Stomach

Overview
In anatomy, the stomach is a bean-shaped hollow muscular organ of the gastrointestinal tract involved in the second phase of digestion, following mastication. The word stomach is derived from the Latin stomachus, which derives from the Greek word stomachos. The words gastro- and gastric (meaning related to the stomach) are both derived from the Greek word gaster.

Functions
The stomach is a highly acidic environment due to gastric acid production and secretion which produces a luminal pH range usually between 1 and 4 depending on the species, food intake, time of the day, drug use, and other factors. Such an environment is able to break down large molecules (such as from food) to smaller ones so that they can eventually be absorbed from the small intestine. The human stomach can produce and secrete about 2 to 3 liters of gastric acid per day with basal secretion levels being typically highest in the evening.The body produce enzymes to break down different foods.

Pepsinogen is secreted by chief cells and turns into pepsin under low pH conditions and is a necessity in protein digestion.

Absorption of vitamin B12 from the small intestine is dependent on conjugation to a glycoprotein called intrinsic factor which is produced by parietal cells of the stomach.

Other functions include absorbing some ions, water, and some lipid soluble compounds such as alcohol, aspirin, and caffeine.

Another function of the stomach is simply a food storage cavity.

Anatomy of the human stomach
The stomach lies between the esophagus and the duodenum (the first part of the small intestine). It is on the left side of the abdominal cavity. The top of the stomach lies against the diaphragm. Lying beneath the stomach is the pancreas, and the greater omentum which hangs from the greater curvature.

Two smooth muscle valves, or sphincters, keep the contents of the stomach contained. They are the esophageal sphincter (found in the cardiac region) dividing the tract above, and the Pyloric sphincter dividing the stomach from the small intestine.

The stomach is surrounded by parasympathetic (stimulant) and orthosympathetic (inhibitor) plexuses (anterior gastric, posterior, superior and inferior, celiac and myenteric), which regulate both the secretory activity and the motor activity of the muscles.

In humans, the stomach has a volume of about 50 mL when empty. After a meal, it generally expands to hold about 1 litre of food, but it can actually expand to hold as much as 4 litres. When drinking milk it can expand to just under 6 pints, or 3.4 litres.

Sections
The stomach is divided into four sections, each of which has different cells and functions. The sections are:

Blood supply
The lesser curvature of the stomach is supplied by the right gastric artery inferiorly, and the left gastric artery superiorly, which also supplies the cardiac region. The greater curvature is supplied by the right gastroepiploic artery inferiorly and the left gastroepiploic artery superiorly. The fundus of the stomach, and also the upper portion of the greater curvature, are supplied by the short gastric artery

Layers
Like the other parts of the gastrointestinal tract, the stomach walls are made of the following layers, from inside to outside:



Glands
The epithelium of the stomach forms deep pits. The glands at these locations are named for the corresponding part of the stomach:

Different types of cells are found at the different layers of these glands:

Control of secretion and motility
The movement and the flow of chemicals into the stomach are controlled by both the autonomic nervous system and by the various digestive system hormones:

Other than gastrin, these hormones all act to turn off the stomach action. This is in response to food products in the liver and gall bladder, which have not yet been absorbed. The stomach needs only to push food into the small intestine when the intestine is not busy. While the intestine is full and still digesting food, the stomach acts as storage for food.

Diseases of the stomach

 * Dyspepsia
 * Stomach ache
 * Peptic ulcer
 * Achlorhydria
 * Hypochlorhydria
 * Linitis plastica
 * Zollinger-Ellison syndrome
 * Gastroparesis
 * GERD
 * Borborygmus

Historically, it was widely believed that the highly acidic environment of the stomach would keep the stomach immune from infection. However, a large number of studies have indicated that most cases of stomach ulcers, gastritis, and stomach cancer are caused by Helicobacter pylori infection. One of the ways it is able to survive in the stomach involves its urease enzymes which metabolize urea (which is normally secreted into the stomach) to ammonia and carbon dioxide which neutralizes gastric acid and thus prevents its digestion. In recent years, it has been discovered that other Helicobacter bacteria are also capable of colonizing the stomach and have been associated with gastritis.

Having too little or no gastric acid is known as hypochlorhydria or achlorhydria respectively and are conditions which can have negative health impacts. Having high levels of gastric acid is called hyperchlorhydria. Many people believe that hyperchlorhydria can cause stomach ulcers. However, recent research indicates that the gastric mucosa which secretes gastric acid is acid-resistant.

Differences among animals
In ruminants, such as bovines, the stomach is a large multichamber organ which hosts symbiotic bacteria that produce enzymes required for the digestion of cellulose from plant matter, primarily cellulase. The partially digested plant matter passes through each of the intestine chambers in sequence, being regurgitated and rechewed at least once in the process.

In some animals (such as cats and dogs), the pH of the lumen is lower, usually between 1 and 2. In contrast, the human stomach pH is usually between 1 and 3.