Digestive System

The system of organs that breaks down food so the body can use the nutrients.

What is your favorite food? Tacos? Broccoli? Ice-cream? Whatever you enjoy, your digestive system turns the food you eat 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 you healthy and gives you the energy you need to do all that you do during the day like run, play, work, and learn. Your amazing digestive system works for you every day of your life. Let's learn more about it.

The digestive system is a complex process that actually begins with your mouth. In fact, the salivary glands in your mouth start working before you even take a bite. When you see, smell, or even think about the food you're about to eat, your mouth starts to water. Your salivary glands, located under the tongue and in the back of the mouth, add saliva to your mouth to begin the digestive process. When you take in pieces of food, your teeth bite, chew, grind, and turn that solid food into something you can swallow. Saliva helps moisten the food and begins to break down carbohydrates and fats right there in the mouth. The tongue pushes the food around as you chew and sends the mashed-up food to the throat when you are ready to swallow. This mass of chewed-up food is known as a bolus.

After swallowing the bolus, the esophagus takes over for a while. Food is pushed down the esophagus, a stretchy, hollow tube that is about 10 inches (25 cm) long. During the act of swallowing, the epiglottis (a flap of cartilage at the back of the throat) closes over the opening to the windpipe (or trachea, the tube that carries air) so that food cannot go 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 were 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 down in waves. These waves — known as peristalsis — are the process that moves food all the way through the body.

From the esophagus, the 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. The stomach is a stretchy, muscular bag that actually churns, mashes, and adds special chemicals, or gastric juices, to the bolus. These gastric juices and acids help to break down food particles and kill any harmful bacteria that might be present. 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 motion, together with the gastric juices, breaks the food down into a thick, soupy liquid known as chyme. At the other end of the stomach, another sphincter holds the chyme in the stomach until it is just the right time to send it to the small intestine.

The small intestine is the longest part of the digestive system. Although it is all coiled up inside your body, if you stretched it out it would be 22 feet (6.5 meters) long! It is called “small” because it is only one inch (3 cm) wide. The small intestine folds and turns many times in the abdomen. As the chyme flows through the small intestine by peristalsis, it is further broken down into smaller and smaller nutritional parts until it is mostly liquid. Digested nutrients are absorbed into the bloodstream through the walls of the small intestine, and carried throughout the body to provide fuel for energy, growth, and cell repair. The lining of the small intestine is covered with tiny finger-like projections called villi, which provide a large surface area for absorbing nutrients into the body. Within the digestive system, about 90% of nutrient absorption takes place in the small intestine.

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 is 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, an oblong organ located just behind the stomach, secretes chemicals known as enzymes into the small intestine. These enzymes cause chemical changes that help digest fats, proteins, and carbohydrates. The enzymes from the pancreas work with chemicals from the liver and small intestine to break down food parts into nutrients the body needs, which are then absorbed into the bloodstream. The pancreas also produces insulin, which is needed to move sugars from the blood to the body's cells. Take a look at this picture showing the location of the pancreas.

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, also called the colon. The large intestine is shorter and wider than the small intestine – about 5 feet long (1.5 meters) and 3 inches wide (7.6 cm) in the average adult. It makes about three gentle turns in the abdomen. The job of the large intestine is to absorb water, vitamins, and minerals from the undigested food. Necessary bacteria that live there help digest any remaining nutrients. As the water leaves the undigested material, what remains becomes more solid as it keeps moving along to the end of the large intestine.

At the end of the digestive process, the remaining unused waste is stored in the rectum. The rectum's job is to receive the undigested waste products, known as stool or feces (or poop!). The rectum stores it there while sensors let the brain know that waste is ready to leave the body as a bowel movement. The urge to use the bathroom is the signal that the process is just about done. The muscles of the rectum move the waste products out through the anus, the final part of the digestive tract. The anus consists of sphincter muscles that hold onto the waste until you use the toilet. At that point, the anus allows the feces to exit the body, and then be flushed away.

The digestive process can be thought of as a four-part process: Ingest, Digest, Absorb, Excrete. The mouth and esophagus do the work of ingestion, the stomach works on digestion, absorption occurs in the small intestine, and the large intestine prepares what's left for excretion. The process from beginning to end, from first bite to the last trip to the bathroom, can take between 24 and 72 hours (1 to 3 days), although it happens faster with certain foods. Carbohydrates, such as rice or simple sugars, are digested quickly, while fats and proteins take longer. Food particles may remain in the stomach for 1 to 5 hours, in the small intestine for 2 to 6 hours, and up to 36 hours in the large intestine. Most people eat several times within that time frame, so the process can seem to be constantly ongoing.

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. Harmful 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.

