Biology / Digestion


Digestion is the transformation of food into assimilable substances, carried out in the digestive or digestive tract through two types of processes: mechanical and chemical .

Mechanical Digestion

Mechanical digestion is performed with chewing , swallowing and movements that occur in the digestive tract, called peristaltic movements or peristalsis.

Chewing and Swallowing

In digestion, in its mechanical process, foods are chewed and reduced to very small pieces, with the help of teeth and tongue. The contact of food with saliva facilitates its passage through the digestive tract.

After chewing and salivation, the food cake is swallowed. During swallowing the soft palate is retracted upwards and the tongue pushes the food backwards, throwing it into the pharynx , which contracts and projects the food bolus into the esophagus .

When we swallow, the epiglottis closes the glottis, preventing food from going to the trachea .


The esophagus is a muscular conduit that performs involuntary contractions called peristaltic movements or peristalsis , leading the food bolus to the stomach , where the chemical process of digestion begins.

Chemical digestion

In chemical digestion , foods are broken down into smaller particles thanks to the action of enzymes present in the digestive juice, undergoing changes in their chemical composition.


In the stomach, peristaltic movements mix the bolus with gastric juice, produced by the glands of the gastric mucosa. This juice contains hydrochloric acid , which maintains stomach acidity, providing a favorable condition for the work of enzymes in digestion.

Pepsin , the main enzyme in the stomach, acts on protein transformation, enhancing chemical digestion. The gastrin hormone (produced in the stomach when food comes in contact with its walls) regulates the action of pepsin, which transforms large molecules (polypeptides) into smaller molecules (dipeptides).

The food juice resulting from chemical digestion is called the chyme . The passage from the chyme to the intestine is controlled through the valve called the pylorus .

Small intestine

In the small intestine most digestion and assimilation of nutrients occurs. Two regions are distinguished in it: the duodenum and the jejunum ileum.

In the duodenum, secretions from the liver and pancreas are released, which, together with the enteric or intestinal juice, act on the chyme (food bolus that looks like a white mass after undergoing gastric digestion).

  • Bile: is the secretion of the liver, stored in the gallbladder , which is released into the duodenum through the common bile duct. Bile does not contain digestive enzymes, but bile salts (mainly water and sodium bicarbonate) that separate fats into microscopic particles, facilitating the action of pancreatic enzymes on lipids .
  • Pancreatic juice: It is produced by the pancreas. Trypsin is one of the enzymes produced in the pancreas that acts on proteins. It only becomes active when it arrives in the duodenum and joins the enteric juice into chymotrypsin .
  • Intestinal or enteric juice: is produced by the intestinal mucosa. It has enzymes that complete the digestion of lipids, proteins and carbohydrates .

At the end of the process performed in the duodenum, the set of substances forms a viscous white liquid, called kilo , that goes to the jejunum.

In the jejunum ileum most of the nutrients resulting from the digestion process are absorbed into the blood and carried to all cells of the body. What is not absorbed - water and the food mass, made up mostly of fiber - pass into the large intestine.

Dietary fiber is therefore essential for the formation of feces and the proper functioning of the intestines.

Large intestine

The large intestine absorbs water and mineral salts that the small intestine has not assimilated into digestion. Undigested material forms the feces that accumulate in the rectum (final part of the large intestine) and are later pushed out by peristaltic movements through the canal of the anus .