Anatomy and physiology of stomach

The stomach is one of the important parts of the digestive system located on the upper portion of the abdomen. Our stomach forms a junction between the tail end of the oropharynx with the duodenum- the first segment of the small intestine. The size depends upon the position of our body and the amount of food inside it, but an empty stomach measures about 12 inches long and 6 inches across at its most wide portion-body. It works as a temporary space for food and also assists in the partial digestion of the nutrients particularly, proteins. It is a hollow-muscular space, relatively thicker, rough, stronger and appears like J-shaped. Our stomach is generally referred to as gastric canal, where gastro refers to the stomach. The human stomach is highly acidic due to HCL secretion. The pyloric sphincter controls the movement of partly digested food (chyme) from the stomach into the duodenum while peristalsis performs this function through the rest of the intestines. The outer layer of the stomach forms itself into many uneven folds with each fold containing some undifferentiated edges known as rugae.

Location of  the stomach

 

Our abdominal cavity is divided into 4 segments or quadrants: right upper quadrant, left upper quadrant, right lower quadrant and left lower quadrant. The stomach lies in the left upper quadrant close to the spleen . As you can see in the image above, part of the liver also lies close to the stomach.  From the top, the extension of the last segment of oesophagus leads to the upper portion of the stomach while it enters into the duodenum with its lowest segment-tail. Oesophagus, otherwise known as a food pipe forms a continuous channel between the mouth and the stomach. The area where the oesophagus comes in contact with the stomach is called the gastro-oesophagal (GE) junction. 

Regions of  the stomach

The stomach is anatomically divided into 5 regions; Cardia region, fundus, body, antrum and the pylorus.

Cardia region

The cardia region is the first part of the stomach lies just beneath the oesophagus. It is a segment into which the food enters from the lower end of the oesophagus. The word cardia sphincter here refers to the sphincter formed by coiling of circular and longitudinal muscles into a thick plug-like structure. It helps to prevent the stomach contents from going back up into the oesophagus.

The fundus

The word fundus is derived from the Latin, meaning bottom. It lies close to the left side of the cardia region forming a curvature-like channel that adheres close to the diaphragm.

Body 

The body is the most fundamental portion of the stomach and is the larger part of responsible for mixing and partial digestion of food. The body is relatively thicker, stronger and has many layers of muscles arranges differently.

The antrum

The antrum is the lower part of the stomach that releases partly digested food into the pylorus. It is otherwise known as the pyloric antrum.

The pylorus

It is the distal segment or the tail end of the stomach that enters into the small intestine. It has a pyloric sphincter formed by a thick ring of muscles located in the pyloric region. The pyloric valve aid in emptying of stomach content into the duodenum (first part of the small intestine). In the absence of food, our stomach deflates inward and rugae become more apparent.

Curvatures of the stomach

Curvatures form the borders and angular areas of the inner regions of the stomach. The 2 curvatures formed by the stomach are the greater curvature and the lesser curvature. Greater curvature forms the lateral, long and convex portions of the stomach. Originating at the cardiac notch, it arches behind and passes inferiorly towards the left but it curves to the right to continue to reach the pyloric antrum. On the other hand, the lesser curvature leads to the shorter, concave, and medial facade of the stomach. The angular notch forms the junction of the body and pyloric region and is the most inferior part of the lesser curvature.

Microscopic anatomy of the stomach

 

 

The layers of the stomach comprises  4 layers from inside: serosa, muscularis, submucosa, and mucosa. The serosa is the fibrous membrane that forms the innermost layer of the stomach. Just above this, the muscular or muscular layer exists. The muscular layer has a unique rearrangement of thick and elastic muscular bands in a longitudinal and circular manner. Sub-mucosa, situated above the muscularis is made up of many connective tissues. It also supplies nerves, blood, and lymph to the stomach. Mucosal layer that lies just above the submucosa is a thin lining and provides space necessary for a variety of gastric glands that secrete HCL. Let us go in detail bout each layer.

Serosa

The serosa is a smooth membrane, consists of a thin layer of connective tissues and an epithelial layer below. The cells in this layer are smaller and it has several serous cavities secretes a lubricating fluid. The connective tissues are associated with many blood vessels and nerves that lie above secretory cells. It also acts as a binding layer that holds the serous membrane with other structures.

