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Digestive System: Facts

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Taco and Hot Sauce

What is your favorite food? Tacos? Broccoli? Ice-cream? Whatever you enjoy – your digestive system turns the food into usable energy inside your body. The food contains nutrients such as carbohydrates, proteins, vitamins, minerals, fats, fibers, salts, and water. It is through digestion that these nutrients are broken down, absorbed and used by our bodies. This keeps us healthy and gives us the energy we need to do all we do during a day like run, play, work, and even to eat again. The energy that food produces is measured in calories. If you’d like to know more about nutrition, Science Trek has a site for you to visit.

The Mouth

Boy Eating Hot Dog

The digestive system is a complex process that actually begins with your mouth. Our teeth bite, chew, grind and turn that solid food into something we can swallow. At the same time that we are chewing, your saliva gland adds saliva to your mouth to begin the digestive process. The saliva gland is located under the tongue and in the back of the mouth. Saliva helps carbohydrates and fats to start breaking down right there in the mouth. The tongue pushes the food around as we chew and also sends the food down the throat to the esophagus when we are ready to swallow. This mass of chewed up food is known as bolus.

The Esophagus

Esophagus and Stomach

After swallowing the bolus, the esophagus takes over for a while. Food is pushed down the esophagus – a long hollow tube – past your heart and lungs on its route to the stomach. During the act of swallowing, the epiglottis (a flap of cartilage at the back of the throat) closes over the voice box which leads to the windpipe to stop food from going into the lungs. A muscle known as a sphincter at the top of the esophagus relaxes to let food pass. If you have ever choked while eating, you probably got to swallowing a little too fast and your epiglottis didn’t close in time to stop food before it tried to enter into the lungs. So you coughed. That’s the body’s quick reaction to food in the wrong location!! When eating, muscles in the esophagus continue to push food on through in waves. These waves – known as peristalsis - are the process that moves food all the way through the body.

The Stomach


From the esophagus, bolus enters the stomach. Another sphincter holds the entryway to the stomach closed and then opens to allow small amounts of bolus into the stomach. This is all controlled by the body. The stomach is a muscular sac that actually churns, mashes and adds special chemicals, or gastric juices, to the bolus. The lining of the stomach is protected from the gastric juices by a mucus covering so that the gastric juices don’t try to digest the stomach itself. The muscles of the stomach work in peristalsis to move the bolus around and churn it. This churning along with the gastric juices, break the food down some more into a thick liquid known as chyme. On the other end of the stomach is another sphincter that holds the chyme in the stomach until it is just the right time to send the chyme to the small intestine.

The Small Intestine

Small Intestine

The small intestine is about 18 feet long in the average adult, but it is small in width compared to the large intestine. The small intestine folds and turns many times in the abdomen. (Center of image to the left.) As the chyme flows through the small intestine by peristalsis, it is further broken down into smaller and smaller nutritional parts. The vitamins, minerals, proteins, carbohydrates, fats, water, and salts from the food pass through hair-like projections all along the interior lining of the small intestine and into the bloodstream. These hair-like projections are known as villi.

Helping Organs – Liver, Gall Bladder and Pancreas

Liver and Gall Bladder
The large organ in this image is the liver and the small green organ is the gall bladder.

At the top of the small intestine is a duct or tube that allows a chemical known as bile to be added to the material in the small intestine. This bile was created in the liver, stored in the gall bladder until it is needed and then passed off to the small intestine for the purpose of digesting fats. Without it the fats would not break down and would be unusable in the nutrition process. This is a complex process that the body controls on its own. Most of the bile is recycled back into the bloodstream, back to the liver and stored in the gall bladder again until needed.

The pancreas also sends chemicals known as enzymes into the small intestine near the same location as the gall bladder does. The pancreas is about 6 inches long and is located behind the stomach. The job of the pancreas is to regulate carbohydrates, proteins, amino acids, and fats in the body. This organ knowns which of several chemicals and how much of each to add to the chyme to break these food parts into nutrients the body can use. Many of the chemicals actually change while in the small intestine as part of this process. It is here that insulin is produced. People with diabetes have an inability to produce insulin and so the carbohydrates in their food do not break down as required by the body. This can cause many health issues. Fortunately, science has found ways to help replace insulin through other methods. Here’s a great picture of the location of your pancreas.

