Comment fonctionne le système digestif ?

How does the digestive system work?

Our digestive system is a formidable machine, as powerful as it is complex. From the mouth to the large intestine, chemical and physical processes take place in order to extract the maximum of nutrients from the ingested food while protecting the organism from potential dangers.

If one's goal is to maintain good health over the long term by avoiding disease, it is interesting to understand the process of digestion.

Indeed, problems with the digestive system are often signs of a malfunction somewhere in the process. In this article, we will see together the different stages of digestion and how it all works.

The mouth and salivary glands

The first stage of the digestion process takes place in the mouth. Here, the purpose of chewing is to grind food. The salivary fluid produced by the glands is a mixture of secretions that help lubricate and break down food.

Chewing also has the effect of sending a signal to the stomach telling it to prepare its own secretions for the arrival of the “food bolus” created in the mouth.

During swallowing, the tongue pushes the food bolus into the esophagus, which takes over.

Did you know ?

We produce up to one liter of saliva per day!

Three pairs of salivary glands (in addition to those located under the tongue), allow it to be produced in sufficient quantity.

Young woman eats her lunch on the couch

The esophagus

Muscles in the wall of the esophagus create synchronized waves that one after another slowly propel food into the stomach.

In this process, called peristalsis, the muscles behind the food bolus contract, pressing it forward, while the muscles in front of it relax, allowing it to move forward without resistance.

Thus, it is possible to swallow food, even while doing a handstand!

Good to know : The esophagus is made up of several layers of muscle fibers, allowing the passage of food.

When the bolus of food reaches the lower end of the esophagus, the pressure exerted by the food signals a muscular valve - the lower esophageal sphincter - to relax and let the food enter the stomach.

The esophagus is hermetically isolated from the stomach because it does not have a wall protecting it from the acids contained in the stomach.


The stomach is an organ with strong muscular walls. When food reaches the stomach, it is mixed with acid and powerful enzymes that break it down.

These stomach juices also work to kill bacteria that may be in food.

Enzymes and acids mix with food that has already begun to break down in the mouth and esophagus, and thanks to the muscles present in the stomach, are transformed into "chyme"; it is a semi-liquid mass that is expelled from the stomach and sent to the intestines during digestion.

Drawing illustration of a digesting human stomach

How ?

By movements of muscular contraction, the stomach pushes the mass thus formed in the direction of the small intestine.

At the end of the stomach, the pyloric sphincter, which opens onto the small intestine, only opens a few millimeters. Thus, the larger pieces remain in the stomach to liquefy a little more.

The small intestine

The small intestine is made up of distinct segments, the duodenum, the jejunum and the ileum.

It is in the small intestine that most of the digestion takes place, and where all the remaining nutrients are absorbed.

Illustration hand drawing large intestine and small intestine digestive system

The duodenum

The duodenum performs a good part of the chemical digestion, as well as a small part of the absorption of nutrients; the main function of the jejunum and ileum is to complete chemical digestion and absorb these nutrients along with water and vitamins.

The secretion of digestive enzymes in the stomach usually causes the production of chemicals by the pancreas and liver, including the famous bile, which are used in the duodenum.

The jejunum

The jejunum, like other parts of the small intestine, is responsible for absorbing nutrients from food into the bloodstream. The jejunum is able to absorb these nutrients because it is lined with finger-like projections called villi.

The villi absorb nutrients in the form of minerals, electrolytes, carbohydrates, proteins and fats. Nutrients are absorbed into the bloodstream where they can be used as a source of energy by the whole body.

The ileum

The chyme then passes into the last, longest part of the small intestine, called the ileum. Its main function is to absorb vitamin B12, bile salts and all products of digestion that have not been absorbed by the jejunum.

Consequently, the ileum presents an extremely large surface both for the adsorption of enzymatic molecules and for the absorption of the products of digestion.

What remains of the food when it reaches the end of the ileum is a combination of water, electrolytes – like sodium and chloride – and waste products, like plant fibers and dead cells that break off of the wall of the digestive tract.

It is also along the small intestine that we find nearly 100 billion bacteria divided into several hundred species. Lining the walls of the digestive tract, they play a major role in digestion and disease prevention .

The large intestine

The large intestine is a large muscular tube about 7 to 10 cm in diameter that extends from the cecum (the first part, connected to the small intestine) to the rectum. It is about 1.5m long.

The slurry of digested food travels from the small intestine to the colon through the ileocaecal valve and cecum, where it mixes with the beneficial bacteria of the colon.

Vitamins are sometimes mistakenly thought to be nutrients that are only absorbed higher up in the digestive tract, but the colon plays a very important role in the absorption of vitamins necessary for good health. These vitamins are actually produced by healthy bacteria in the colon via fermentation.

The chyme moves through the four regions of the colon for several hours through peristalsis. In some cases, this process can become much faster due to the stronger waves of peristalsis that follow a heavy meal.

Did you know ?

Diseases affecting the large intestine such as irritable bowel syndrome (IBS) or Crohn's disease can lead to partial removal of this part of the digestive tract.

As it passes through the large intestine, more water and nutrients are absorbed and the mixture turns into a stool. These stools are then stored in the sigmoid colon until the body passes them out through the rectum and anus.

Stools are mostly made up of food debris and bacteria. When the descending colon is full, it empties its contents into the rectum.

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