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The Human Gastrointestinal Tract

The Human Gastrointestinal Tract
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  • 0:37 Mouth
  • 1:16 Swallowing
  • 2:37 Stomach
  • 3:55 Small Intestine
  • 6:31 Large Intestine
  • 7:38 Lesson Summary
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Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Would you like to know what happens to your lunch after you eat it? Watch this video as we take a trip through your gastrointestinal tract and learn when and where the foods you eat get broken down and separated into usable nutrients and wastes.

GI Tract

Did you ever wonder what happened to that cheeseburger that you had for lunch? When exactly did it start getting digested? Where did most of the digestion take place? Was it in your stomach or somewhere else? And how did that big cheeseburger get broken down into pieces that were small enough to enter your bloodstream?

There are answers to these questions, and you'll find all of them when you learn about the human gastrointestinal tract, or GI tract, which is the continuous tube through which food is broken down. Let's take a look.

Mouth

Your GI tract doesn't waste any time digesting, or breaking down, the foods you eat. In fact, as soon as your first bite of cheeseburger enters your mouth, the process begins. Your teeth help to break down large pieces of food into smaller, more manageable pieces. Then, your tongue helps move the food around so it can be mixed with saliva. Saliva contains salivary amylase, which is an enzyme that moistens the food and begins the digestion of starches, like the bun of your burger. By this point, the food is a softened food mass that's ready to be swallowed.

Swallowing

Swallowing is a fairly basic process. The food mass gets pushed to the back of your mouth where it enters the pharynx, which you might think of as your throat. At this point, it's ready for a straight descent through the tube that connects the pharynx with your stomach, called your esophagus, but first it has to be maneuvered past a tricky spot. I'm talking about your windpipe.

Did you ever swallow wrong and end up coughing? That's because food accidentally entered your windpipe, which has its entrance near the bottom of the pharynx. Fortunately, this doesn't happen very often, because when things go right, the action of swallowing causes the epiglottis, which is a flap of cartilage that covers the entrance to the windpipe, to tip over. The epiglottis closes the door to your windpipe until the food passes by.

Your food mass is now on the way down your esophagus. It's being pushed along by wavelike muscular contractions known as peristalsis. At the bottom, it encounters one of a number of gateways found in your GI tract - this one is known as the lower esophageal sphincter. It's a ring of muscle that leads from the esophagus to the stomach. It closes off after food passes so the stomach contents don't go backward and return up to the esophagus.

Stomach

Your glob of cheeseburger has now made it to your stomach. Your stomach is sort of like a water ride for your meal as food gets tossed around and mixed with gastric juices by muscular contractions in a process known as churning. One of the gastric juices that gets secreted when food arrives in your stomach is pepsin. Pepsin is a digestive enzyme in your stomach that breaks down proteins like meat and cheese.

We also find that hydrochloric acid, or HCl, makes up part of the gastric juices in your stomach. You might recall from chemistry class that HCl is a strong acid. This acid helps in the early stages of food digestion, but you might be wondering how you could have such a harsh chemical inside your body without causing damages. Fortunately, your stomach is lined with mucus that protects your stomach lining against the harsh acid. While it's not fun to think of food being in contact with mucus, you should be glad it's there to protect you.

Food can stay in your stomach for two to six hours, depending on what you eat. But surprisingly, this is not the area of the GI tract where most of the digestion takes place. That comes next as we move into the small intestine.

Small Intestine

When the food mass is ready to leave the stomach, it leaves in small squirts through another gateway. This gate is called the pyloric sphincter, which is a ring of muscle that controls the passage of food out of the stomach. On the other side of the sphincter is the first part of the small intestine, which is called the duodenum. It's in your duodenum where most of the chemical digestion of food takes place because this is where important digestive organs, like your pancreas, liver and gallbladder, secrete their digestive juices.

When the food mass, that was once your delicious cheeseburger enters your duodenum, your pancreas gets to work. For starters, it secretes bicarbonate, which is a base that helps neutralize the acidic gastric juices from the stomach. Bicarbonate not only makes the gastric juices less acidic, it also helps activate important digestive enzymes that come from the pancreas.

Your pancreas secretes digestive enzymes that break down all of the macronutrients, which are carbohydrates, proteins and fats. If you think back on what we've discussed, you'll see that this is the first time fat is being broken down. Because fats haven't been broken down before they enter the duodenum, they require a bit more work than the other two nutrients.

Fat tends to glob together, which makes it hard to break down. So, when fat, like the grease in your cheeseburger, comes into the small intestine, your liver and gallbladder pitch in to help. Your liver makes bile, which is a yellowish-green substance that breaks large fat globs into smaller fat globs. This makes the fat particles more manageable, so the pancreatic digestive enzymes can get to them more easily. Any leftover bile is then taken back up into the gallbladder, which you can think of as a bile storage sac. Unused bile stored in the gallbladder can be used in the future, so in a way this is like a recycling program for bile.

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