The human stomach is a major organ of the gastrointestinal tract and found in the upper left quadrant of the abdomen. This organ system is involved in digestion. Starting with the entry of food via the mouth, it travels down the esophagus to the stomach, where specialized enzymes and cells carry out the organ’s functions.
Structure of the stomach
The stomach is often referred to as a hollow sac, and it is capable of expanding as it fills with food. The exact size and extent of stomach distension differs from person to person, but generally it can hold between 1 and 4 liters (or approximately a gallon) according to HyperTextbook. The organ is connected to the end of the esophagus via the esophageal sphincter and the beginning of the small intestine, called the duodenum, by the pylori sphincter. The sphincters are muscular valves that control the transit of food from the stomach. The wall of the stomach is made of layers of smooth muscle with interior ridges called rugae, also known as gastric folds. The longitudinal rugae of the body of the stomach form gastric canals.
Parts of the stomach
The stomach appears as a “J” shape and is subdivided into four sections. The cardia is the section attached to the esophagus and is so-named because of its proximity to the heart. The fundus is the top curve. The body, or corpus, of the stomach is the main central portion of the organ where food mixes with enzymes and acid to be broken down. The pylorus is the section connected to the intestines. Tubules within the lining of the stomach, called gastric glands, secrete enzymes and acid.
The wall of the stomach consists of layers. The interior layer is the mucosa - consisting of the simple columnar epithelium lining the stomach, the connective tissue layer known as the lamina propria, and smooth muscle known as muscularis mucosae. The lamnia propria can contain lymphatic cells, such as lymphocytes. Moving outward is the submucosa, which contains nerves (Meissner’s plexus). The third layer from the inside is the muscularis externa, which consists of three layers of smooth muscle: inner oblique, middle circular, and outer longitudinal. The outer muscle layer contains another nerve plexus (Auerbach’s plexus), which is responsible for peristalsis. The outer layer of the stomach wall is the serosa, which encloses the organ and secretes serosal fluid to protect it from friction within the abdominal cavity.
Cells of the stomach
The numerous gastric glands in the stomach wall contain several different types of cells. Parietal cells secrete hydrochloric acid and intrinsic factor. The acid is produced via a hydrogen/potassium ATPase, a transmembrane protein that pumps hydrogen ions into the contents of the stomach. Chief cells secrete pepsinogen, the precursor to the enzyme pepsin. When food arrives in the stomach, endocrine cells in the stomach secrete gastrin, which stimulates pepsinogen secretion. Cells of the mucus membrane also secrete mucus, which acts as a protectant against the gastric juices. When this barrier is defeated, ulcers form.
Function of the stomach
Food that arrives in the stomach has already been broken down into smaller pieces by chewing. The acid and enzymes in the stomach work on these pieces to break them down further. Pepsin acts in the first step in breaking down food proteins to amino acids for use in the body’s cells. Pepsin works best in the acidic environment of the stomach. The smooth muscle of the stomach contracts, driven by the parasympathetic nervous system, to agitate the food pieces, aiding in digestion. What results is a mixture called chyme, which goes on to the small intestines for further digestion and nutrient absorption.
However, what happens in the stomach is not just about breaking up food. It is necessary for the absorption of nutrients in the intestines. Intrinsic factor binds to vitamin B12 in the stomach, allowing it to be absorbed when it makes it to the intestines. Little absorption occurs in the stomach itself – water, some ions, and certain drugs (e.g., alcohol and aspirin) are absorbed directly into the blood stream via the lining of the stomach. The acid in the stomach also acts as a first line of defense against ingested pathogens.
The human stomach
The stomach is just one stop in the human digestive tract. Here chewed food is broken down and partially dissolved so that the intestines can later do their work to absorb nutrients and process the waste products. The anatomy of the stomach gives it a unique ability to secrete acid and enzymes for this purpose without damage under healthy conditions. To better understand the stomach's place in relation to other anatomy, see the three-dimensional images at Healthline.