The Large Intestine

Large Intestine

At this point, whatever the body has not used is said to be undigested. This can include fibrous parts of food, extra nutrients the body didn’t need, bile, germs and bacteria and lots of water. This material moves from the small intestine to the large intestine. The large intestine is much shorter than the small intestine – about 5 feet long in the average adult. It makes about three gentle turns in the abdomen. Inside the large intestine, necessary bacteria that live there help to break down this undigested food further which also helps to create additional vitamins to be used by the body. The water and a few last minute amounts of nutrients are absorbed out of the undigested food in the large intestine. The large intestine is also called the colon.

The Waste Product

Toilet Paper

From here, we come to the end of the digestive process. The final unused waste is now ready to leave the body. It has done its job of giving the body nutrients and water and is ready to be removed. It heads to the rectum where the urge to use the restroom is the signal that things are just about done. The final step in this process is the anus – another sphincter muscle that holds on to the undigested food parts. When we use the restroom, the anus allows this material to leave the body and we flush it away. This waste is known as stool or feces.

Beginning to End

Digestive System

The digestive process from beginning to end can take up to fifteen hours. From the very first bite to the last trip to the bathroom, the food enters and exits within that fifteen hour block. It can spend as much as five hours in the stomach alone. For most people, they have eaten again within that fifteen hour time frame, so the process can seem like it never ends.

Tummy Troubles

Basket of Fruit

So what happens when we have stomach issues that cause us pain or sickness? Here are just a few of the things that can be out of sorts in the digestive system.

Constipation – this is when it is hard to go to the bathroom. Not enough water, stress, not eating enough fiber or a slowed digestive process can cause food to move too slowly through the large intestine and too much water is then removed. This causes the material to get hard. While there are other reasons for constipation, eating more fiber and drinking more liquids will often solve this problem.

Diarrhea - this is when the stool is too liquid and it causes frequent and often uncontrollable trips to the bathroom. Bad bacteria can be the cause of this problem. Sometimes too much fiber is the cause. Usually this will clear up on its own, but when it doesn’t a trip to a doctor would be in order.

Sick Person

Gas – don’t laugh!!! We all have gas. Gas is produced when the food material is breaking down in the intestinal system. Food such as milk, fruits and vegetables are actually rotting inside the intestines and this will create gas. When we expel the gas, it often has an odor. It can sometimes create pain, as well. Avoiding some of these foods may help. Avoiding gum chewing, fizzy drinks and talking while you eat can also keep air out of the digestive system. But even with this, the average human can pass gas up to 21 times per day. Learn more about gas at NIDDK.

Nausea – the feeling of being sick to the stomach. Nausea can be caused by all sorts of things; from car sickness, to ear infections, to unclean food or water. Solving the cause will usually clear up the nausea. Nausea can result in vomiting, which can also solve the problem. Nausea can also be caused by other diseases and conditions. Some are very serious.

Dragon with an Upset Tummy

Vomiting – also called throwing up. Most of us have had it happen to us. It isn’t pleasant, but will often help us to feel better by removing the problem from our stomachs. When we throw up, the sphincter muscles of the stomach and the muscular parts of the esophagus help to push the contents up and out of the stomach. This is often to clear the offending contents away and we often feel better. As with nausea, vomiting can be caused by many things. This is another one of those things that can be a symptom of a serious illness.

Heartburn – this is about the esophagus and stomach, not the heart. Stomach acid can bubble up past the sphincter connecting the stomach to the esophagus. This can happen due to gas, a weak sphincter, eating too much or too fast, or something called GERD (gastroesophageal reflux.) A burning sensation will happen in about the middle of the abdomen and can sometimes travel up to the back of the throat. Antacids will sometimes clear this up. If they do not, this requires a doctor’s help. Learn more about GERD at kidshealth.org.


