It is the organ that receives food from the esophagus that is introduced through the mouth . Inside, the digestive process begins, made possible both by the presence of digestive enzymes and by the periodic contraction of the stomach itself. From here the food then transits into the intestine , where the digestive processes can continue allowing the absorption of the nutrients that are present in the ingested foods.
What is the stomach?
It is an organ that is located on the left side of the upper abdomen area . From the anatomical point of view it can be considered an enlargement – from the bag shape – of the digestive tract . The stomach is in fact directly connected both to the esophagus, from which it receives food which passes through a structure called the lower esophageal sphincter (or cardial valve), and to the small intestine – from which it is separated from the pyloric sphincter – a muscular valve that opens and closing regulates its emptying.
25-28 cm long and 10-12 cm wide , it is an elastic structure that varies in shape and size depending on the food that is present inside. This is made possible by the folded folds that form its wall: their distension extends its surface, giving this organ a capacity of 1000-1500 ml.
Its wall is made up of three layers or cassocks : the gastric mucosa , the muscle cassock and the serous cassock .
The gastric mucosa is the innermost layer , secretes the gastric juices and creates the acidic environment that is typical of the stomach, while producing the mucus that allows the stomach to protect itself from digestion. It can in turn be divided into three layers : the mucosa (the epithelium that lines the internal wall), the muscularis mucusae (a slightly dense layer of smooth muscle fibers) and the submucosa (a connective tissue intertwined with the enteric nervous system).
Outwards, a layer of muscles follows which – by contracting – allows the mixing of food (the muscle tunic, which in turn can be divided into three layers: longitudinal, circular and oblique) and the outermost covering layer (the serous tunic ), which completely surrounds the organ.
The characteristics of these layers change according to the area of the stomach that is taken into consideration. The organ can in fact be divided into several parts: the bottom (the upper part), the body (the central part that acts as a tank for swallowed food), the cardial antrum and the pyloric antrum (which correspond, respectively, near the cardial valve and near the pylorus sphincter). The channel through which the esophagus connects to the stomach is called cardias , while the orifice that connects the stomach and duodenum is called the pylorus . The upper zone takes the name of small curvature, while the lower one is called large curvature.
In the mucous membrane of the fundus and body there are the glands that produce gastric juices , while the prepiloric glands produce most of the mucus that protects the gastric wall from digestion. The circular layer of the muscular tunic is contiguous to that of the esophagus , but is absent in the fundus. Its thickness increases in the pyloric cave . The oblique layer, on the other hand, is clearly present in the bottom and in the small curvature, but disappears continuing towards the pyloric antrum.
What is the stomach for?
Its main function is to allow the transit of food to the intestine while participating in its digestion. Specifically, gastric juices and enzymes begin the digestion of fats and proteins by breaking them down into the building blocks (respectively, fatty acids and amino acids). The digestion of carbohydrates inside the stomach is instead severely limited due to the acidic environment present in it.
The digestive functions are facilitated by the contractions made possible by the gastric muscles , which mix up the contents of the stomach . In this way, in a maximum time of five hours, the organ is able to digest the solid food that comes from the esophagus, reducing it to a semifluid substance ( chyme ) which is sent to the intestine; this thanks to the opening of the pyloric sphincter , which closes immediately in order to prevent the chyme from returning to the stomach.
Digestion is made possible by the gastric glands , which secrete the three basic components of gastric juice. The first is pepsinogen , which after being transformed into pepsin deals with the breakdown of proteins into amino acids. The second is hydrochloric acid , indispensable for pepsin to perform its function. Finally, the intrinsic factor: produced by the gastric glands is also fundamental for the absorption of vitamin B12 in the intestine and iron.
At the stomach level, molecules such as water and alcohol can also be directly absorbed .