In this article we will discus about the anatomical structure of human stomach with the help of suitable diagrams.
The shape of the normal stomach is generally like the letter J. Sometimes the long axis may be slanting from left to right (steer horn type) or it may be even horizontal. The junction of the oesophageal mucosa with that of the stomach is abrupt. The oesophago-cardiac line of junction is irregular or zigzag and is often referred to as the Z or ZZ line.
At the pylorus, the mucous membrane of the stomach makes junction with that of the duodenum. The capacity of the average stomach is about 1.12 -1.70 litres (2-3 pints). It can be subdivided into three parts- the fundus, the body and the pylorus, each of which contains a particular type of gland.
The cardiac area is the zone, 1 to 4 cm wide that guards the oesophageal orifice, also known as cardiac sphincter. The fundic area is the largest area of the stomach accounting for 60 to 80% of total mucosal surface, interposed between the cardiac and the pyloric areas The lower part of fundic area is separated from the pylorus by a sharp angle on the lesser curvature; called the incisura angularis.
The junction of the pyloric and fundic area is not sharply demarcated and is frequently known as the transitional zone. The pylorus is limited on the left by the incisura and on the right by the pyloric sphincter. The circular fibres of pyloric sphincter guards against backflow of small intestinal contents into the stomach. The pyloric area is about 15 percent of the total gastric mucosal area.
It is subdivided into two parts:
(a) The pyloric antrum (pyloric vestibule) which is the short, comparatively wider, proximal chamber, and
(b) The pyloric canal which is narrow tubular passage, about 3 cm long, ending in the pyloric sphincter (Fig. 9.19).
Histology:
Histologically, stomach consists of the same four layers but with certain characteristic differences. The outer serous coat consists of peritoneum. The muscular coat consists of three layers the outer longitudinal, the middle circular and the inner oblique layer. Next comes the submucous coat, and then come the layer of museularis mucosae and a supporting stroma of connective tissue.
This layer of muscle also consists of an outer longitudinal and an inner circular layer. Finally comes the mucous membrane which is thrown out into large folds called rugae when the stomach is empty and these folds tend to disappear when the stomach is distended.
Hence, this folded arrangement of the mucous membrane is a great protective device to prevent damage from stretch. The surface of the gastric mucosa is marked by slight linear depression into small polygonal areas, called areae gastricae, giving the surface a mammilated appearance; the areas measure from 1 to 6 mm in diameter.
The surface is constantly covered by a thick layer of tough tenacious mucus secreted by the surface epithelial cells, the layer varying in thickness from 0.5 mm to 2.5 mm. Beneath the surface layer of epithelium are located simple tubular glands which remain arranged like parallel tubes opening upon the surface.
These glands secrete gastric juice. Histologically, as well as functionally, the glands (Fig. 9.20) are not all same. They differ in different parts of stomach but they are all tubular in structure extending to the museularis mucosae where they terminate in a blind bulbar end, known as the fundus of the gland. The main tubular part is the body. The neck of the gland connects the body to the isthmus which communicates with the gastric crypt. The total number of glands has been estimated at about 35,000,000.
i. Near the Cardiac Orifice:
The cardiac glands which are short and tortuous are mostly surface mucous cells and occupy a small zone around the orifice and secrete mucus although a few pepsinogen cells are also present.
ii. In the Fundus and Body of Stomach:
The fundic glands tend to be straight, slender structures with narrow lumen.
The following types of cells are found to compose these glands:
a. The Mucous Neck Cells:
They secrete mucus which is soluble and have been found to constitute a part of soluble mucinogen droplets of the gastric juice under electron microscope. According to some authors they possibly secrete intrinsic factor.
b. Chief (Zymogenic) Cells in the body of the Glands or the Peptic Cells:
These cells are basophilic and secrete pepsin and contain zymogen granules which are precursors of pepsin. Histologically, the cells appear as a typical protein secreting one. In addition to pepsinogen, the chief cells probably produce gastric rennin, and a gelatin-splitting enzyme, known as gelatinase.
