Anatomy and Physiology of Stomach

Stomach is a J shaped organ situated in the alimentary canal. It digests food before transferring it to the small intestine for absorption.

Location of Stomach

The anatomical structure known as the stomach is situated in the superior abdominal region, inferior to the costal margin. It is between the thoracic region and small intestine. The stomach is located in the left hypochondriac part of the abdominal cavity, mostly in the left hemiabdomen.

Structure of the Stomach

The stomach is built in a way that helps it break down food mechanically and chemically, controls how it moves into the small intestine, and keeps it safe from its own acidic environment.

Cardia: The upper stomach, near the oesophagus, is called the cardia. The esophagus-stomach junction is curved

Fundus: Above the cardia, the stomach has a dome-shaped fundus. It temporarily stores food in the upper left abdomen

Body: The stomach's corpus is the body. It runs from the fundus to the lower pylorus. Muscular contraction and relaxation combine and ground food

Pylorus: The lower stomach, near the small intestine, is the pylorus. It has the pyloric antrum and canal. The pyloric sphincter, a muscular ring, releases partly digested food from the stomach into the small intestine.

Rugae: The stomach lining has folds. These folds stretch the stomach to digest more food.

Gastric glands: Millions of microscopic gastric glands release gastric juice, which contains hydrochloric acid and enzymes like pepsin for food digestion

Histology of Stomach

The microscopic structure and organisation of the stomach wall's tissues is called stomach histology. Each stomach layer contains specialised cells and functions. The stomach's histology includes specialised tissues and cell types that work together to aid digestion, gastric juice secretion, stomach lining protection, and food movement through muscular contractions.

From inside out, the stomach wall's primarily layers are:

Mucosa

The stomach's deepest layer directly contacts gastric contents. Three main parts:

a. Epithelium

Stomach epithelial cells are columnar. These cells generate pits in the stomach that form gastric glands. Mucus from epithelial cells protects the stomach from acidic damage.

b. Gastric Glands

Gastric glands are located within the mucosa and extend into the underlying layers. They are responsible for secreting gastric juice, which contains digestive enzymes, hydrochloric acid, and intrinsic factor. The gastric glands consist of several different cell types:

Mucous neck cells

These cells secrete mucus, which helps protect the stomach lining

Parietal cells: Parietal cells secrete hydrochloric acid and intrinsic factor. Hydrochloric acid responsible for formation of an acidic environment for efficient enzyme function and kills microorganisms in food. Intrinsic factor is necessary for vitamin B12 absorption.

Chief cells: Pepsinogen, an inactive version of the enzyme pepsin, is secreted by chief cells. The acidic environment of the stomach later activates pepsinogen, turning it into pepsin, which breaks down proteins.

Enterochromaffin Cells: Histamine is secreted by these cells, which induces the release of stomach acid.

c. Lamina Propria

The lamina propria is a connective tissue layer located beneath the epithelium. It contains blood vessels, lymphatic vessels, and immune cells

Submucosa

The submucosa lies beneath the mucosa and is composed of connective tissue. It contains larger blood vessels, lymphatic vessels, and nerves that supply the stomach. It also houses Meissner's plexus, a network of nerves involved in regulating the gastrointestinal functions.

Muscularis Externa

The muscularis externa is responsible for the movement and mixing of food in the stomach. It consists of three layers of smooth muscle:

a. Inner oblique layer: This layer consists of muscle fibers that are arranged diagonally. The oblique layer allows the stomach to contract and churn the food, aiding in digestion.

b. Middle circular layer: The circular layer consists of muscle fibers that encircle the stomach. Contraction of these muscles helps to further mix the food.

c. Outer longitudinal layer: The longitudinal layer consists of muscle fibers that run parallel to the long axis of the stomach.

Serosa

Stomach serosa is the outermost layer. Mesothelium and connective tissue form a serous membrane. The serosa protects and anchors the stomach in the abdomen.

Functions of the Stomach

Food Storage

The stomach stores food. It slowly releases food from the oesophagus into the small intestine for processing and absorption.

Mechanical Digestion

Stomach muscles contract and relax to mix food with gastric juice and break it down. Mechanical churning improves digestion.

Chemical Digestion

Gastric juice contains enzymes and acids that break down food. Pepsin, the major enzyme in gastric juice, breaks proteins into peptides.

Production of Intrinsic Factor

The stomach produces intrinsic factor, a glycoprotein needed for vitamin B12 absorption in the small intestine. Intrinsic factor prevents vitamin B12 from breakdown, facilitating its absorption.

Defense Against Pathogens

Hydrochloric acid in the stomach kills bacteria and other microbes in meals, reducing infection risk.

Formation of gastric juice

Gastric juice is produced by stomach glands. Food breakdown and digestion start with it. Gastric juice is made by gastric gland cells secreting different chemicals.

Process overview

Mucous-neck cells

Mucus from these cells protects the stomach lining. Mucus protects the stomach from gastric fluid and digestive enzymes.

Parietal cells secrete hydrochloric acid (HCl) and intrinsic factor. Gastrin and neuronal impulses regulate HCl secretion. Hydrochloric acid is formed when parietal cells pump hydrogen ions (H+) into the stomach lumen. HCl acidifies food, denatures proteins, activates digestive enzymes, and kills microorganisms. Small intestine vitamin B12 absorption requires intrinsic factor.

Chief cells

Chief cells release pepsinogen. HCl from parietal cells activates pepsinogen in the stomach. Activated pepsinogen becomes pepsin in acidic pH. Pepsin breaks proteins into peptides.

Enterochromaffin-like cells (ECL cells)

ECL cells are specialised cells in the stomach glands that release histamine. Histamine is a paracrine signaling molecule that causes parietal cells to release stomach acid. It makes the stomach make more hydrochloric acid, which makes the gastric juice more acidic.

These secretions from mucous neck cells, parietal cells, chief cells, and ECL cells collectively form the gastric juice. Gastric juice also contains other components such as water, electrolytes, and various enzymes. The coordinated actions of these secretions help create an optimal environment for the digestion of food in the stomach.

Composition of gastric juice

Gastric juice is a complex mixture released by stomach cells. Gastric juice contains:

Hydrochloric Acid (HCl): Gastric juice is mainly contains HCl, released by gastric gland parietal cells.

Pepsinogen: The inactive form of pepsin.

Gastric lipase: promotes fat digestion.

Mucus: Gastric mucous neck cells produce mucus.

Intrinsic Factor: Parietal cells secrete this glycoprotein.

Electrolytes: Gastric juice contains chloride, potassium, and sodium ions. These electrolytes maintain electrolyte balance.

Water: It is the main content of gastric juice. It dissolves and dilutes digestive enzymes and acids, helping them digest food.

Functions of Gastric Juice

Acidic Environment

Hydrochloric acid (HCl) in gastric juice produces an acidic environment in the stomach. This is responsible for optimal enzyme activity of pepsin.

Antimicrobial Action

The high acidity of gastric juice inhibit the growth of bacteria, viruses, and other pathogens that may be present in food, which prevents gastrointestinal infections.