The stomach (in ancient Greek στόμαχος, stòmachos, from which the lat. stomachus; in Latin also ventriculus) is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach is the organ that receives from the esophagus the food introduced through the mouth. Inside the stomach the digestive processes begin, made possible both by the presence of digestive enzymes and by the periodical contraction of the stomach itself. From here the food then passes into the intestine, where the digestive processes will continue allowing the absorption of the nutrients present in the ingested food.
The stomach is located in the left side of the upper abdomen. From an anatomical point of view it can be considered an enlargement, with a sac-like shape, of the digestive tract. In fact, the stomach is directly connected both to the esophagus, from which it receives the food that passes through a structure called the lower esophageal sphincter (or cardial valve), and to the small intestine, from which it is separated by the pyloric sphincter, a muscular valve that opens and closes to regulate the emptying of the stomach.
About 25-28 cm long and 10-12 cm wide, it is an elastic structure that can vary in shape and size depending on the food present inside. This is made possible by the folded folds that make up its wall: their distension extends its surface, giving the stomach a capacity of 1000-1500 ml.
The wall of the stomach is formed by three layers or tonaches: the gastric mucosa, the muscular tonaca and the serous tonaca.
The gastric mucosa is the innermost layer, it secretes the gastric juices and creates the acid environment 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 inner wall of the stomach), the muscolaris mucusae (a not very dense layer of smooth muscle fibers), and the submucosa (a connective tissue intertwined with the enteric nervous system).
Followed, towards the outside, by a layer of muscles that by contracting allows the mixing of food (the muscular tonaca, which in turn can be divided into three layers: longitudinal, circular and oblique) and the outermost layer of coating (the serous tonaca), which completely surrounds the organ.
The characteristics of these layers varies according to the area of the stomach taken into consideration. In fact, the organ can be divided into several parts: the fundus (the upper part), the body (the central part which acts as a reservoir for the swallowed food), the cardial antrum and the pyloric antrum (which correspond, respectively, to the area near the cardial valve and the area near the pyloric sphincter). The channel through which the esophagus joins the stomach is called cardias, while the orifice that connects the stomach and duodenum is called pylorus. The upper area is called the small curvature of the stomach, while the lower area is called the great curvature of the stomach.
In the mucous tonaca of the fundus and body are the glands that produce gastric juices, while the prepyloric glands produce most of the mucus that protects the gastric wall from digestion. The circular layer of the muscular tonaca is in continuity with that of the esophagus, but is absent in the fundus. Instead, its thickness increases in the pyloric antrum. The oblique layer, on the other hand, is clearly present in the fundus and in the small curvature, but disappears as it continues toward the antrum pylori.
The main function of the stomach is to allow the transit of food to the intestine while participating in its digestion. In particular, gastric juices and enzymes start the digestion of fats and proteins by breaking them down into their building blocks (fatty acids and amino acids, respectively). The digestion of carbohydrates in the stomach, on the other hand, is limited due to the strongly acidic environment present inside.
The digestive functions are facilitated by the contractions made possible by the gastric musculature, which stir the contents of the stomach. In this way, in a maximum time of five hours, the organ is able to digest the solid food coming from the esophagus, reducing it to a semi-fluid substance (the chyme) which is sent towards the intestine thanks to the opening of the pyloric sphincter, which closes immediately after 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 converted to pepsin is involved in the breakdown of proteins into amino acids. The second is hydrochloric acid, which is necessary for pepsin to perform its function. Finally, the intrinsic factor. Produced by the gastric glands, it is also essential for the absorption of vitamin B12 in the intestine and of iron.
Molecules such as water and alcohol can also be absorbed directly in the stomach.
The human stomach is an abdominal muscular-mucosal organ, unequal and paramedian, with the shape of an elongated sac, flattened in an antero-posterior sense, which occupies topographically the regions of the left hypochondrium and epigastrium; however, it must be emphasized that it presents considerable variability in shape and position both between living and cadaver, and depending on the physical constitution, its filling and the position assumed: in fact, it shows a greater vertical axis in the longilineal while in the brevilineal tends to assume a greater horizontal axis. It has an anterior and posterior face, a concave right margin or small curvature and a convex left margin or large curvature.
