high in fat leave the stomach more slowly than foods high in either protein or carbohydrates.

Fiber also reduces the rate at which gastric contents empty into the small intestine. As a result, meals with adequate fiber depress the rate at which carbohydrates elevate blood glucose levels as well as prolong the sense of satisfaction or satiety generated by a full stomach.

By moderating the rate at which chyme passes into the small intestine, where carbohydrates are digested and absorbed. Overall, an additional three to ten hours is needed for your meal to traverse the large intestine and complete its journey.

An additional one to two days may pass before residues that are mostly fiber leave your body. Chewed food is swallowed as a lump, or bolus, which the muscles of the gastrointestinal tract push in a wavelike motion past the epiglottis, through the esophagus, and into the stomach.

Swallowing causes a temporary relaxation of the LES, which returns to a contracted state after the bolus passes into the stomach. Gastroesophageal reflux disease GERD happens when stomach contents pass back through the LES into the esophagus, causing heartburn and regurgitation. GERD treatment includes behavioral modification and medications that reduce stomach acid content.

The stomach continues the breakdown of foods that started with chewing. Hydrochloric acid in the stomach denatures food proteins, making them more digestible, and inhibits bacterial growth, which reduces the risk of foodborne illness.

Gastrin, somatostatin, and ghrelin manage stomach function, while pepsinogen is activated to make pepsin, which begins the enzymatic breakdown of protein. Stomach contractions move the mixture of food and gastric juices into the small intestine, where further digestion takes place.

The vast majority of the nutrients that we get from our food and drink are absorbed in the small intestine. An amazing list of hormones, enzymes, emulsifiers, and carrier molecules makes this possible.

Even though fat, carbohydrates, and protein are absorbed in the small intestine, much work remains for the large intestine, where fiber supports beneficial bacteria, water is conserved through absorption, and digestive residues are prepared for excretion. The small intestine is the primary site for the digestion and eventual absorption of nutrients.

In fact, over 95 percent of the nutrients gained from a meal, including protein, fat, and carbohydrate, are absorbed in the small intestine. Alcohol, an additional source of energy, is largely absorbed in the small intestine, although some absorption takes place in the mouth and stomach as well.

Three organs of the body assist in digestion: the liver, the gall bladder, and the pancreas. The liver produces bile, a substance that is crucial to the digestion and absorption of fat, and the gall bladder stores it.

The pancreas provides bicarbonate and enzymes that help digest carbohydrates and fat. The liver, gall bladder, and pancreas share a common duct into the small intestine, and their secretions are blended.

If the common duct becomes blocked, as with a gall stone, adequate bile is not available, and the digestion of fat is seriously reduced, leading to cramping and diarrhea. Bicarbonate secreted by the pancreas neutralizes chyme makes it less acidic and helps create an environment favorable to enzymatic activity.

The pancreas provides lipase, an enzyme for digesting fat, and amylase for digesting polysaccharides carbohydrate. The small intestine produces intermediate enzymes, such as maltase, that digest maltose and peptidase to break down proteins further into amino acids.

The villi are fingerlike projections from the walls of the small intestine. They are a key part of the inner surface and significantly increase the absorptive area.

A large surface area is important to the speed and effectiveness of digestion. Some medical treatments, such as radiation therapy, can damage villi and impair the function of the small intestine. Diseases also affect villi health. One sign of chronic alcoholism is blunted villi that lack adequate surface area, resulting in poor absorption of nutrients.

Someone in the advanced stages of alcoholism often experiences diarrhea due to reduced water and sodium absorption, poor eating habits that limit vitamin C intake coupled with an increased loss in urine, and zinc deficiency due to poor absorption. Cells in the villi are continuously exposed to a harsh environment and, as a result, have a short life-span of about three days.

Adequate nutrition is required for optimal health and to ensure that new cells are ready to replace aging ones. Insufficient protein in the diet depresses cell replacement and reduces the efficiency of absorption, thereby further compromising overall health.

This is a significant issue for people who have experienced starvation. A quick introduction of large amounts of food can result in cramping and diarrhea, further threatening survival.

Enzymes are biological catalysts that speed up reactions without being changed themselves. Enzymes produced by the stomach, pancreas, and small intestine are critical to digestion. For example, carbohydrates are large molecules that must be broken into smaller units before absorption can take place.

