Stomach

 Stomach

The stomach is the dilated portion of the alimentary canal and has three main functions: It stores food (in the adult it has a capacity of about 1500 ml), it mixes the food with gastric secretions to form a semi fluid chyme, and it controls the rate of delivery of the chyme to the small intestine so that efficient digestion and absorption can take place.

The stomach is situated in the upper part of the abdomen, extending from beneath the left costal margin region into the epigastric and umbilical regions.

Much of the stomach lies under cover of the lower ribs. It is roughly J-shaped and has two openings, the cardiac and pyloric orifices; two curvatures, the greater and lesser curvatures; and two surfaces, an anterior and a posterior surface.

The stomach is relatively fixed at both ends but is very mobile in between. It tends to be high and transversely arranged in the short, obese person (steer-horn stomach) and elongated vertically in the tall, thin person (J-shaped stomach). Its shape undergoes considerable variation in the same person and depends on the volume of its contents, the position of the body, and the phase of respiration.

Parts

The stomach is divided into the following parts-

Fundus: This is dome-shaped and projects upward and to the left of the cardiac orifice. It is usually full of gas.

Body: This extends from the level of the cardiac orifice to the level of the incisura angularis, a constant notch in the lower part of the lesser curvature.

 Pyloric antrum: This extends from the incisura angularis to the pylorus.

Pylorus: This is the most tubular part of the stomach. The thick muscular wall is called the pyloric sphincter, and the cavity of the pylorus is the pyloric canal

Lesser Curvature

It forms the right border of the stomach and extends from the cardiac orifice to the pylorus. It is suspended from the liver by the lesser omentum.

Greater Curvature

It is much longer than the lesser curvature and extends from the left of the cardiac orifice, over the dome of the fundus, and along the left border of the stomach to the pylorus. The gastrosplenic omentum (ligament) extends from the upper part of the greater curvature to the spleen, and the greater omentum extends from the lower part of the greater curvature to the transverse colon.

Cardiac Orifice

It is where the esophagus enters the   stomach. Although no anatomic sphincter can be demonstrated here, a physiologic mechanism exists which prevents regurgitation of stomach contents into the esophagus.

Pyloric Orifice

It is formed by the pyloric canal, which is about 1 in. (2.5 cm) long. The circular muscle coat of the stomach is much thicker here and forms the anatomic and physiologic pyloric sphincter.

The pylorus lies on the transpyloric plane, and its position can be recognized by a slight constriction on the surface of the stomach.

Function of the Pyloric Sphincter

The pyloric sphincter controls the outflow of gastric contents into the duodenum. The sphincter receives motor fibers from the sympathetic system and inhibitory fibers from the vagi. In addition, the pylorus is controlled by local nervous and hormonal influences from the stomach and duodenal walls. For example, the stretching of the stomach due to filling will stimulate the myenteric nerve plexus in its wall and reflexly cause relaxation of the sphincter.

 

Mucous membrane of the stomach

It is thick and vascular and is thrown into numerous folds, or rugae, that are mainly longitudinal in direction. The folds flatten out when the stomach is distended.

The muscular wall of the stomach

It contains longitudinal fibers, circular fibers, and oblique fibers.  The visceral peritoneum completely surrounds the stomach. It leaves the lesser curvature as the lesser omentum and the greater curvature as the gastrosplenic omentum and the greater omentum.

Relations

Anteriorly: The anterior abdominal wall, the left costal margin, the left pleura and lung, the diaphragm, and the left lobe of the liver.

Posteriorly: The lesser sac, the diaphragm, the spleen, the left suprarenal gland, the upper part of the left kidney, the splenic artery, the pancreas, the transverse Mesocolon, and the transverse colon.

Blood Supply

Arteries- The arteries are derived from the branches of the celiac artery.

The left gastric artery arises from the celiac artery. It passes upward and to the left to reach the esophagus and then descends along the lesser curvature of the stomach. It supplies the lower third of the esophagus and the upper right part of the stomach.

The right gastric artery arises from the hepatic artery at the upper border of the pylorus and runs to the left along the lesser curvature. It supplies the lower right part of the stomach.

The short gastric arteries arise from the splenic artery at the hilum of the spleen and pass forward in the gastrosplenic omentum (ligament) to supply the fundus.

The left gastroepiploic artery arises from the splenic artery at the hilum of the spleen and passes forward in the gastrosplenic omentum (ligament) to supply the stomach along the upper part of the greater curvature.

The right gastroepiploic artery arises from the gastroduodenal branch of the hepatic artery. It passes to the left and supplies the stomach along the lower part of the greater curvature.

Veins- The veins drain into the portal circulation. The left and right gastric veins drain directly into the portal vein. The short gastric veins and the left gastroepiploic veins join the splenic vein. The right gastroepiploic vein joins the superior mesenteric vein.

Nerve Supply

The nerve supply of stomach includes sympathetic fibers derived from the celiac plexus and parasympathetic fibers from the right and left vagus nerves. The anterior vagal trunk, which is formed in the thorax mainly from the left vagus nerve, enters the abdomen on the anterior surface of the esophagus. The trunk, which may be single or multiple, then divides into branches that supply the anterior surface of the stomach.

A large hepatic branch passes up to the liver, and from this a pyloric branch passes down to the pylorus. The posterior vagal trunk, which is formed in the thorax mainly from the right vagus nerve, enters the abdomen on the posterior surface of the esophagus. The trunk then divides into branches that supply mainly the posterior surface of the stomach. A large branch passes to the celiac and superior mesenteric plexuses and is distributed to the intestine as far as the splenic flexure and to the pancreas.

The sympathetic innervation of the stomach carries a proportion of pain-transmitting nerve fibers, whereas the parasympathetic vagal fibers are secretomotor to the gastric glands and motor to the muscular wall of the stomach. The pyloric sphincter receives motor fibers from the sympathetic system and inhibitory fibers from the vagi.