Gas – We all have gas. Gas enters your digestive tract in two ways: when you swallow air while eating or drinking, and when bacteria in your large intestine breaks down undigested food material. This process creates gases such as hydrogen, carbon dioxide, and methane. When the gas is expelled, it may have a bad odor. Some people try to avoid foods that tend to cause gas, such as beans, broccoli, onions, and fizzy drinks. But remember that passing gas is normal. The average human passes gas up to 25 times a day. Learn more about gas at KidsHealth.

Nausea – This is 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 relieve the nausea.

Vomiting – This is also known as “throwing up.” Most of us have had it happen to us. It isn't pleasant, but it 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 clears the offending contents away and we often feel better. As with nausea, vomiting can be caused by many things. In most cases, it is not serious, but if it persists, it should be treated by a doctor.

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 GER (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 about GER at KidsHealth.

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 were 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 a lot of other problems. 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. Learn more at KidsHealth.

Food sensitivity – Also known as food intolerance, this is similar to a food allergy but less serious. A true food allergy involves the immune system, while a food intolerance affects only the digestive system. When a person has a sensitivity to a certain food, they tend to feel unwell after eating it. Symptoms may be a stomach ache, nausea, vomiting, or diarrhea. These reactions are not life-threatening but can be very uncomfortable. Some of the more common foods involved are dairy products, spicy foods, and wheat gluten. For example, one food sensitivity is known as lactose intolerance, where the body has trouble digesting the lactose sugar contained in milk, yogurt, and ice cream. People sometimes become more sensitive to food as they get older. They might have loved spicy foods when they were younger, but as they age they find that their digestive system “complains” so they choose to 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.) Doctors used to think this little organ had no real purpose, but today it is believed that the appendix stores good bacteria that can help your digestive system recover after you've been sick. However, when it gets infected, the appendix swells up and needs to be removed. A sick appendix will cause pain, nausea, vomiting, fever, and other symptoms of illness. People usually recover quickly from an appendectomy (surgical removal) and can get along fine without an appendix.

Learn more about digestive diseases from the National Institutes of Health.

How do doctors and scientists know what is happening in the digestive system? One of the first to observe the digestive process in motion was Dr. William Beaumont. In 1822, a man named Alexis St. Martin was accidentally injured in the stomach by a gunshot wound. Dr. Beaumont treated him immediately, but after three years, a small opening leading into his 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.

Today, doctors who are experts on the digestive system are called gastroenterologists. They can observe a patient's digestive tract through the use of special X-ray scans. They can also examine a person's esophagus, stomach, and intestine through a procedure called an endoscopy, where a long, flexible tube with a tiny camera is inserted down the throat and the camera transmits images to a video monitor. That's much better than having to observe through a hole in the stomach!

All animals have digestive systems that allow them to take in the nourishment they need. There are many variations of digestive systems in the animal kingdom, with different animals' systems being adapted to their specific feeding behavior and diet. For example, jellyfish have a digestive cavity with just one opening that acts as both mouth and anus. Spiders digest their food outside their bodies by secreting chemicals onto their prey, turning it into a liquid soup before sucking it up into their mouths. Frogs, on the other hand, have most of the same digestive organs that humans do.

In vertebrates, there are four main types of digestive systems. Bears, pigs, dogs, and cats have systems similar to humans in that they have one simple stomach. This is known as a monogastric digestive system.

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

Ruminant digestive systems are found in cattle, sheep, goats, deer, and giraffes. They have four different compartments in their stomachs to help store and digest the high-fiber foods they eat, such as hay and grasses. Each compartment has a special function that helps digest these tough foods. When ruminant animals eat, they chew their food only a little and then swallow it into their first stomach compartment, known as the rumen. Later, they regurgitate or bring the moist, softened food back into their mouths to chew it further. This is called chewing their cud. After thoroughly chewing the cud, they swallow again and the food passes back through the stomach chambers to be broken down further. Other animals, such as horses, rabbits, hamsters, and camels have pseudo-ruminant digestive systems. These animals also eat lots of fiber but lack four-chambered stomachs.

Learn more about these four digestive systems; the diagrams will help you understand how each system is different. Then see if you can figure out which digestive system some of your favorite animals have.

• 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 4-8 cups (1-2 liters) of saliva every day.
• The average human eats about a ton of food per year – yes, that’s 2000 pounds per year.
• Your digestive system measures about 30 feet (9 meters) from mouth to anus, which is almost as long as a bus.
• The stomach is about the size of a tennis ball when it's empty, but expands to the size of a soccer ball following an extra-large meal.
• Because of the action of peristalsis, food would get to your stomach even if you were standing on your head.

Before your digestive system does its work, your first step is choosing the food you eat. You can help keep your digestive system healthy by eating a diet that includes high-fiber foods such as fruits, vegetables, and whole grains, and by drinking plenty of water. Learn more about food science at Science Trek's Nutrition site.