The small curvature forms the posterior-superior margin of the stomach; it extends to the right and inferiorly and then rises more gently at the level of the angular incisura, and descends further ending at the level of the pylorus. On it insert anteriorly the hepato-gastric ligament, which connects liver and stomach, and continuing in the hepato-duodenal ligament constitutes the small omentum.
The great curvature is four to five times longer than the small curvature (about 40 cm), it originates at the cardial incisura and then goes up forming the dome-shaped margin of the bottom of the stomach, reaching the esophagus, then, starting from the apex of the fundus (the point of maximum convexity, just below the nipple) it directs inferiorly and medially until it reaches the intermediate groove, which divides the pyloric antrum from the pyloric canal. The great curvature is lined anteriorly by the peritoneum, while laterally, further to the left, the two anterior and posterior peritoneal laminae join to create the gastrolienal ligament, which connects the stomach wall to the splenic hilum. Posteriorly it is related to the body and tail of the pancreas, as well as to a portion of the left lobe of the liver; also on the wall of the great curvature is created the gastrocolic ligament, which expanding from the great curvature to the transverse colon, the right colic flexure, and the duodenum constitutes the anterior root of the great omentum.
The anterior or upper face of the stomach is lined by the peritoneum and has relationships with the diaphragm, with the spleen, determining the gastric face, with part of the left and square lobes of the liver, and with the transverse colon. The posterior or inferior face of the stomach contracts relations with the left adrenal gland, with the body and tail of the pancreas, with the aorta and the lienal and hepatic arteries, and with the portal vein. It is completely lined by peritoneum except near the cardia where it is in contact with the diaphragm.
In the stomach we recognize four main portions (fundus, body, pyloric antrum, pyloric canal) and two orifices (cardias, pylorus).
- Cardias (lat. pars cardiaca): represents the orifice that connects stomach and esophagus, externally this junction is not covered by peritoneum and does not present any thickening of the muscular tonaca. The cardias allows the passage of food soaked with saliva (food bolus) in one direction only, from top to bottom, and prevents reflux into the esophagus thanks to a series of mechanisms such as maintaining a certain muscle tone and oblique fibers of the internal muscular tonaca of the stomach that constitute a virtual valve that occludes the lumen. The demarcation between the esophagus and the stomach is represented by the Z line, constituted by a part of gastric mucosa that deepens for some centimeters inside the esophageal lumen and then ends with a zigzag profile and a squamous or columnar structure. The mucosa of the cardia is raised into characteristic ridges called “mucosal rosettes” that help prevent gastroesophageal reflux.
- Fundus (lat. fundus ventriculi/stomachs): it is a glandular portion, which rests posteriorly on the diaphragm, distinguishable by tracing an imaginary horizontal line starting from the cardiac incisura. It corresponds radiologically to the gastric bubble, i.e. the part of the stomach full of air and therefore radiolucent, as it is not reached by radiological contrast. Its projection on the thoracic wall is called Traube’s semilunar space, delimited inferiorly by the inferior margin of the 9th costal cartilage and by the xiphoid process of the sternum, superiorly by the 5th-6th rib, laterally to the left by the costal arch and to the right by the anterior margin of the liver. The mucosa of the fundus of the stomach possesses temporary folds that disappear altogether when it is distended.
- Corpus (lat. corpus ventriculi/stomach): is the largest portion, glandular in shape with a vertical axis, slightly inclined to the right, and narrowed at the bottom. It lies between the base of the fundus of the stomach and the angular incisura. The mucosa of the body of the stomach has permanent gastric folds, which are especially widespread in the postero-medial, medial, and antero-medial regions, i.e., in the area proximal to the small curvature. The inner walls of the great curvature possess folds with more convoluted patterns and increasingly defined and prominent as one moves from the bottom of the stomach to the border with the pyloric antrum. This slows the passage of fluids and the food bolus. It is hypothesized that liquids can transit faster along the small curvature than the large curvature, which is why the folds of the former together form what is called the “short gastric pathway.”
