Digestion: how does it take place?

Understanding the digestion process

In the digestive tract, food undergoes a series of mechanical and chemical breakdowns that cut up the nutrients.

The nutrients resulting from digestion pass through the porous wall of the intestine to enter the blood and lymph: this is absorption.

The blood carries the nutrients to all the organs and these nutrients enter the cells where they are used as building blocks or as a source of energy, the undigested food being expelled as waste.

The passage of food allows a review of the different organs of the digestive system.

Food undergoes a mechanical transformation (mouth and stomach) then a chemical transformation (stomach, intestines). Digestive juices act as molecular scissors to produce nutrients: proteins, lipids, carbohydrates, vitamins and minerals.

In the mouth, food is chewed, cut into small pieces and crushed by the teeth. At the same time, it is soaked with saliva. This food bolus will be pushed by the tongue to be swallowed.

The mouth is connected to the stomach by the oesophagus, which in its upper part, the pharynx, directs the passage of food.

By contractions of the oesophageal walls, food moves forward and passes into the stomach through a valve: the cardia.

In the stomach, the food bolus is churned for three or four hours to reduce it to a pulp. At the same time, gastric juices (acids and enzymes) are produced, reaching up to two litres per 24 hours.

The small intestine, composed of the duodenum, jejunum and ileum measuring seven metres long, collects this pulp.

The duodenum receives digestive juices from the pancreas and the gallbladder, the intestinal cells secreting intestinal juice in turn. The pulp or chyme is broken down into nutrients by these juices and these elements will pass into the blood: this is absorption.

The thin inner intestinal wall, richly vascularised, offers a large surface area for nutrient absorption. The journey ends in the large intestine (1.5 metres) containing thousands of bacteria which will allow the fermentation of indigestible food which, combined with reabsorbed water, will be expelled.