Fundus of the Stomach

The fundus of the stomach is the dome-shaped superior portion of the stomach located above the level of the cardiac orifice. It plays a passive yet important role in the digestive process, primarily functioning as a reservoir for ingested food and gas. Its rounded structure allows for accommodation without a significant rise in intragastric pressure, a process referred to as "receptive relaxation." Radiologically, it is often visible as the gastric air bubble beneath the diaphragm on imaging studies.

Structure

The fundus forms the uppermost part of the stomach and is continuous inferiorly with the body of the stomach and medially with the cardia. It is anatomically defined as the portion of the stomach that lies superior to a horizontal line drawn across the cardiac orifice. Its wall is composed of the same four layers found throughout the stomach:

• Mucosa: Contains closely packed gastric glands rich in parietal (oxyntic) cells that secrete hydrochloric acid and intrinsic factor, and chief cells that secrete pepsinogen.
• Submucosa: Comprised of connective tissue containing blood vessels, lymphatics, and nerves.
• Muscularis externa: Consists of three layers — inner oblique, middle circular, and outer longitudinal muscle layers.
• Serosa: The visceral peritoneum covering the outermost layer of the fundus.

Gastric Glands in the Fundus

Gastric glands in the fundus are deep, straight, and densely packed, particularly abundant in acid-secreting parietal cells. These glands open into gastric pits that occupy about one-fourth of the mucosal thickness in this region.

Location

The fundus is located in the upper left quadrant of the abdomen and lies:

• Posterior and superior to the cardiac orifice
• In contact with the left dome of the diaphragm
• Inferior to the diaphragm, separated from the pleural cavity by a thin layer of muscle and peritoneum
• Lateral to the esophagus, often extending posteriorly in the supine position

It is typically filled with swallowed air or gas produced by digestion, making it visible as a dark crescent-shaped area on upright abdominal radiographs or chest X-rays, known as the gastric air bubble.

Function

The fundus has several functional roles that support the early phases of digestion:

• Reservoir for ingested material: It accommodates food and liquid entering from the esophagus before active churning begins in the body of the stomach.
• Receptive relaxation: Upon swallowing, vagal stimulation causes the fundus to relax and expand to receive food with minimal pressure increase — essential for volume accommodation.
• Gas storage: Acts as a temporary reservoir for swallowed air, which can later be expelled through belching or absorbed gradually.
• Acid and enzyme production: Parietal and chief cells within the fundic glands begin the chemical breakdown of proteins and provide intrinsic factor for vitamin B12 absorption.

Clinical Significance

Although the fundus is not as commonly affected by disease as other parts of the stomach, its anatomical and physiological features still play roles in several conditions:

• Hiatal hernia: The fundus may herniate through the esophageal hiatus in sliding or paraesophageal hernias, potentially leading to reflux or strangulation.
• Gastric volvulus: In cases of organoaxial volvulus, the fundus can rotate anteriorly, which may compromise blood flow and cause acute obstruction.
• Fundic gland polyps: Benign polyps may arise from the fundic glands, sometimes associated with proton pump inhibitor use or familial polyposis syndromes.
• Gastric cancer: Tumors may occasionally originate in the fundus; early detection is difficult due to its silent location and lack of early symptoms.
• Radiological landmark: The fundus’s gas-filled appearance is important in radiographic evaluation, and absence of a normal gastric bubble can suggest pathology such as diaphragmatic hernia or gastric distension.

Surgeons and gastroenterologists must be aware of the fundus’s boundaries during procedures such as fundoplication, sleeve gastrectomy, or partial gastrectomy to avoid disrupting essential gastric functions.