- Pyloric antro (lat. pars pylorica): is a cylindrically shaped portion that runs laterally and superiorly to the body. It is included between the angular incisura and the intermediate sulcus. Its internal mucosa is mostly smooth, but in a contracted state, relevant folds can be seen at the border with the pyloric canal; these are longitudinal folds, more similar to those of the short gastric route than to those of the great curvature.
- Pyloric canal: it is a hemispherical shaped portion, included between the intermediate sulcus and the pylorus, it runs inferiorly and laterally to the pyloric antrum.
- Pylorus (lat. pylorus): it is a muscular sphincter that connects the stomach to the duodenum, whose position can be determined according to the narrowing of the pyloric canal. It is formed by thickened circular smooth muscle fibers intertwined with some oblique muscle fibers.
The gases generated by bolus digestion tend to rise and concentrate in the fundus of the stomach, which represents the most cranial area of the organ. In humans the stomach has a capacity of 0.5 L if empty, and has an average capacity, if completely full, of about 1-1.5 L. After a normal meal, it generally expands to contain about 1 L of bolus, but can also expand to contain up to 4 L and more, compressing however the other organs of the abdominal cavity, and often of the thorax.
All major gastric arteries are derived from the celiac trunk of the abdominal aorta. This branches into 3 large arteries, on the left the lienal artery, on the right the common hepatic artery, and again on the left and upward the left gastric artery. Because of its large and branched arterial circulation, stomach ischemia is infrequent. However, numerous variations of the gastric arteries are possible.
- The left gastric artery, beginning at the celiac trunk, ascends to the cardia, where it branches into an esophageal branch that ascends to the esophagus, then curves and follows the course of the small curvature of the stomach, staying within the peritoneum. Along the small curvature it sends branches to both the upper and posterior faces of the stomach, thus contributing to the vascularization of the cardial area, sometimes a small part of the fundus of the stomach, and the upper part of the fundus of the stomach in the anterior and posterior faces.
- The right gastric artery is a branch of the hepatic artery propria; it runs superior to the gastroduodenal artery, then descends to the level of the pylorus and follows the small curvature, anastomosing with the left gastric artery. Like the left gastric artery, it sends branches to the upper part of the pyloric canal, pyloric antrum, and body of the stomach, in their anterior and posterior faces.
- The short gastric arteries, varying in number from 5 to 7, are small arteries branching from the lienal artery, near the hilum of the spleen, they ascend until they reach anteriorly and posteriorly the fundus of the stomach, anastomosing with branches of the left gastric artery and the left gastroepiploic artery.
- The left gastroepiploic artery is the largest branch of the lienal artery and departs from it in the lower area of the posterior aspect of the spleen, staying within the gastrolienal ligament. From here it follows the course of the great curvature of the stomach by sending branches to the anterior and posterior faces, which anastomose with the short gastric arteries and the left gastric artery. It then irrigates the lower part of the fundus of the stomach. Some of its branches, however, penetrate the great omentum, irrigating its upper part.
- The right gastroepiploic artery is a large branch of the gastroduodenal artery. It follows the great curvature of the stomach, sending branches anteriorly and posteriorly to the lower area of the pyloric canal, pyloric antrum, and part of the body of the stomach, and then anastomosing with the left gastroepiploic artery.
- The gastroduodenal artery is the main branch of the common hepatic artery. It courses inferiorly and posteriorly to the pylorus and duodenum, and then branches into the right gastroepiploic artery and the antero-superior pancreatic-duodenal artery.
- The posterior gastric artery is not always present; when it is, it is a branch of the lienal artery that runs posterior to the body of the stomach and branches out into the upper part of the stomach and the fundus of the stomach.
The gastric veins drain primarily into the portal vein, lienal vein, and superior mesenteric vein. As with arteries, variations in venous distribution are frequent and numerous.
