The Digestive Processes

Digestive processes allow food to enter the body, move through the digestive tract and transform nutrients into simple molecules that can be absorbed by the body. They also enable the removal of non-digestible food residue.

The main digestive processes are listed here.

The organs of the digestive tract are where one or more digestive processes take place. The following picture and table match the organs of the digestive tract with the digestive processes that take place in them.

The propulsion of food is carried out by two mechanisms: deglutition and peristalsis. Food is propulsed in one direction only: from the mouth to the large intestine.

Mechanical digestion of food is a process that facilitates chemical digestion. Reducing the size of the food pieces increases the surface area of contact between the nutrients and the digestive enzymes, which helps break them down into simpler molecules.

Here are four processes of mechanical digestion.

Mastication, also known as chewing, is a movement of the jaws that allows the teeth to grind food.

During chewing, the cheeks and closed lips hold the food between the teeth while the tongue mixes saliva with the chewed food.

Once well mixed with saliva, the food is compacted by the tongue and forms a mass known as the food bolus or alimentary bolus.

Since lipids are fats, they are not soluble in chyle, which mainly consists of water. The insolubility of lipids in water hinders their breakdown into simpler molecules that can be absorbed by the body.

This is why the emulsification of fats is essential.

The emulsification of fats is performed in the small intestine by the bile produced by the liver. More specifically, the bile salts contained in the bile surround the fats in a soap-like manner and enclose them in numerous tiny water-soluble capsules. The fat cluster is transformed into fine lipid droplets that are distributed in the chyle.

Once the fat has been emulsified, it becomes easier to break down the lipids into simpler molecules that can be absorbed by the body.

During chemical digestion, nutrient molecules are broken down into simpler molecules that can be absorbed by the body. For example, a carbohydrate can be transformed into a simple sugar (glucose, fructose, etc.); a lipid can be transformed into glycerol and fatty acids; and a protein can be transformed into amino acids.

The image below shows the main nutrients that make up food, as well as the simple molecules made accessible through chemical digestion.

Chemical digestion is performed by digestive enzymes found in the digestive juices.

As food moves through the digestive tract, digestive juices are produced by certain digestive glands and added to the contents of the digestive tract.

The following table shows the digestive juices, the digestive glands that secrete them, where they are released and their main roles in the chemical digestion of food.

Water, vitamins and minerals are nutrients that are small enough to be absorbed without undergoing processing. Other simple molecules such as amino acids, glucose, glycerol and fatty acids are made available through mechanical digestion of ingested food and chemical digestion of nutrients.

Most absorption happens in the small intestine. Folds, called villi, line the inner wall of the small intestine. The surface of the cells lining the villi is also covered with microscopic projections, called microvilli. This configuration favours the absorption by greatly increasing the contact surface between the contents of the small intestine and its wall. Nutrients and simple molecules then pass through the wall into capillaries and lymphatic vessels.

When the contents of the small intestine flow into the large intestine, the absorption of nutrients and simple molecules is largely complete. While 90% of water is absorbed by the small intestine, the large intestine absorbs some of the remaining water from undigested food. The large intestine also allows the absorption of vitamins, some of which are synthesized by the thousands of bacteria it contains. These bacteria form the bacterial flora of the large intestine.