The digestion

The digestion is the transformation of foods, previously ingested into simpler substances that can be absorbed and utilized by the cells.

Throughout the digestive tract, food is transformed into different substances. Digestion can be of two types:

• Mechanical digestion. The food is fragmented, kneaded, mixed, etc.
• Chemical digestion. Some substances, such as  digestive enzymes, are responsible for breaking down food, aided by other chemicals. These substances are secreted by the walls of the digestive tract and by the adjacent glands .

Digestion begins in the mouth, when the amylase or ptyalin enzyme contained in saliva begins to chemically digest carbohydrates. In addition, the teeth and the tongue facilitate this digestion with their mechanical action.

But the most important digestion processes occur in the stomach and small intestine, where the pancreas and liver also play a role in digestion.

Stomach

In the stomach, it is a widening of the digestive tract that connects with the esophagus through the cardia (unlike the pylorus, it is always open) and with the small intestine through a valve, the pylorus, which prevents the backward movement of food.

Modificado de By Stomach_diagram.svg:Indolences at en.wikipediaderivative work: Mcstrother (Stomach_diagram.svg) [CC BY 3.0], from Wikimedia Commons

The stomach produces gastric digestion, in which the glands that line the inner walls produce gastric juice, which when mixed with the food bolus forms a more fluid mass called chyme.

The gastric juice contains:

• Hydrochloric acid: with bactericidal action and favoring the action of pepsin.
• Pepsin: it is a digestive enzyme that digests proteins.

To prevent gastric juice from damaging the proteins in the stomach lining, the stomach is lined with mucus.

The stomach is made up of layers of muscles that allow (mechanical) movements to be made so that the chyme mixes well. The longer it is in the stomach, the more gastric juice is secreted and the more it is digested, until the pylorus opens and the chyme passes into the small intestine. There are foods, such as fats and proteins, that need to be in the stomach for a long time, while others, such as carbohydrates , need to be in the stomach for less time.

Activity: Actions of the stomach in digestion.

3D model: Anatomy of the stomach.

Small intestine

The small intestine is a tube that is about 7 meters long that connects the stomach (pylorus) with the large intestine (ileocecal valve). The small intestine is divided into three parts: the duodenum, the jejunum, and the ileum.

In the first part, the duodenum, about 25 cm long, the intestinal cells secrete intestinal juice, with a large amount of enzymes that digest food, breaking it down so that it can be absorbed and passed into the blood.

In addition to the intestinal juice, they reach the intestine through the bile duct, the pancreatic juice and bile (from liver) which assist digestion, forming a slurry called chyle.

The intestinal juice and pancreatic juice containing lipases, amylases and proteases that attack lipids, carbohydrates and proteins, respectively.

The bile emulsifies the fats, converting them into small droplets, thus facilitating the action of the lipases in the pancreatic juice.

The action of all these enzymes allows the chemical digestion of all foods.

Peristaltic movements continue to occur throughout the small intestine.

Pancreas

The pancreas is an organ located in the abdomen, below and behind the stomach that has dual functions:

• Endocrine secretion: produces hormones such as insulin and glucagon, as will be seen in the topic of the endocrine system.
• Exocrine secretion: produces pancreatic juice, which together with bile mixes with intestinal juice to form chyle.

Modificado en la obra de CNX OpenStax [CC BY 4.0], via Wikimedia Commons

Liver

The liver forms bile that is stored in the gallbladder.

The bile contains no enzymes but some bile salts to emulsify fat, leaving as small droplets that can be attacked by digestive juices.

In addition, bile contains other pigments, such as bilirubin, which is red in color, and biliverdin, which is green, from the breakdown of hemoglobin, which give the characteristic color of stool.

The liver is responsible for removing different substances from the blood that may be harmful to the body, such as alcohol, making them harmless.

Activity: Digestion in each part of the digestive system.

3D model: Anatomy of the liver.