- The left gastric vein drains blood from the abdominal part of the esophagus, via the esophageal tributary vein, from the right side of the fundus of the stomach, from the right upper part of the body of the stomach. It makes a sort of undulating semicircle from the small curvature, receives the esophageal tributaries at the level of the cardia, and then passes anteriorly to the celiac trunk, the inferior vena cava, descends, and finally flows into the portal vein.
- The right gastric vein drains blood from the upper left side of the body of the stomach, the pyloric antrum, and the pyloric canal. It follows the course of the small curvature and then rises vertically and heads posteriorly towards the portal vein. At the level of the pylorus it also receives blood from the prepyloric vein.
- The short gastric veins, numbering 4-5, drain blood from the fundus of the stomach and part of the body of the lower stomach, and then flow into the lienal vein.
- The left gastroepiploic vein drains blood from the lower part of the body of the stomach as well as portions of the great omentum. It runs along the great curvature and confluences with the right gastroepiploic vein, with which it anastomizes.
- The right gastroepiploic vein drains blood from the right lower part of the body of the stomach, the antrum, and the pyloric canal. It courses along the great curvature receiving blood from the left gastroepiploic vein and then flows into the superior mesenteric vein. It may receive blood from the superior pancreaticoduodenal vein before flowing into the superior mesenteric.
- The posterior gastric vein, when present(s) run at the posterior gastric artery and drain blood from the posterior middle part of the body of the stomach and part of the fundus of the stomach, draining into the lienal vein.
The lymphatic vessels of the stomach represent a part of the continuous lymphatic network in the upper abdomen, in particular they are in continuity with the esophageal and duodenal ones, but also with the pancreatic, hepatic and splenic ones. The lymphatic vessels proper can be described as distributed in three zones. The first drains the upper part of the anterior and posterior faces of the fundus, body, pyloric antrum, and pyloric canal. These vessels flow into the cardial lymph nodes (at the cardia, within the peritoneum) and suprapyloric lymph nodes (superior to the pylorus), which in turn drain lymph into the celiac lymph nodes (anterior to the celiac trunk). The second drains the lower part of the anterior and posterior faces of the body, antrum, and pyloric canal.
These vessels flow into the subpyloric lymph nodes (located inferior to the pylorus and its right side) and the right gastroepiploic lymph nodes. A third zone drains the lymph from the left lower part of the body of the stomach and the left part of the fundus of the stomach. Its vessels drain into the left gastroepiploic lymph node, which in turn drains lymph into the lienal lymph nodes, located at the hilum of the spleen. Lymph drained throughout the stomach eventually flows to the celiac lymph nodes and from there to the chyle cistern.
Sympathetic innervation of the stomach is derived from the celiac plexus, the hepatic plexus, and the large and small splanchnic nerves, corresponding to the anterior branches of the thoracic nerves T5 through T12. The nerves of the celiac plexus reach the stomach following the course of the arteries, which they innervate and surround, and then distribute to the organ. Sympathetic nerve branches arising from the celiac plexus tend to distribute on the posterior (lower) face of the stomach and at the antrum while those of the hepatic plexus on the anterior (upper) face and at the fundus. The sympathetic system of the stomach is vasoconstrictor for its vessels, inhibitory for the gastric muscles, while it makes the pylorus contract, they also transmit sensitivity and pain.
Parasympathetic innervation of the stomach is derived from branches of the anterior and posterior vagus nerves. The anterior vagus nerve, which derives from the left vagus nerve and esophageal plexus, is tightly adherent to the outer muscular layer of the esophagus and divides into hepatic, gastric, and pyloric branches at the small curvature. Some anterior gastric branches detach from the anterior vagus to innervate fundus and part of the body of the stomach (upper face) while the main branch, called anterior gastric great nerve, runs in the small omentum and the great curvature, issuing branches to the body of the stomach and to the antrum, whose course follows that of the right and left gastric arteries, and then branches to the pylorus (pyloric nerves). The posterior vagus nerve runs into the adventitia tonaca of the esophagus and is less tightly adhered to it. It emits gastric branches that run behind the cardia and distribute to the lower face of the stomach and antrum, and celiac branches that join with the celiac plexus. Other nerve plexuses are located in the external muscular tonaca (Auerbach’s plexus) and submucosa (Meissner’s plexus). The stomach parasympathetic is responsible for stomach motility, glandular secretion, and pyloric release.