Muscularis

Muscularis of the stomach,  also known as tunica muscularis, is the bed of muscles found beneath the submucosa. It is divided into  3 layers: inner oblique muscles, middle circular muscles and outer longitudinal muscle sheaths. The action of these 3 layers provides an elastic and free movement required for mechanical digestion and movement of food. The organization of the muscularis differs among different regions of the stomach. In the cardia,  the layers are well-developed to form sphincter that prevents acid reflux from the stomach into the oesophagus. In the fundus region, the muscle layers are poorly developed as this layer is less active in moving the food. Conversely, the body of the stomach is very thick as it contains all the 3 muscle layers, except in the anterior and areas parts of the stomach where the longitudinal layers are absent. The muscularis is generally referred to as muscularis externa. It is highly developed in the pyloric region so it actively participates in the propulsion of chyme into the duodenum. The thickened circular layers of these muscles forms the pyloric sphincter.

Submucosa

The submucosal layer is located in between the mucosa and the muscularis. It is made up of several connective tissues, blood vessels, and nerves. Connective tissues support other tissues surrounding the mucosa and also aids in connecting submucosa with the muscular layer. 

Mucosa

The gastric mucosa is the outermost portion that comes in contact with the food. Mucus layer contains a number of glands and the gastric pits. They are responsible for the secretion of gastric enzymes and hormones. Mucosa measures about 1 mm thick and it feel-like smooth, soft, and velvety. It consists of simple columnar epithelium, lamina propria, and the muscularis mucosae. The distribution of mucosal cells widely varies across various regions of the organ.

Cells in the stomach

A variety of cells are found across different layers and surfaces of the stomach. But 4 of the most important are the mucous cells, epithelial cells, parietal cells, G-cells and the chief cells.  Mucous cells protect the epithelium against the acidic nature of the stomach while the parietal cells secrete hydrochloric acid essential for digestion of proteins and the chief cells secrete an enzyme called pepsin. Lastly, the G-cells secrete a hormone called gastrin.

Functions of stomach

  1. The stomach acts as a temporary reservoir of food.

  2. It helps in mixing of food with its secretions as this can ease the process of digestion.

  3. It secretes a highly acidic gastric fluid called HCl. On average, our stomach can synthesize 2.4 Litres of gastric secretions in a day in which  Hcl is the primary component. These secretions also contain one of the most important enzyme pepsin that helps in breaking the complex proteins into simple units. Being in an empty stomach for a long time causes gastritis as Hcl starts affecting the normal lining of the mucosa in the absence of food.

  4. Stomach help breaking down the large particles of food into simpler and more absorbable ones.

  5. Hcl present in the stomach juices kills most of the harmful bacteria enters through food. 

  6. The muscular layer of the stomach produces a series of alternative contractions and relaxations called peristalsis. Peristalsis helps in the propulsion of partly digested food in a single direction but in rare conditions, the reversal of peristalsis can cause vomiting.

  7. Stomach synthesizes many hormones that help in the digestion process.

Gastric emptying

The duration within which the stomach gets emptied is called gastric emptying. Gastric emptying in a typical adult ranges between 6-8 hours, however, in children it is much faster as their metabolism is very quick. Stomach emptying depends upon the size and capacity of the stomach, which may vary based on the age-groups, gender, race, body built etc. Gastric emptying is confirmed by a test called gastric emptying test( GET). The test measures the time required for food to completely empty from the stomach and enter into the small intestine. 

Blood supply to the stomach

The stomach is supplied by a number of arteries derived from the celiac trunk. The celiac trunk is the first major visceral branch of the abdominal aorta, which is a segment of descending aorta that comes out from the main aorta. Some part of the stomach is supplied by the left and right gastric arteries branched out from the celiac trunk and the common hepatic arteries.

Nerve supply

Nerves help in the contraction and relaxation of the stomach. Our stomach receives nerves from the autonomic nervous system that forms parasympathetic nerve supply arises from the vagus nerve. Thoracic nerves T6-T9 from the spinal cord allows coeliac plexus via the greater splanchnic nerve.

Read More

  1. Structure of tongue

  2.  Digestion, a life-process in human beings

  3.  Ph in daily life 

  4. Human body systems

Questions

  1. Which is the largest segment of the stomach?

  2. What are greater and lesser curvatures of the stomach

  3. Explain about the gastric emptying.

  4. Describe the 4 segments of the stomach.

  5. Identify the organs surrounding the stomach.

  6. Explain the features of 4 layers of the lining of the stomach.

  7. Why is stomach juice acidic?

  8. Describe the nerve supply and the blood supply of the stomach.

  9. Enumerate the functions of the stomach

  10. The stomach predominantly help in the digestion of which nutrient?

  11. Explain the microscopic anatomy of layers of the stomach

  12. What are the roles of Hcl in the stomach?

 



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