Stomach is a hollow organ situated just below the diaphragm on the left side in the abdominal cavity. Volume of empty stomach is 50mL. Under normal conditions, it can expand to accommodate 1 L to 1.5 L of solids and liquids. However, it is capable of expanding still further up to 4 L.
Organs associated with the Stomach-
- Anteriorly —left lobe of liver and anterior abdominal wall
- Posteriorly —abdominal aorta, pancreas, spleen, left kidney and adrenal gland
- Superiorly — diaphragm, oesophagus and left lobe of liver
- Inferiorly — transverse colon and small intestine
- To the left — diaphragm and spleen
- To the right—liver and duodenum
Part of Stomach-
In humans, stomach has four parts
- Cardiac region
- Body or corpus
- Pyloric region.
Cardiac region -
Cardiac region is the upper part of the stomach where esophagus opens. The opening is guarded by a sphincter called cardiac sphincter, which opens only towards stomach. This portion is also known as cardiac end.
Fundus is a small dome shaped structure. It is elevated above the level of esophageal opening.
Body or corpus -
Body is the largest part of stomach forming about 75% to 80% of the whole stomach. It extends from just below the fundus up to the pyloric region
Pyloric region has two parts, antrum and pyloric canal. The body of stomach ends in antrum. Junction between body and antrum is marked by an angular notch called incisura angularis. Antrum is continued as the narrow canal, which is called pyloric canal or pyloric end. Pyloric canal opens into first part of small intestine called duodenum. The opening of pyloric canal is guarded by a sphincter called pyloric sphincter. It opens towards duodenum. Stomach has two curvatures. One on the right side is lesser curvature and the other on left side is greater curvature.
Stomach wall is formed by four layers of structures-
- Outer serous layer- Formed by peritoneum
- Muscular layer- Made up of three layers of smooth muscle fibres, namely inner oblique, middle circular and outer longitudinal layers.
- Submucus layer- Formed by areolar tissue, blood vessels, lymph vessels and Meissner nerve plexus.
- Inner mucus layer- Lined by mucus secreting columnar epithelial cells. The gastric glands are situated in this layer. Under resting conditions, the mucosa of the stomach is thrown into many folds. These folds are called rugae. The rugae disappear when the stomach is distended after meals. Throughout the inner mucus layer, small depressions called gastric pits are present.
Gastric glands -
The basis of their location in the stomach classified into three types
- Fundic glands or main gastric glands or oxyntic glands- Situated in body and fundus of stomach
- Pyloric glands-Present in the pyloric part of the stomach
- Cardiac glands- Located in the cardiac region of the stomach.
Arterial blood is supplied to the stomach by branches of the coeliac artery and venous drainage is into the portal vein.
The sympathetic supply to the stomach is mainly from the coeliac plexus and the parasympathetic supply is from the vagus nerves. Sympathetic stimulation reduces the motility of the stomach and the secretion of gastric juice vagal stimulation has the opposite effect.
Function of Stomach-
- Mechanical Function-
- Food is stored in the stomach for a long period, i.e. for 3 to 4 hours and emptied into the intestine slowly. The maximum capacity of stomach is up to 1.5 L. Slow emptying of stomach provides enough time for proper digestion and absorption of food substances in the small intestine.
Formation of Chyme-
- Peristaltic movements of stomach mix the bolus with gastric juice and convert it into the semisolid material known as chyme.
2. Digestive Function-
- Pepsin Pepsin is secreted as inactive pepsinogen. Pepsinogen is converted into pepsin by hydrochloric acid. Optimum pH for activation of pepsinogen is below 6.
- Action of pepsin Pepsin converts proteins into proteoses, peptones and polypeptides. Pepsin also causes curdling and digestion of milk (casein).
- Gastric Lipase
- Gastric lipase is a weak lipolytic enzyme when compared to pancreatic lipase. It is active only when the pH is between 4 and 5 and becomes inactive at a pH below2.5. Gastric lipase is a tributyrase and it hydrolyzes tributyrin (butter fat) into fatty acids and glycerols.
|Pepsin||Hydrochloric acid||Proteins||Proteoses, peptones and polypeptides|
|Gastric lipase||Acid medium||Triglycerides of butter||Fatty acids and glycerols|
|Gelatinase||Acid medium||Gelatin and collagen of meat||Peptides|
|Gastric amylase||Acid medium||Acid medium||Dextrin and maltose (negligible action)|
3. Protective Function-
- Protects the stomach wall from irritation or mechanical injury, by virtue of its high viscosity.
- Prevents the digestive action of pepsin on the wall of the stomach, particularly gastric mucosa.
- Protects the gastric mucosa from hydrochloric acid of gastric juice because of its alkaline nature and its acidcombining power.
4. Excretory Function-
Pepsinogen is synthesized from amino acids in the ribosomes attached to endoplasmic reticulum in chief cells. Pepsinogen molecules are packed into zymogen granules by Golgi apparatus.
5. Hemopoietic Function-
Intrinsic factor of Castle, secreted by parietal cells of gastric glands plays an important role in erythropoiesis. It is necessary for the absorption of vitamin B12 (which is called extrinsic factor) from GI tract into the blood. Vitamin B12 is an important maturation factor during erythropoiesis. Absence of intrinsic factor in gastric juice causes deficiency of vitamin B12, leading to pernicious anemia.