The stomach, like all organs of the gastrointestinal tract, possesses four tonaches, respectively, from the innermost to the outermost: mucosa, submucosa, muscular, and serosa.
The mucosa of the stomach is formed by a superficial epithelium that is in contact with the lumen of the organ, by a lamina propria of connective tissue and by the muscolaris mucosae. Its color varies from red in the fundus and body of the stomach, to a pinkish-red at the pylorus. The mucosa is raised in folds of different shapes depending on the area of the stomach considered, some of these are temporary (fundus, pyloric antrum), in this case they are wrinkles of the submucosa that appear during contraction, others are permanent (body), in this case they are real folds of the mucosa.
The lining epithelium consists of a simple monolayered cylindrical epithelium with secretory and lining functions. Its constituent cells are called “mucoid” cells. They have an average life span of 3 days. They have a highly developed Golgi apparatus in a perinuclear position and a central nucleus. On the cis side of the Golgi apparatus, there are numerous protosecretory vesicles, containing the neutral proteoglycans characteristic of the mucosal secretion of these cells. These proteoglycans serve the function of protecting the mucosal epithelium from acidic pH and the action of gastric juice enzymes. The secretions of the epithelial cells are directed toward the apical pole of the cell. The epithelium has numerous invaginations about 0.2 mm in diameter and with an irregular lumen, called gastric dimples, at the base of which the gastric glands of the stomach open, which then deepen into the lamina propria. The gastric glands, which pour their secretions into the crypts of the gastric areolae, are distinguished into three different types: cardial glands, main gastric glands, and pyloric glands. The gastric glands differ in structure according to the different regions of the stomach being considered.
The cardial glands are located in a band approximately 3 cm from the cardia. These glands are simple tubular or compound tubular and produce a mucous secretion containing neutral glycoproteins. Intermixed with the cardial glands there may also be some major gastric glands. Sometimes glands of this type are found in the esophagus, in which case they constitute an ectopic.
The main gastric glands are located in the fundus and body of the stomach and are the most prevalent in the organ. They are located within the gastric dimples, for each dimple there are 3 to 7 tubular glands. The part of each gland connected to the bottom of the gastric dimple is called the isthmus, immediately below it is the collar, then the body and finally the base of the gland. This regional distinction is useful in understanding the distribution of cells that contribute to forming these complex glands. There are at least five cell types in each gland, some of which possess subtypes.
- Superficial mucous cells carpet the walls of the gastric dimple, and are therefore diffuse in the isthmus and the apical part of the collar. They have a cylindrical and elongated shape, with a brush-like rim evident at the apical pole of the cell that protrudes toward the lumen and is the same in which mucus is secreted. The nucleus, roundish or oval, is located at the base of the cell, in a central position is located a developed Golgi apparatus, while in the apical portion are concentrated large vesicles containing mucus that are about to be secreted. Numerous are the mitochondria, often of elongated and bastoncellular shape, rather developed the smooth endoplasmic reticulum.
- The mucous cells of the collar constitute the main type of cells in this area of the gastric gland. They possess a cylindrical shape, either more squat than that of the superficial mucosal cells or prismatic, and an equally developed brush-like orifice. The nucleus is oval, with the major axis perpendicular to that of the cell. The Golgi apparatus is centrally located and well developed, while the smooth endoplasmic reticulum tends to concentrate around the nucleus, enveloping it. Mitochondria are quite numerous and mucin vesicles, smaller than those of superficial mucous cells and with chemically different acidic proteoglycans, tend to concentrate at the apical zone of the cell. These cells are completely replaced every 3 days because of the highly acidic environment of the stomach.
