The digestive system breaks down food into the colorful nutrients that give energy for the body to serve.
The digestive tract ( also called the gastrointestinal tract or GI tract) is the concave tube that food passes through when it’s swallowed and digested, and which expels the waste products of digestion as coprolite.
The stomach (or stomach pouch) is the pouch-shaped portion of the digestive tract between the esophagus and the duodenum. The stomach receives food chewed in the mouth and swallowed in the esophagus. In humans, the organ is shaped like a capital J, in adulthood it is 15 cm high, contains 0.5 l when empty, and can hold up to 4 liters. The stomach is anatomically related to the liver (right), the spleen (left), the pancreas (back), the diaphragm (top) and the intestines (bottom). It is located above the mesocolon (supra-mesocolic level).
The branch of medicine that treats the stomach is Gastroenterology.
The stomach ensures digestion through its mechanical (stirring) and chemical functions by mixing food with gastric juices (water, hydrochloric acid, enzymes). For an ideal digestion, the pH of the stomach is between 1.5 (during the night) and 5 (at the beginning of digestion): the gastric enzymes work at an acid pH.
The product of the transformation by the stomach is a paste, called chyme, which pours into the duodenum through the pylorus. It is in the duodenum that the acidic chyme is neutralized by the bases produced by the bile and the pancreatic juice. This neutralization produces assimilable mineral mariners, some of which pass into the body directly through the wall of the duodenum thanks to hormones produced by the adrenal glands, the mineralocorticoids.
The duration of digestion in the stomach varies between 3 and 7 hours.
The stomach is characterized from the front by a J shape and has an opening at the top, the cardia, which allows the junction with the esophagus. It includes the lower esophageal sphincter and the pylorus at its exit towards the duodenum below. There are two curvatures:
the small curvature (right) vascularized by the left and right gastric arteries and veins;
the large curvature (left) vascularized by the left and right gastroepiploic arteries and veins.
The stomach is composed of three parts, whose mucous membranes all secrete protective mucus against self-digestion, from top to bottom:
The mucosa of the fundus and stomach has gastric glands on a quarter of their thickness, these glands are the place where acid is made, produced by parietal or oxynlitic cells1. The wall produces pepsins (probably 8 different ones according to chromatography). Under the effect of these secretions, proteinaceous foodstuffs begin their transformation. This decomposition plays a major role in the transformation of food particles by reducing their size.
Between the antrum and the body, there is a zone of permanent stenosis, a sort of functional sphincter limiting the passages from top to bottom. The stomach is made up, from the surface to the depth, of several layers: the peritoneum covers it almost entirely on the surface, then come three muscular layers (longitudinal, oblique and circular), and the internal mucous membrane placed on its connective chorion. To control the work of its muscles, the stomach has its own parietal nervous system, located a little lower than the middle in the greater curvature, and this pace maker gives rhythm to the contractions of the lower part (antrum and pylorus: the pump). It is modulated by the sympathetic and parasympathetic systems.
At the level of the oeso-gastric junction, there is the Hiss angle which prevents acid reflux upwards that would cause burns of the oesophagus: this one is not protected against the hydrochloric acid secreted at pH 0.9 in the stomach. Reflux can go into the mouth and damage the teeth. In passing, it can also irritate the respiratory tract through false respiration – especially at night or when bending over – produce coughing, and perhaps also defensive apneas.
The inferior part (pylorus) includes the pyloric sphincter muscle, which allows the rhythmic exit of gastric chyme into the duodenum. The main role of this muscle is to restrict the passage of food particles larger than 0.5 mm. It is only at the end of digestion that the stomach empties what has not been reduced by the work of enzymes and powerful mechanical stirring, in desperation, entrusting the unfinished work to its main follower : the pancreas, the most competent organ of digestion, capable like the stomach of reducing the size of food particles and proteins, but also of continuing the digestion of fats (lipids) and complex sugars (carbohydrates and starch, for example) begun in the oral cavity (ptyalin for complex sugars and lingual lipase for fats).
The second important role of the pylorus is counting calories: contrary to the accepted view, an efficient stomach is a slow stomach. The biggest digestive disorders are due to the invasion of the intestine by a large mass of food not processed by the stomach. The extreme model is the dumping syndrome after gastrectomy. Another harmful effect is the massive arrival of calories in the blood (normal pancreatic digestion is explosive and the resorption in the duodenum in the first half hour is 10% of sugars, 7% of lipids and 4% of proteins on the total of the meal) with various consequences (hyperglycemia and then reactional hypoglycemia, hypercholesterolemia and hypertriglyceridemia, etc.).
Caloric richness is analyzed in the duodenum and gives rise to feedbacks regulating pyloric ejection. The aim is to give the stomach a pantry role for daily activity.
Triggered by various mechanisms after the entry of food (local hormones such as gastrin, parasympathetic nervous system with the termination of the two cranial pneumogastric nerves) the work of the ejecting part is done at a rate of three waves per minute during digestion. The waves collide with the pylorus, a small amount of ejection goes into the bulb of the duodenum, the rest goes up towards the dam constituted by a permanent stricture zone located between the antrum and the upper part (fundus – body).
If the ejecting part is always the place of a muscular tonus keeping it flattened when the stomach is empty, the upper part is always open (it is lodged under the half-dome of the left diaphragm, under the heart), even in the absence of food, and forms a pocket of air sent back during the filling of the meals. This upper part, contrary to the antrum, is the place of an adaptive relaxation to the volume of the meal: it relaxes to receive, and the food settles in horizontal layers.
The zone of stricture maintained by the muscular tone forms, like a sphincter, a controlled passage of the digestive contents: the passage towards the antrum gives various results according to the moment of digestion. Liquids, even when drunk at the end of a meal, come out first. The food mixed by the culinary preparation and the masticatory crushing progressively decant: the semi-pasty part remains at the bottom while the fats rise, being of lesser density. They will be the last to come out.
The three parts of the stomach are :
- The fundus: hydration of the food bolus ;
- The body: it includes the parietal cells (border cells) which synthesize hydrochloric acid (HCl), the mucus cells for the protection of the stomach wall and the main cells which secrete pepsinogen (precursor of pepsin). The body cells are sensitive to gastrin (increases secretions);
- The antrum: predominantly motor wall to allow a good mixing of food, produces gastrin.
- The pylorus is a communication orifice between the stomach and the duodenum
The stomach includes:
- The cardia which surrounds the entrance orifice,
- The upper pole or fundus or large tuberosity, forms a dome that receives the gases contained in the food,
- The body is the middle portion,
- The antrum narrows to form the pylorus,
- The pylorus which constitutes a rise of chyme towards the duodenum.
Gastric secretions are produced by specialized gastric glands that have different secretory cells depending on their location in the stomach.
Thus, the glands of the cardia produce mucus while those of the body produce mucus, pepsinogen (an inactive enzyme that is converted into active pepsin), hydrochloric acid and gastric hormones. The pylorus produces mainly gastrin.
The stomach is the site of many diseases, congenital or acquired:
Ulcers, following an infection with Helicobacter pylori, stress or taking non-steroidal anti-inflammatory drugs (aspirin etc)
Upset stomach (in dogs)