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Digestion in the Stomach
After involving the mouth and esophagus, food digestion continues in the stomach or gastric cavity. Similar in shape to a bag, the stomach is the site of important digestive transformations. Its two orifices the cardia and the pylorus communicate, respectively, with the esophagus and the duodenum.
The gastric content, which can reach the considerable volume of three liters, is attacked by acid substances poured into it by glands specialized in secreting hydrochloric acid. For this reason very low pH levels are reached inside the stomach and normally between 0.9 and 3.5.
In a healthy individual this extreme acidity is completely harmless, thanks to the presence of a very resistant layer of internal mucosa. However, it may happen that this structure yields to the attack of acids causing the formation of more or less serious lesions. These lacerations, which are called gastric ulcers, usually arise due to the prolonged overproduction of hydrochloric acid or following bacterial infections such as that caused by Helicobacter pylori.
Despite this disadvantage, hydrochloric acid plays an essential role in the digestion of food. Moreover, thanks to its remarkable antibacterial efficacy, it is able to protect the whole organism from food diseases. Its strong acidity also makes some minerals such as calcium and iron more soluble.
The most important characteristic of hydrochloric acid, however, lies in its ability to activate an enzyme essential for the digestion of proteins. This substance called pepsin is produced in an inactive form, called pepsinogen. Only in the presence of an acidic environment can pepsin exert its proteolytic action by degrading proteins into short chains of amino acids.
Based on the amount of bolus that reaches the stomach, the gastric walls have the ability to relax and contract to increase or decrease their capacity.
In addition to an internal mucous layer, the gastric cavity is in fact enveloped by a muscle tissue that relaxes and contracts based on the presence of food.
Muscle fibers become more numerous at the pyloric sphincter, an anatomical structure that separates the stomach from the duodenum by opening and closing according to reflex mechanisms.
After the stomach is completely emptied, spontaneous contractions run through, the intensity and frequency of which increases as fasting persists. These cramps, distinctly felt by the subject in conditions of prolonged fasting, cease upon ingestion of food when the muscles slowly relax to receive the food bolus.
After undergoing the attack of gastric acids and enzymes, the food bolus coming from the esophagus takes the name of chyme and continues its path passing the pyloric sphincter and entering the first section of the small intestine.
Digestion in the Duodenum
Gastric emptying is a rather slow process, influenced by many elements such as the content of fat, fiber and protein.
After exiting the stomach through the pyloric sphincter, the chyme slowly pours into the duodenum. This first part of the small intestine, approximately 25-30 cm long, is extremely important for the correct digestion of food.
The duodenum is in fact connected to important excretory ducts which in the presence of chyme pour into it a whole series of juices rich in digestive enzymes.
After leaving the stomach, the chyme is composed of an acidic and semifluid mixture containing unaltered fats in their structure, proteins and partially digested carbohydrates. In this intestinal tract all the various digestive processes must be completed since the final part of the intestinal tract is almost exclusively assigned to absorption.
Upon entry into the duodenum, the chyme is attacked by enzymes produced by two important organs, the pancreas (pancreatic juices) and the liver (bile).
MORE: Digestion in the pancreas and "intestine"