- Parietal (oxyntic) cells are dispersed primarily in the body of the gastric gland, but may also be found more rarely in the collar or isthmus. They are very large cells, pyramidal in shape and with a characteristic structure, in fact their apical surface invaginates into numerous canaliculi covered with microvilli that contain proton (H+) and potassium (K+) pumps on the plasma membrane. These microvilli appear to form and cleave continuously according to the secretory activity of the cell. The canaliculi are connected to a tubulo-vesicular system that pervades the cytoplasm of the cell. The flow of protons and chlorine ions out of the parietal cells determines their main function, i.e. the secretion of hydrochloric acid, which contributes to maintain the gastric pH around values between 1 and 3. The parietal cells have an eosinophilic cytoplasm very rich in mitochondria, of mainly smooth endoplasmic reticulum, which is concentrated at the base of the cell, while the nucleus is centrally located and roundish in shape. The Golgi apparatus is present but not as developed as that of mucous cells. In addition to secreting hydrochloric acid, they produce intrinsic factor, a protein essential for cobalamin (vitamin B12). They are completely replaced every week.
- The main (zymogenic) cells are found in the base and body of the gastric glands. They are prismatic or cuboidal cells, with a less developed brush-like rim than mucosal cells and a strongly basophilic cytoplasm due to the abundance of ribosomes and RNA. Their nucleus is roundish or oval, euchromatic, and placed in the basal area of the cell, just above the wrinkled endoplasmic reticulum that is concentrated instead immediately below the nucleus and at its sides. Mitochondria are scarce, while the Golgi apparatus is quite developed. The zymogen granules, round and electrondense, are concentrated in the apical pole of the cell, but can be easily found also in a central position, above the nucleus. They contain the digestive enzymes pepsinogen and lipase.
- Neuroendocrine cells are widespread in the basal zone and body of the gastric glands, in every area of the stomach, including the cardia and pylorus. They are highly variable in shape, sometimes vaguely pyramidal, possessing irregularly shaped, though often rounded, nuclei. In the cytoplasm there are mitochondria, a highly developed Golgi apparatus, several cisternae of wrinkled endoplasmic reticulum. Small secretory vesicles (0.3 µm in diameter) with strongly electrondense contents are concentrated in an area close to the nucleus. Based on the vesicular content, the neuroendocrine cells of the stomach are distinguished into G cells (gastrin), δ cells (somatostatin), and ECL cells (histamine). They also secrete factors that control gut motility and glandular secretion.
- Stem cells are undifferentiated cells, often found in mitotic phase at the level of the isthmus and especially the body of the gastric glands. They are cylindrical in shape with a poorly developed brush-like rim. Their central position in the gastric glands allows them to differentiate and migrate either to the isthmus or to the base of the gland, differentiating into different possible cells according to the stimuli received from the environment and interactions with neighboring cells.
Pyloric glands are branched tubular glands (usually consisting of 2-3 tubules) located at the base of the pyloric antrum. They consist of mucous cells, neuroendocrine cells such as G cells (which secrete gastrin) and others that secrete serotonin, some parietal cells and major cells.
The lamina propria consists of loose connective tissue, bundles of collagenous and elastic fibers, with fibrocytes, macrophages, eosinophilic granulocytes and plasma cells. In the lamina propria are a large number of blood capillaries, aggregates of lymphoid tissue, sometimes in the form of true gastric lymphatic follicles, and nerve plexuses with sensory and motor endings. It is separated from the epithelium by a weakly PAS-positive basement membrane.
The muscolaris mucosae of the stomach is a layer of smooth muscle cells, located below the lamina propria. There are three layers, an inner circular one that sends bundles of smooth muscle fibers between the glands, facilitating the emptying of the secretion into the dimples and then into the gastric lumen, a longitudinal one that continues with the inner circular one and finally an outer circular one, discontinuous with respect to the other two.
The submucous membrane of the stomach is composed mainly of loose connective tissue, with numerous elastic and collagen fibers. It is abundant and allows the formation of transient mucosal folds. Macrophages, eosinophilic granulocytes, lymphocytes, plasma cells, and fibroblasts are present. The elements of the immune system can sometimes aggregate to form true lymphoid follicles with a germinal center. Accommodates Meissner’s submucosal nerve plexus.