c. Oxyntic (Parietal) Cells:
They are found pressed at the bed by chief cells and are oval in shape. They secrete hydrochloric acid (HCl). The cytoplasm of oxyntic cells are stained with acid dyes. Under electron microscope, secretory canaliculi are found to be lined by microvilli from the apical border, deep into cells.
d. Argentaffin (Kulchitsky or Enterochromaffin) Cells:
According to some, a few argentaffin cells, strainable with silver salts and responsible for the secretion of vasoconstrictor serotonin, are also present in fundic glands.
iii. In the Pyloric Region:
Here the glands are fewer in number and shorter in length. They are devoid of both peptic and oxyntic cells. They are entirely lined by cells, resembling the mucous cells, as found in the neck of the fundic glands and secrete alkali and mucus.
The combined secretion of the above glands forms the gastric fluid. High concentration of ribonucleo-protein is present in the enzyme-producing cells. A good number of mitochondria and a golgi apparatus have been readily demonstrated.
It is to be noted that the secretion of the body and the fundus of the stomach is acid in reaction, and rich in enzyme and chlorides, whereas the secretion of pylorus is alkaline in reaction, rich in mucus and poor in enzyme and chloride.
iv. The Surface Lining:
The surface lining of the stomach consists of columnar cells and goblet cells (mucus-secreting).
Functions of Stomach:
i. Mechanical Functions:
a. Stomach receives food material and acts as a reservoir of food.
b. The movements of stomach help in the proper mixing of food with the digestive juices and also help to propel the food into the duodenum.
ii. Secretion:
Stomach secretes gastric juice which acts as a digestive fluid (Fig. 9.21). The HCI of gastric juice acts as an antiseptic against swallowed bacteria.
iii. Digestion:
With the help of gastric juice, stomach digests protein up to peptone stage. It also digests fats to some extent with gastric lipase. Gastric rennin coagulates milk. HCI causes some hydrolysis of foodstuffs.
iv. Absorption:
Small quantities of water, saline, alcohol, glucose and certain drugs are absorbed from the stomach.
v. Excretion:
Stomach excretes certain toxins (such as those of uraemia), certain alkaloids like morphine and certain other substances.
vi. Stimulatory Functions:
Stomach manufactures two chemical substances which act as stimulants.
They are as follows:
a. Gastrin:
This substance is manufactured by the pyloric mucous membrane and is a true peptide hormone. It acts as a stimulant for gastric secretion. This is responsible for the second phase of gastric secretion.
b. Castle’s Intrinsic Factor:
The normal gastric juice as well as the gastric mucous membrane contains an intrinsic factor. The extrinsic factor is vitamin B12 taken along with food and the intrinsic factor interacts with it and helps in its absorption. Vitamin B12 exerts great influence on the normal development and maturation of red blood cells.
c. Release of Enterogastrone:
There is also release of enterogastrone (enteron, gastron, chalone) from the mucosa of the upper small bowel which inhibits gastric secretions.
The intrinsic factor, as suggested by Castle, is an enzyme like unidentified substance secreted by the stomach. It is present in the gastric juice as well as in the gastric mucous membrane. The optimum pH for its action is 7 and it is inactivated at temperature above 45°C. It does not necessarily run parallel with the amount of HCI or pepsin in the gastric juice.
So that in some cases the intrinsic factor may be present even if there be no HCI or pepsin and vice-versa. The site of formation of the intrinsic factor varies in different species. In pigs it is obtained from pylorus and beginning of duodenum. In human beings it is present in the fundus and body of the stomach.
d. Reflex Function:
Various reflexes are initiated from the stomach.
They are as follows:
i. Gastrosalivary Reflex:
Irritation of stomach stimulates salivation.
ii. GastroiIlial Reflex:
About half an hour after meal, increased peristaltic movements occur in the last part of ileum.
iii. Gastrocolic Reflex:
Mass peristalsis is initiated in the colon about half an hour after taking food.
iv. Presence of food in the stomach reflexly stimulates pancreatic secretion and probably also bile expulsion.