The object of digestion is to break down large molecules into smaller units, and the first stop is the mouth. Picture a hamburger poised to enter your mouth, or oral cavity. The first bite, or even the mouth-watering anticipation of the first bite, starts the mechanical and chemical processes of digestion.

As you chew your mouthful of hamburger, the teeth tear and crush the bread and meat thus beginning mechanical digestion, which increases the surface area available for the enzymes to begin their work of chemical digestion. While we chew, the salivary glands near the jaw and under the tongue release saliva, which both lubricates the mouth and esophagus and contains salivary amylase, an enzyme that starts digesting the carbohydrates in the bun, as well as a lipase, which begins digesting fats in the meat. The tongue mixes saliva with the foods in the mouth and pushes food to the back of the mouth to initiate swallowing.

The tongue does more than help mix and swallow the food, though—it’s also part of the reason we enjoy that savory, juicy burger. The tongue contains taste buds that identify or sense foods on the basis of specific flavors or tastes. Food is a com-bination of five primary tastes: sweet, sour, salty, bitter, and umami, a more recently recognized savory flavor that’s harder to identify but may be an element of your burger, as it’s found in meats, seafood, cheese, and some vegetables. The number of taste buds we have declines with age, which explains why children—whose tongues are coated in taste-sensing cells—are often averse to strong flavors and may prefer to eat their burgers plain (or with a little ketchup) than with spicy mayonnaise or hot sauce.

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These taste buds don’t just tell us if something is too bitter or salty; they also create signals that tell the rest of the GI tract to prepare for the next steps of digestion.

Once the hamburger has been chewed and coated in saliva, it becomes a soft, moist lump of food known as a bolus, which is swallowed and passed through the throat. It then enters the esophagus, a roughly 10-inch-long muscular tube that transports the bolus of burger from the mouth to the stomach, relying on gravity and peristalsis. Positioned at the junction of the esophagus and the stomach is a circular muscle (sphincter) that normally functions as a one-way valve (as do the other sphincters in the body). As food nears the lower-esophageal sphincter, it relaxes to let food pass into the stomach; otherwise, it stays tightly closed to prevent foods and secretions from the stomach from moving backwards into the esophagus.

The bolus then enters the stomach, a muscular, J-shaped sack that can accommodate up to four cups of food and generally takes about two to four hours to empty. The stomach secretes gastric juices that contain hydrochloric acid (HCL), which help to unfold the proteins in the hamburger through chemical digestion. The gastric juices also contain enzymes such as proteases, which digest proteins, and lipase, which continues to digest fat. Importantly, the gastric juices also contain mucus, which lubricates the food and protects the stomach lining from acidity. The particularly forceful peristaltic contractions of the stomach vigorously churn the hamburger bolus, fragmenting it into fine particles that are dispersed throughout the gastric fluid. This essentially grinds the food into a semi-liquid mass called chyme. A few teaspoons at a time, chyme is passed along to the small intestine, moving through the partially relaxed pyloric sphincter that functions as a sieve, allowing only small food particles (generally smaller than 1 mm) to pass through. (INFOGRAPHIC 3.6)

Question 3.6

How quickly the stomach empties into the small intestine depends on the composition and quantity of the foods and fluids you consume. A hamburger typically spends 24 to 72 hours going from the mouth to the anus, but this transit time can change because of illness, medication, how active you are, and even your emotional state. Food with more fiber, for instance, slows emptying from the stomach, helping you feel full. But as fiber passes into the large intestine, it can also stimulate propulsive contractions, which speeds up the transit of the intestinal contents through the rest of the digestive system. These combined effects of fiber are important and help explain why nutrition professionals recommend healthy amounts of vegetables, fruits and other high-fiber foods in the diet.