The stomach has a complex smooth muscular structure, which completely envelops it in several layers. The inner layer consists of smooth muscle fibers arranged obliquely from a region called the collar of Helvetius, located at the cardial incisura. The fibers descend from the cardial incisura on the anterior surface of the body of the stomach until they reach the pyloric antrum. However, these fibers are rather scattered and do not constitute a particularly dense layer. The middle muscular layer has circular fibers, which wind concentrically at the bottom of the stomach, and then wrap around the body of the stomach, the antrum, and the pyloric canal (continuing into the duodenum) in a postero-anterior direction. The outer muscle layer of the stomach has longitudinal fibers.
The external esophageal muscle fibers at the level of the cardia are divided into two large bundles, one running along the fundus of the stomach (going to cover the circular musculature) and then along the great curvature until the pyloric canal, the other running along the small curvature, then joining the previous one at the level of the pylorus, and continuing as a single muscle layer in the duodenum. A window in the anterior aspect of the stomach thus remains devoid of the external longitudinal layer and is covered only by the internal oblique layer and the overlying middle circular layer. The pylorus consists of a thickening of the middle circular musculature and oblique fibers that deepen between the circular ones. The muscular layers of the stomach are covered by the visceral peritoneum, with which they are closely united. Muscle contraction is regulated by a network of amyelinated nerve fibers of the enteric nervous system, located between the muscle layers to form the myenteric plexus of Auerbach.
The serous tonaca is formed by the visceral peritoneum, which almost completely covers the stomach. It is therefore composed of mesothelium and a sub-mesothelial layer of loose connective tissue with elastic fibers and collagen fibers. Serosa is not present at the insertions of the great omentum and the small omentum on the great curvature and the small curvature, because of the separation of the peritoneal leaflets by blood vessels and nerves. There is also a triangular area behind the cardia not covered by the peritoneum and in direct contact with the diaphragm.
The main role of the stomach is to digest into linear filaments the protein molecules ingested with food (denaturation), through the action of hydrochloric acid and some enzymes, in order to allow their absorption in the small intestine.
The stomach’s function is to store ingested food while awaiting its digestion, to stir it up inside through its movements and finally to convey it gradually in the form of chyme into the duodenum and then into the rest of the digestive system. The food entering the stomach due to the effect of gastric motility and the force of gravity is placed over that previously ingested, in what is physiologically considered the “oral” portion of the stomach, that is the bottom and the upper two thirds of the body. Through sensitive endings that terminate in the brainstem and efferent fibers directed back to the stomach (“vagus” reflex) this senses its degree of filling, reducing its muscle tone and facilitating its distension if there is food in entry. The stomach is capable of containing up to 1,5 liters of chyme.
Food, after being attacked by hydrochloric acid and gastric enzymes, turns into a semi-fluid and opaque substance of different consistency (depending on the amount of water in relation to the consistency of the ingested food) called chyme. The gastric juice which is an integral part of chyme is secreted by the gastric glands located in all the walls of the stomach except for a small part of the small curvature.
In the stomach, weak stirring waves (peristaltic waves) are generated every 15-20 seconds and propagate from the body to the antrum, becoming progressively more intense. Stirring waves are generated from slow waves resulting from the intrinsic ability of stomach smooth muscle to maintain a basal electrical rhythm, consisting of fluctuations in the membrane potential of smooth muscle fibrocells on the order of 5-15 mV. Some stirring waves are particularly intense and, propagating in all directions, originate a circular peristaltic contraction that pushes food from the body of the stomach toward the antrum and pylorus.
