Anatomy and Physiology of Stomach: Definition, Function, Structure, Diagram, & Facts

What Is the Stomach?

The stomach is a muscular, hollow organ of the digestive system. Located between the oesophagus and the small intestine, the stomach performs mechanical digestion by the contraction of muscles and chemical digestion accompanied by compounds formed from gastric secretions. It prepares food for absorption in the intestine.

Location and External Anatomy of the Stomach

The anatomy of the stomach is listed below:

• The stomach is situated in the upper left quadrant of the abdominal cavity between the oesophagus and the small intestines.

• It is located sub-diaphragmatic (below the diaphragm).

• It is positioned ventral to the spleen and pancreas.

External Anatomy

• Fundus: The dome-shaped upper part.

• Body: The middle part of the stomach which grinds and mechanically integrates the food with the stomach acid.

• Pylorus: The two muscular rings near the junction of the stomach and the first part of the small intestine through which the partially broken-down and mixed food is allowed into the latter

• Greater Curvature: The longer, convex and more prominently placed curve from the termination of the fundus to the level of the umbilicus.

• Lesser Curvature: The short, inverted ‘U’ shaped portion of the right side of the stomach.

Histology of the Stomach Wall

The stomach consists of four layers:

Mucosa

• The deepest layer is covered by simple columnar epithelium.

• Responsible for the secretion of gastric juice and mucus.

• It is composed of the following lined with gastric pits and gastric glands.

Submucosa

• A layer of connective tissue that is rich in blood supply, nerves and lymphatics which supply the mucosa with nourishment.

Muscularis Externa

• Including three layers of smooth muscles: oblique muscles, inner circular muscles and outer longitudinal.

• Responsible for churning the food and peristalsis.

Serosa

• The fourth layer is the layer of connective tissue covered by a layer of mesothelium.

• This layer acts as a protective layer that minimizes abrasion between the stomach and other organs.

Gastric Glands and Cells

Types of cells present in stomach are:

Parietal Cells

• They are found in the gastric glands and release hydrochloric acid.

• Intrinsic factor which plays a great role in the digestion of proteins and absorption of vitamin B12.

Chief Cells

• Secrete pepsinogen which turns into pepsin, an enzyme that helps in protein digestion.

Mucous Cells

• These cells secrete the mucus which coats the stomach lining.

• Preventing it from being burnt by acid and mechanical breakdown.

G Cells

• G cells are located in the pyloric region, some may be found in the fundic glands as well.

• Release gastrin which excites the secretion of HCl and the contractions of the stomach.

Physiology of The Stomach

The physiology of the stomach includes:

Mechanical Functions

• Churning and mixing of food

• Formation of semi-fluid chyme

• Peristalsis moves chyme to pylorus

Chemical Functions

• Hydrochloric Acid (HCl): Secreted by parietal cells, activates pepsin, kills microbes

• Pepsinogen: Inactive enzyme released by chief cells and is converted into pepsin (active form) in an acidic environment and initiates protein digestion.

• Mucus: Produced by mucous cells, protects the stomach lining.

Regulation Of Gastric Secretion

The regulation of gastric secretion occurs by:

Phases of Gastric Secretion

• Cephalic Phase: Activated by sight, the smell or thought of food.

• Gastric Phase: Started by the presence of food in the stomach.

• Intestinal Phase: Feedback regulation by chyme in intestine.

Hormonal Regulation

• Gastrin: Secreted by G-cells, gastrin promotes the production of gastric acid and increases the contractions of the stomach.

• Somatostatin: Produced by D cells, suppresses gastric acid secretion and avoids the overproduction of acid.

Nervous Regulation

• Vagus Nerve: Acts to stimulate parasympathetic nerves– gastric acid, stimulating movements/contraction.

• Enteric Nervous System: Local control capable of regulating gastrointestinal functions.

Role of the Stomach in Digestion

The stomach plays a major role in the:

Protein Digestion

• In the stomach, proteins are firstly digested by pepsin, secreted by the chief gland in the form of an inactive precursor the pepsinogen, which is activated by HCl.

• Pepsin cleaves large protein molecules into smaller peptides which are then further split in the small intestine.

• As proteins are digested, the food is converted into a semiliquid with the help of gastric juices.

• It increases the protein’s digestibility in the small intestine.