The small intestine is the primary site for the digestion of food and the absorption of nutrients and as we shall see, it is where digestion goes awry in those with celiac disease. The small intestine has three sections: the duodenum, the first portion of the small intestine after the stomach; the jejenum, the middle portion; and the ileum, the last and longest portion. The small intestine isn’t actually “small” at all; it is a coiled hollow tube approximately 20 feet long and one-and-a-half inches wide, and several structural features give it a surface area approximately equal to the size of a tennis court. For example, the internal circular folds of the small intestine carry fingerlike projections called villi that increase its surface area for absorption. These villi are densely covered with fine, hairlike projections called microvilli that further increase surface area and the efficiency of absorption. Because these structures give the lining of the small intestine a brushlike appearance (when viewed with a microscope), it is often called the brush border. (INFOGRAPHIC 3.7)

Question 3.7

Secretions from accessory organs such as the pancreas and gallbladder play an important role in the digestion of the hamburger within the lumen of the small intestine. Chyme that enters the small intestine from the stomach is very acidic, and if it doesn’t get neutralized, it denatures and inactivates the enzymes required for digestion. The pancreas releases pancreatic juice that contains bicarbonate (baking soda), a base that neutralizes the gastric acids in chyme. Refer to INFOGRAPHIC 3.3.

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The liver and the gallbladder also help to digest the many lipids found in the hamburger. The liver produces bile—which is stored in the gallbladder, a small, pear-shaped sac below the liver in the right upper abdomen—that contains substances critical for effective lipid digestion. The hormone cholecystokinin (CCK) is released from the small intestine in response to the hamburger’s fats and protein in the small intestine; it stimulates the gallbladder to release bile and the pancreas to secrete juice into the lumen of the small intestine. In addition to bicarbonate, pancreatic juice contains enzymes—lipases, proteases, and amylase—that break the hamburger’s large fat, protein, and carbohydrate molecules into smaller ones. Enzymes located in the brush border complete the digestion of the burger’s carbohydrates and proteins.

To enter the mucosal cells lining the GI tract, water and small amounts of a few other nutrients can pass directly through the cell membrane by simple diffusion. The cell membrane serves as the boundary that holds the content of body’s cells in place and keeps their internal structures safe, so that cells function properly. The membrane also serves as a semi-permeable filter through which nutrients can enter and wastes can be excreted. Many nutrients enter the cells by facilitated diffusion, which requires a specific transport protein to help each of these nutrients move through the cell membrane. The entry of even other nutrients to cells depends on active transport, which requires both a transport protein and energy to bring them across a membrane. (INFOGRAPHIC 3.8)

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Question 3.8

Once inside the cells of the brush border of the small intestine, nutrients must reach the areas of the body where they are needed. The circulatory system which includes both the blood and the lymphatic system distribute vital nutrients to the tissues and organs. For example, carbohydrates, amino acids, minerals, and water-soluble vitamins enter directly into blood vessels, where they are transported to the liver before reaching other organs. Most fats (and some vitamins) first enter lymphatic vessels before they find their way into the blood. As a result, they reach the liver only after circulating throughout the rest of the body. (INFOGRAPHIC 3.9)

Question 3.9

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For people with celiac disease, however, these last few steps do not progress properly, and the delicate and essential process of absorption becomes disrupted. The gluten found in the hamburger bun sparks an immune reaction that triggers the person’s immune cells to attack other body cells. These so-called autoimmune cells begin destroying the villi of the small intestine. This leaves a flattened surface on the small intestine that compromises digestion and absorption of nutrients. Over time, this damage to the intestinal tract increases the risk for certain types of intestinal cancers, including intestinal lymphomas, which could help explain the higher mortality rate among people suffering from celiac disease.

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From the small intestine, any undigested nutrients that remain in the chyme are passed into the large intestine, which consists of the cecum, colon, and the rectum. Here, little digestion or absorption take place; there are no villi. Secreted mucus protects and lubricates the lining, making it easier for everything that’s leftover to be excreted as feces. (INFOGRAPHIC 3.10)

Question 3.10

But before that happens, the large intestine will extract electrolytes (sodium, chloride, and potassium), some fatty acids, vitamins (K, biotin, and folate), and water. In addition, the large intestine contains more than 1,000 species of bacteria that feed on undigested fiber and starch; since humans have no enzymes that digest dietary fiber, these bacteria perform some of that function, producing gas and short-chain fatty acids in the process. Some bacteria play a role in preventing disease, reducing the activity of other bacteria that may cause it.