Peristaltic contractions are generated by action potentials unlike slow waves. In the latter case, however, the contraction does not cause significant amounts of chyme to pass from the antrum to the duodenum through the pylorus since the latter is particularly narrow and the stirring waves have the effect of contracting it rather than distending it. Thus the vast majority of chyme tends to be pushed back into the antrum or body rather than into the duodenum. Although only a very small proportion of the chyme previously present in the stomach passes into the duodenum after each circular peristaltic contraction the process is useful for gastric reshuffling. Antral circular peristaltic contractions, on the other hand, are primarily responsible for gastric emptying. They are waves that originate in the antrum of the stomach and propagate towards the pylorus, tending with time to originate higher and higher in the stomach reaching the body. This particular motility allows the progressive pushing of the food in the body towards the antrum.
Even though in this case only few milliliters of chyme go beyond the pylorus, it should be considered that these waves are repeated in time and little by little they succeed in emptying the stomach. Besides that, by means of the chyme “rejected” by the pylorus barrier, they collaborate in the gastric mixing. Another type of rhythmic peristaltic contractions characteristic of the body of the stomach are the hunger contractions. As is easy to guess from their name and common experience, these are muscular contractions that occur when the stomach is deprived of food to digest for many hours or days. The hunger contractions can overlap generating a single tetanic contraction that can last for several minutes and be cause of pain for the subject (the so-called “hunger pangs”).
The pylorus is the sphincter of the stomach, a structure where the layer of circular muscles doubles in thickness compared to the rest of the stomach. The pylorus is almost always slightly contracted but it is never completely closed and liquids can easily pass through it, unlike food that has not yet been well digested and transformed into chyme of the right semi-fluid consistency. Gastric emptying is mainly determined by signals from the duodenum and stomach and is regulated in such a way that the rate of emptying is adapted to the absorption capacity of the small intestine.
Increased gastric content due to food ingestion, for example, facilitates emptying because the distension of the gastric walls due to food entry activates the myonteric plexus, which increases the frequency of antral circular peristaltic contractions and pyloric distension. A second factor that aids gastric emptying is the secretion of gastrin, which is increased when the gastric walls are distended by food and when proteins are digested. Gastrin modestly increases gastric motility and in particular circular peristaltic contractions of the antrum; it also promotes glandular secretion of the stomach walls. Duodenal factors that trigger gastric emptying are primarily responsible.
Three types of reflexes originate from the walls of the duodenum and act by inhibiting gastric emptying. A first modality is activation of the enteric nervous system of the duodenum, which gives rise to inhibitory enterogastric reflexes. This activation can take place according to various factors such as the degree of distension of the duodenal walls, excessively acidic pH, the amount or osmolarity of chyme entering the duodenum, the significant presence of products of protein and lipid catabolism, and irritation of the gastric mucosa.
Inhibition is then possible by extrinsic fibers that reach the spinal cord and then go to the paravertebral orthosympathetic ganglia and then to the stomach wall by means of sympathetic inhibitory fibers; finally, a third modality, of lesser importance, by means of vagal fibers that go to the brainstem and there inhibit the excitatory stimuli of the vagus nerve itself. All three types act by inhibiting circular peristaltic contractions and causing the pylorus to contract.
Protein digestion is carried out by lytic enzymes, chymosin also called rennin or labferment as it is specific for milk casein, by the presence of a mild gastric lipase and pepsin (proteins are broken down into smaller chains, called polypeptides). Moreover there is the absorption of water, of some ions and liposoluble compounds such as alcohol, acetylsalicylic acid, caffeine and not less important the sterilization of the ingested food by hydrochloric acid. Pepsin works only in a low pH environment. This is always guaranteed by the presence of hydrochloric acid.
The combination of all these elements is called gastric juice, which is activated even if we think of eating (in fact “watering” occurs, i.e. salivation is stimulated). The walls of the stomach, in the absence of protective factors, would be damaged by the intrinsic acidity of the gastric juice, but the stomach secretes a substance, mucin, which avoids this problem. The thick musculature finally guarantees the movements of stirring of the food, which during the permanence in the stomach, that can vary from one to three hours, are transformed in chyme.
Another function is the absorption that begins in the cells of the stomach: the absorption is minimal, a minimal amount of water, some short chain fatty acids, and some drugs such as aspirin are absorbed and finally alcohol is absorbed.