Food allergies – not everyone gets along with all food. If a person has an allergy to a food, it means that their body is reacting to the food as if it was an invading organism or bacteria. The immune system tries to fight it and gives off chemicals in the process. This can lead to rashes, swelling of the throat, difficulty breathing, vomiting, diarrhea, and lot of other nasties. Serious reactions can mean that a person will need to go to the emergency room. If the person knows, in advance, that they have a food allergy they should avoid that food. If by accident, they should eat the offending food, some people keep medication on hand to help them overcome the reaction. Strawberries, milk, peanuts, shellfish and eggs are often the cause of a food allergy. Food allergies can be very serious and even life threatening. Yup, there’s more at kidshealth.org.

Loaf of Bread

Food sensitivity – different, but similar to allergies. When a person has a sensitivity to a food, they will also get reactions. These are less life threatening, but can also be very uncomfortable. Nausea, vomiting, diarrhea, stomach discomfort, and a general feeling of not being well will be the symptoms. There can be lots of foods that cause this. Milk, dairy, spicy food and wheat gluten are some of the more common foods involved. People often get more sensitive as they get older. They might have loved eating spicy food when they were in their twenties, but now that they are in their sixties they find that their digestive system complains and so they avoid them.


Appendicitis – this is when the appendix gets infected. The appendix is a small organ about the size of your little finger that is attached to the large intestine. (See the small organ in the lower left of the image.) Science isn’t sure why we have one. It might have had a purpose at one time in history, but no longer seems to have a job. When it gets infected, it swells with the infection and the appendix needs to be removed. A sick appendix will cause pain, nausea, vomiting, fever and other symptoms of illness. Most people recover from the removal, or appendectomy, very quickly. Find out more about appendicitis at kidshealth.org.

The Man With the Hole in His Stomach

Cowboy Sheriff

In 1822 Alexis St. Martin was accidently injured in the stomach by a gunshot wound. Dr. William Beaumont treated him immediately, but after three years, a small opening leading into this stomach was still not healed. Through this opening, Dr. Beaumont was able to observe the digestive process. His experiments and understanding of digestion became the basis for modern day knowledge of how the digestive process works.

Digestion in Other Animals

Hen and Chick

Up to this point, our discussion has been about the digestive system in humans. Because, honestly, people are more likely to use a computer to study science than say a bear, right? But bears, pigs, dogs, and cats have digestive systems similar to humans in that they have one simple stomach. This is known as being monogastric.

Cow Eating Grass

There are many variations of digestive systems in the animal kingdom. Avian digestive systems are found in birds such as chickens, turkeys, and geese. Avian digestive systems include additional organs such as the crop and the gizzard. The crop is used for storing food for later digestion – a kind of internal pantry. The gizzard is a special stomach that grinds food. Birds will eat small bits of rock which ends up in the gizzard along with food. The rocks help to grind the food material which is helpful because birds have no teeth. Some birds, like owls, regurgitate a pellet of hair, bones, and feathers left over after the digestion consumes the food portion of their meals.

Giraffe Eating Leafs

Ruminant digestive systems are found in cattle, sheep, goats and deer. These animals eat food that is high in fiber like grasses. They eat it with very little chewing. Because of this, these animals have the ability to regurgitate their food and chew their food a second time. This is called chewing their cud. They have different compartments in their stomach for the storing and processing of food at the time of the first eating and then again later after they’ve chewed it another time.

Learn more about four digestive systems - great diagrams to help you understand how each system is different.

Take a look at the digestive systems of some of your favorite animals at Animalsmart.org.

Fascinating Facts About Digestion

One Ton Weight
  • Seahorses, platypuses, and carp have no stomachs at all.
  • The rumbling noise your stomach sometimes makes is caused by peristalsis – it’s louder when your stomach is empty.
  • It takes about 7 seconds from the time you swallow for the bolus to reach the stomach.
  • Our bodies create a little over 7 cups of saliva each day.
  • The acid in your stomach is strong enough to dissolve metal.
  • The average human eats about a ton of food per year – yes, that’s 2000 pounds per year.
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