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Digestive System I: The Upper Gastrointestinal Tract

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  • 0:42 Mouth
  • 1:23 Throat
  • 2:13 Stomach
  • 3:23 Pepsin
  • 6:45 Summary
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Lesson Transcript
Instructor: Samuel Wright
Did you know that there are digestive enzymes in your saliva? It's true. As soon as you put a piece of food in your mouth, the digestive process begins. Join us with this first of two lessons about the human digestive system, where we'll follow food through the upper gastrointestinal tract from the mouth through the stomach.

Upper Gastrointestinal Tract

All is calm in the rainforest. A beautiful butterfly lands on a flower in search of nectar, but unbeknownst to the butterfly, a well-concealed predator is only inches away. In a fraction of a second, the chameleon strikes out with its tongue, snatching the butterfly from the flower and drawing it into its mouth. The chameleon begins to chew - wait, chameleons don't chew! But little boys like Timmy, who pretend to be chameleons, do.

The Mouth

The digestive process begins when amylase in saliva starts to break down food
Salivary glands

As Timmy chewed the butterfly and physically broke it down into smaller pieces, his salivary glands went to work, releasing saliva containing amylase - a digestive enzyme that breaks large polysaccharides into smaller ones. Amylase is capable of breaking every other bond between sugars in a polysaccharide. Between chewing (which physically breaks the butterfly into smaller pieces and creates greater surface area for enzymes to contact) and amylase (which begins breaking complex carbohydrates down into smaller polysaccharides), the digestion of the butterfly has already begun.

The Throat

The tongue then forms the chewed-up and saliva-soaked food into a ball and directs it to the back of the mouth. Here, it enters the pharynx (which you may remember is the junction of the mouth and nasal airways that leads down the throat). However, unlike the air that passes through the larynx on its way to the trachea and lungs, the food must enter the esophagus (the tube that serves as a passage for food from the pharynx to the stomach). When Timmy swallows, his larynx moves up, and at the same time, a flap called the epiglottis, flips downward to cover the larynx. This prevents the food from continuing down the airway. Instead, the food is directed into the esophagus, where muscle contractions push the food down into the stomach.

The Stomach

The stomach is a fairly large organ with an elastic wall. It is capable of stretching to make room for over a half-gallon of food and fluid in the average person. The stomach wall is lined by a layer of epithelial cells and has lots of deep pits that lead to the gastric glands. The epithelial cells inside these glands secrete the different components of gastric juice. This juice liquefies food and continue the process of chemically breaking it down into smaller components so it can be absorbed by the body.

Epithelial cells lining the stomach wall secrete gastric juice
Stomach

One major component of gastric juice is hydrochloric acid, which you may remember is a very strong acid. It can be represented by the abbreviation HCl. Because of its acidic properties, HCl kills most bacteria found in food. It also dissolves most types of tissues that we eat into a liquid form that is much more accessible to digestive enzymes. Unfortunately, HCl inactivates amylase, which leaves the polysaccharides only partially digested.

Pepsin

Speaking of digestive enzymes, another major component of gastric juice is pepsin. Pepsin is a digestive enzyme that breaks proteins down into smaller peptides. You can remember the function of pepsin if you remember that pepsin breaks proteins into peptides..

But this function of pepsin raises a fundamental question: how does a cell make an enzyme that breaks down proteins, without the enzyme damaging the proteins in the cell itself?

The answer is that the cell makes an inactive form of pepsin named pepsinogen, which is several amino acids longer than active pepsin. The extra amino acids block the active site of pepsin, causing it to be inactive inside the cell where it can damage other proteins. However, the HCl in the gastric juice removes these amino acids and converts the inactive pepsinogen into active pepsin once it is out of the cell and inside the stomach. To keep them from digesting components of the secretory cells, most digestive enzymes are produced in an inactive form known as a zymogen (an inactive precursor of an enzyme which requires a change to be activated). In this case, pepsinogen is a zymogen that is activated by HCl and turned into active pepsin.

So why doesn't the pepsin and hydrochloric acid digest the epithelial cells that line the stomach? It turns out that most of the epithelial cells of the stomach secrete large amounts of mucus, which forms a barrier to protect the cells against the acid and pepsin. In addition, the stomach isn't always filled with gastric juice. Between meals, the stomach is empty and pretty much inactive. However, when a person sees, smells or otherwise anticipates eating food, the brain sends a signal to the stomach to start preparing for a meal. As a result of this signal, the epithelial cells secrete a small amount of HCl into the stomach. The smooth muscles of the stomach are also activated and start churning the stomach at a low rate. This churning of the stomach, without food in it, is what causes your stomach to growl when it's getting close to meal time.

When food is seen or smelled, the brain sends a signal to the stomach
Pepsin

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