Absorption in the Stomach

• Limited absorption.

• Some vitamins are absorbed through the simple diffusion. Some drugs and alcohol dissolve and enter the bloodstream.

Protection of Stomach Lining

• The stomach lining has a layer of mucus produced by mucous cells that are on the lining and create a shield from the acidic environment.

• Bicarbonate ions are released to compensate for acid that manages to diffuse through the mucus barrier and thus making stomach lining slightly alkaline.

• The short lifecycle which is about three to five days of the epithelial cells of the area.

• The epithelial cells are connected with tight junctions and do not allow the leakage of enzymes.

Common Disorders Of The Stomach

The common disorders of the stomach include:

Gastritis

Gastritis is the inflammation of the stomach lining and it can be attributed to. More frequently this is caused by the bacterium Helicobacter pylori.

• Causes: NSAIDs, alcohol, stress

• Symptoms: Abdominal ache or discomfort, nausea or vomiting, lack of appetite and even haemorrhage.

Peptic Ulcers

Peptic ulcers are open sores that develop on the lining of the stomach, the top part of the small intestine or the oesophagus. Key causes include: Some of the local pathogens include Helicobacter pylori bacteria.

• Causes: NSAIDs (Non-steroidal anti-inflammatory drugs), excessive acid production.

• Symptoms: Burning stomach pain, bloated abdomen, sickness with vomiting, and in extreme instances, blood.

Treatment Approaches

Treatment: Management of this condition involves the reduction of acid secretion, eradication of H. pylori and promotion of ulcer formation.

• Medications: PPIs, H2-receptor antagonists and antibiotics for H. pylori infection, if any.

• Lifestyle Changes: Not taking NSAIDs, quitting smoking and limiting alcoholic drinks.

• Dietary Adjustments: To reduce the occurrence of hives eating smaller portions of food throughout the day and avoiding triggering foods.

Stomach NEET MCQs (With Answers & Explanations)

Important questions asked in NEET from this topic are:

• Layers of the stomach

• Role of Stomach in digestion

Practice Questions for NEET

Q1. The esophagus opens in which part of the stomach?

1. Pyloric

2. Cardiac

3. Fundic

4. Caecum

Correct answer: 2) Cardiac

Explanation:

The oesophagus terminates at the cardiac region of the stomach, which lies adjacent to the gastroesophageal junction. This specific area is termed "cardiac" due to its closeness to the heart. At this juncture, there is a valve known as the lower oesophagal sphincter (LES). The primary function of the LES is to preclude the regurgitation of gastric contents into the oesophagus, thereby maintaining the integrity of the digestive system and preventing potential issues associated with acid reflux.

Hence, the correct answer is option 2) Cardiac.

Q2. Route through which food enters into small intestine is

1. Cardiac sphincter

2. Hepato pancreatic sphincter

3. Pyloric sphincter

4. Sphincter of oddi

Correct answer: 3) Epiglottis

Explanation:

The pyloric sphincter is also called the "pyloric valve" and is the route by which the contents of the stomach are squeezed out of the stomach as chyme, passing into the first part of the small intestine called the duodenum. The outlet (from the stomach) itself is called the pyloric outlet.

Hence, the correct answer is option 3) pyloric sphincter.

Q3. The pyloric sphincter regulates the opening of

1. Pharynx into oesophagus

2. Oesophagus into stomach

3. Stomach into duodenum

4. Ileum into large intestine

Correct answer: 3) Stomach into duodenum

Explanation:

The pyloric sphincter is a vital structure situated at the juncture of the stomach and duodenum, the latter being the initial segment of the small intestine. Its primary role is to regulate the flow of chyme, a semi-digested food substance, from the stomach to the duodenum. This sphincter is crucial in managing the rate of gastric emptying, ensuring that the stomach's contents are passed into the small intestine in a measured manner. This controlled release allows for efficient digestion and nutrient uptake, as the duodenum is not overwhelmed with a large volume of food at once. Moreover, the pyloric sphincter acts as a barrier to prevent the backflow of duodenal contents into the stomach, thereby maintaining the unidirectional flow that is characteristic of the digestive process.

Hence, the correct answer is option 3) Stomach into duodenum.

Also Read:

• MCQs on Anatomy and Physiology of Stomach

• Pancreas Function in Human Body

• Intestinal Glands

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