How The Digestive System Works

By Wendy Innes. May 7th 2016

Most of us know the basics of the digestive system: we eat food, swallow and the body does the rest. But the exact mechanics of how the digestive system works is something of a wonder. Food is fuel for the human body and exactly how the body takes that fuel in, processes it and discards waste is a complicated production, and if one part of the process is off by just a little, it can lead to great discomfort or serious illness.


Everyone knows how the mouth works. We put food in and chew. But the digestion of the food we eat actually begins here, which is why chewing your food thoroughly is so important. Inside the mouth there are three major pairs of glands that produce saliva:

  • Parotid gland: These glands are located in the cheek and they are located in front of the ears, over the jaw. These are the largest of the saliva glands and produce alpha-amylase. This is responsible for breaking down amylase and amylopectin, the starches found in some foods.
  • Submandibular gland: These glands are located in the floor of the mouth, inside the lower jaw. These glands are responsible for about 70 percent of the volume of saliva that people have in their mouths when eating. In addition to secreting amylase, this gland also secretes mucin, which lubricates the food as it moves on through the digestive system.
  • Sublingual gland: These small glands are also located at the bottom of the mouth, inside the submandibular glands, under the tongue. These glands also produce amylase. These glands only produce about 5 percent of the total saliva volume in a person's mouth.

In addition to these three major saliva glands, there are more than 600 minor ones and their job is to secrete mucus to keep the mouth lubricated and aid in digestion. Some of these glands share excretion ducts with other glands and some have ducts of their own.


The esophagus is the tube that moves food from the throat when it's swallowed down to the stomach. The esophagus wall contains layers of muscles that contract and relax in a wave-like motion to move food into the stomach.

At the bottom of the esophagus is the gastro-esophageal sphincter. This is a valve that is closed most of the time, only opening when stimulated to allow food to pass, or to allow a person to burp or vomit.

Some people have a problem with this valve that allows the gastric juices in the stomach to flow back up the esophagus, causing irritation and the sensation of heartburn. Over time this can cause the lining in the esophagus to erode and cause damage to the esophagus itself.


All of the large hollow organs in the digestive system have smooth muscles that allow the walls of those organs to move, enabling food to move through the digestive tract, and the stomach is no different.

The stomach has three jobs. It stores food and liquid that has been swallowed, it mixes all the food and liquid in the stomach with the gastric juices produced in the stomach and slowly releases food into the small intestines.

Several factors will affect how quickly food is released into the smaller intestine including the kind of food and how efficiently the stomach muscles are working. Carbohydrates spend the least amount of time in the stomach, proteins take longer and fats take the longest for the stomach to mix and release as it is supposed to.

Small Intestine

Most people think that the majority of food digestion occurs in the stomach, but this isn't true. In fact, the small intestine does most of the work, which is probably why it is so long – about 22 feet long to be more accurate. The small intestine is responsible for further breaking down food and extracting nutrients from the food and allowing it to be absorbed into the rest of the body.

The small intestine is separated from the stomach by the pylorus and is made up of three sections;

  • Duodenum: This is a short section of the intestine that takes food from the stomach and mixes it with bile from the liver, gallbladder and pancreas to further digest food. The smooth muscles in the walls of the duodenum then move food into the jejunum.
  • Jejunum: This is the middle section of the small intestines and food is rapidly broken down further and moved through to the ileum.
  • Ileum: This is the last and longest section of the small intestines. It is in this section that the nutrients from food are absorbed into the rest of the body. What is left of the food is then emptied into the large intestine.

Liver And Gallbladder

The liver and gallbladder are two of the organs in the digestive system that are not hollow. They work together to produce and store the bile that is necessary for the small intestines to digest food. The liver makes bile that is then stored in the gallbladder. When the bile is needed it is secreted through the bile duct into the duodenum to digest food.


The pancreas is essential to the digestion of sugars in the body. When people chew their food, some of the saliva glands in the mouth produce amylase, an enzyme that breaks carbohydrates into maltose. The juices from the pancreas then break down the maltose into glucose that the body can use for fuel.

Large Intestine

The large intestine is about five feet long and takes a much more direct path through the body. It is also much wider than the small intestines and it is here that the majority of the liquids that are taken into the body are absorbed. Salt is also absorbed from the large intestine when the body needs it. The large intestine is divided into multiple sections:

Cecum: This is the first section of the large intestine and it is shaped like a pouch. It's about two inches long and its job is to receive the processed liquid from the small intestine. Attached to the cecum at the bottom is the appendix.

Colon: There are four sections of colon that make up the majority of the large intestine. Often people will simply call the large intestine the colon, but in fact, there are four separate parts. This is the area in which water and salts are reabsorbed into the body to prevent dehydration. When someone has diarrhea, it is usually because they have some type of infection that affects how well the colon does its job.

  • Ascending colon: Using the contractions of the smooth muscles found in the large intestine, this part of the colon pushes food up from the cecum into the transverse colon. It is located on the right side of the body, just under the liver.
  • Transverse colon: Digested food travels from right to left along the front abdominal in the transverse colon. It is located in front of the small intestine. The food will end up just under the stomach, where it will pass into the descending colon.
  • Descending colon: The digested food moves through the descending colon on the left side of the abdomen near the spleen where it enters the sigmoid colon.
  • Sigmoid colon: This is the final stop in the colon for digested food. The sigmoid colon is an S-shaped passage that curves back in along the bottom of small intestines and empties into the rectum.


Contrary to popular belief, the rectum is not where waste exits the body. Often, rectum and anus are used interchangeably, but the truth is that feces, the digested food waste of the body, collects in the rectum. It is stored there until it expands enough to send a signal that it's time for a person to go to the bathroom. At this point, the rectum is emptied, and waste passes into the anus, where it leaves the body.


The anus is the cavity through which solid waste leaves the body. When a person is not going to the bathroom, it is empty. Nothing is stored in the anus, it is merely a passageway.

There are many parts to the human digestive system, and they all must work in harmony to do their extremely important job. If not, any number of illnesses can result. If you are experiencing any serious or chronic issues with digestion, you should consult your physician.


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