The stomach (ventricle, gastrectum) is a tubular, muscular hollow organ that serves to store, crush and homogenize the ingested food. The capacity of the stomach in adults is usually between 1200 and 1600 ml, although the external shape of the stomach can vary greatly. Through the oesophagus, food mixed with saliva passes from the oral cavity into the stomach, where the chyme is produced by adding gastric acid. Through peristalsis (an undulating movement pattern of the muscles) the food is mixed with gastric juice and further broken up. After a residence time of 1-6 hours, the chyme is emptied in portions into the following duodenum.
Tasks of the stomach during digestion
The stomach is divided functionally into different sections: the esophagus opens into the upper part, the cardia, followed by the fundus and the corpus, which forms the main part of the stomach. Further down are the antrum and pylorus, the lower opening of the stomach. The stomach wall consists of the structure typical of the organs of the gastrointestinal tract, consisting of smooth muscles and adjacent mucous membrane.
However, in addition to longitudinal and transverse muscles, the tunica muscularis has a third layer of obliquely running muscle fibres (fibrae obliquae). This muscle layer enables strong peristalsis, which serves to mix and comminute the stomach contents. Peristaltic waves serve not only to homogenise the chyme but also to transport it further towards the pylorus, where it is emptied in portions into the duodenum.
The stomach also serves as a reservoir in which food can be stored so that the body’s need for nutrients can be met with a few meals spread out over the day. The regular, portioned emptying of the stomach into the small intestine ensures that the chyme is passed on evenly and “smoothed” to the subsequent sections of the digestive tract. The length of time the chyme remains in the stomach depends on the food taken in: easily digestible foods, such as fruit and carbohydrates, only remain in the stomach for 1-2 hours, while foods rich in fat and protein are difficult to digest and only reach the small intestine after 6-8 hours.
Absorbed fluid flows along the inner wall of the small curvature, the so-called gastric road, directly into the distal part of the stomach. The gastric mucosa continuously produces gastric juice consisting of hydrochloric acid, mucins, bicarbonate, digestive enzymes and intrinsic factor. Due to its low pH value, hydrochloric acid creates a strongly acidic environment in the stomach, which on the one hand serves to kill microorganisms and on the other hand helps digest proteins.
Surface cells of the stomach mucosa secrete bicarbonate and mucus, which protects the stomach mucosa itself from the aggressive stomach acid. After the food has reached the stomach, the increase in volume causes the stomach to expand and increases the secretion of gastric acid. In addition to the mechanical crushing of the food by the peristaltic waves, the first steps of digestion begin with the mixing of the chyme with gastric acid.