Enzymes such as amylase, lactase, and maltase catalyze the breakdown of starches polysaccharides and sugars disaccharides into the monosaccharides, glucose, galactose, and fructose. Proteases such as pepsin and trypsin digest protein into peptides and subsequently into amino acids, and lipase digests a triglyceride into a monoglyceride and two fatty acids.

The digestion of fat poses a special problem because fat will not disperse, or go into solution, in water. The lumen of the small intestine is a liquid or watery environment. This problem is solved by churning, the action of enzymes, and bile salts secreted by the liver and gall bladder.

Bile acts as an emulsifier, or a substance that allows fat to remain in suspension in a watery medium. The resulting micelle, or a droplet with fat at the center and hydrophilic or water-loving phospholipid on the exterior, expedites digestion of fats and transportation to the intestinal epithelial cell for absorption.

Nutrients truly enter the body through the absorptive cells of the small intestine. Absorption of nutrients takes place throughout the small intestine, leaving only water, some minerals, and indigestible fiber for transit into the large intestine.

There are three mechanisms that move nutrients from the lumen, or interior of the intestine, across the cell membrane and into the absorptive cell itself. They are passive, facilitated, and active absorption.

In passive absorption, a nutrient moves down a gradient from an area of higher concentration to one of lower concentration. For this downhill flow, no energy is required. Fat is an example of a nutrient that is passively absorbed. In facilitated absorption, a carrier protein is needed to transport a nutrient across the membrane of the absorptive cell.

For this type of absorption, no energy is required. Fructose is an example of a nutrient that undergoes facilitated absorption. In active absorption, both a carrier protein and energy are needed.

Active absorption rapidly moves a nutrient from an area of low concentration in the lumen to an area of high concentration in the cell and eventually into the blood. Glucose and galactose are examples of nutrients that require active absorption. The large intestine completes the process of absorption.

In the upper large intestine, most of the remaining water and minerals are absorbed. Fiber becomes a food source for resident bacteria that generate gas and acids as by-products as well as some vitamins. Over four hundred different bacteria colonize the colon, or large intestine, and provide the body with vitamin K and vitamin B12 as by-products of their life processes.

The normal flora, or bacteria, that reside in the intestine also resist colonization efforts of other, unfamiliar bacteria. Finally, the residues of a meal move into the rectum and are further concentrated and prepared for expulsion from the body as feces. Did you know that the gastrointestinal tract of a newborn baby is sterile?

Exposure to the world and the first swallow of milk changes everything by introducing bacteria. A breastfed baby tends to have a more stable and uniform microbiota than a formula-fed infant, and this is advantageous.

The protective influence of breastfeeding reduces the incidence of diarrhea and modifies the risk of allergic diseases during childhood. Exclusive breastfeeding during the first six months of life is recommended by the World Health Organization followed by supplemental breastfeeding throughout the first two years of life.

Getting the energy and nutrients that we need from our food and drink is a complex process that involves multiple organs and an array of substances. The small intestine is a muscular tube with villi projecting into the lumen that vastly increase its absorptive surface area. The liver produces bile, which the gall bladder stores and secretes into to small intestine via a common duct.

Bile is an emulsifier that suspends fats in the watery chyme, making enzymatic breakdown possible. The pancreas produces lipase and secretes it into a common duct, where it is delivered to the small intestine. Lipase breaks down large fat molecules into manageable parts.

The large intestine plays an important part in concentrating the residues of digestion and conserving water through absorption. It also is a home for beneficial bacteria that are nourished by fiber that is indigestible for humans. Nutrition for Consumers by University of North Texas is licensed under a Creative Commons Attribution-NonCommercial 4.

Skip to content Increase Font Size. Objectives Describe the role of the mouth, teeth, tongue, epiglottis, and esophagus in chewing, lubricating, and delivering food and drink to the stomach and beyond Explain the cause of heartburn or gastroesophageal reflux disease Associate the small intestine and villi with their digestive role Connect the large intestine to its function 3.

Nutrients as Raw Materials Nutrients are provided by the foods that you eat. Digestion Begins Digestion begins in your mouth as you chew or masticate food and mix it with saliva. Mobility Working together, cheek muscles and the tongue position a lump of food for swallowing.

Tongue and Taste The tongue is instrumental in the perception of taste. Summary Digestion is a process that transforms the foods that we eat into the nutrients that we need. Key Concepts The muscular tube called the epiglottis The esophagus and lower esophageal pressure Introduction to the stomach The Epiglottis The esophagus is a muscular tube that connects the mouth to the stomach.