What is the stomach?

The stomach is the organ that receives food from the esophagus, after it has been chewed and swallowed through the mouth. It is where the digestive process begins, which is made possible by digestive enzymes from the regular contraction of the stomach itself. The food is then passed through the intestines, where the digestive process continues, allowing nutrients to be absorbed from the foods ingested.

The stomach is an organ that is located in the left upper part of the abdominal cavity. From an anatomical point of view, it can be considered an enlargement, as a pouch form of the digestive tract . The stomach is in fact directly connected to both the esophagus  (from which it receives the food that passes through a structure called the lower esophageal sphincter) as well as the small intestine , which separates it from the pyloric sphincter , a muscular valve whose opening and closing regulates the emptying of the stomach.

Being about 25-28 cm long and 10-12 cm wide, the stomach is a resilient structure which can vary in shape and size depending on the food present inside. This is made ​​possible by the folded folds that make up the wall and which stretch to extend the surface, giving the stomach a capacity of 1000-1500 ml.

The stomach wall is made ​​up of three layers or tunics : the gastric mucosa, the muscularis and the tunica serosa. The gastric mucosa is the innermost layer, which secretes digestive juices and creates the typical acidic environment the stomach, while producing mucus that allows the stomach to protect itself from the digestion process. This layer can in turn be divided into three other layers: the mucosa (epithelium that lines the inner wall of the stomach), the muscularis mucusae (a layer of small dense and smooth muscle fibers) and the submucosa (a connective tissue braided with the enteric nervous system).The muscularis is a layer of contracting muscles that allows foods to be mixed and it too can be divided into three other layers: longitudinal, circular and oblique. Finally, the outermost layer of coating, known as the tunica serosa, is responsible for fully enveloping the body.

The characteristics of these layers vary greatly depending on the area of the stomach in question. The organ can in fact be divided into several parts: the fundus (the upper part), the body (the central part which serves as a reservoir for the food swallowed), and the pyloric antrum (which corresponds, respectively, to the area near the cardia valve and the one near the pyloric sphincter). The channel through which the esophagus joins the stomach is called the cardia, while the orifice that connects the stomach and duodenum is known as the pylorus. The upper zone is called the lesser curvature of the stomach, while the lower zone is called the greater curvature of the stomach.

The glands that produce stomach acid are present in the mucosa of the fundus and body, while the prepyloric glands produce most of the mucus that protects the stomach wall from the digestion process.The circular muscle layer is continuous in the esophagus, but is absent in the fundus. Instead, its thickness increases in the pyloric antrum. The oblique layer, however, is clearly present in the bottom and in the small curvature, however disappears continuing towards the pyloric antrum.


What function does the stomach serve?

The main function of the stomach is to allow food to be transported to the intestines and at the same time, undergo the digestion process. Digestive juices and enzymes kick off the digestion of fats and proteins by breaking them down into the substances that form them to begin with (fatty acids and amino acids). The digestion of carbohydrates in the stomach is rather limited due to the strongly acidic environment present inside. Also, the stomach is able to directly absorb molecules such as water and alcohol.


Digestive functions are facilitated by the contractions brought about by stomach muscles, scrambling the contents of the stomach. In this way, a maximum of five hours is needed for the body to be able to digest solid food from the esophagus, reducing it to a semi-fluid substance (chyme ) that is sent to the intestine through the opening of the pyloric sphincter, which closes immediately after in order to prevent the chime in the stomach from returning (flowing back).

The digestive process is made ​​possible by the gastric glands, which secrete the three most fundamental components of gastric juice. The first is the pepsinogen enzyme , which after being converted into pepsin, is responsible for the breakdown of proteins into amino acids. The second is the  hydrochloric acid , for which pepsin is necessary in order to perform its function. Finally, the intrinsic factor produced by the gastric glands is essential for the absorption of vitamin B12 and iron in the intestines.