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The body of the stomach makes up the majority of the familiar kidney-bean-shaped portion of the organ. Situated between the fundus, which is the rounded topmost portion of the stomach, and the pylorus, which is the organ’s bottommost section, the body is where food is physically and chemically broken down during digestion. Partially digested food passes through the stomach after leaving the esophagus and entering the stomach via the cardia. Broken down further to form a substance known as chyme, it exits the body by way of the pylorus, entering the small intestine.
Digestion, a combination of physical and chemical processes to break down ingested food for energy, begins in the mouth. Ingested food is broken down physically via mastication, or chewing, and chemically by enzymes in saliva. Upon swallowing, this mix of chewed food and saliva known as bolus is pushed through the esophagus by a series of smooth muscle contractions called peristalsis, carrying it to the stomach. It enters the stomach through an area beyond the esophageal sphincter called the cardia before being deposited into the stomach. The body is the large distensible space in which the next stage of digestion occurs.
Two substances are released in the body of the stomach that further contribute to the chemical digestion of food. Hydrochloric acid, known as stomach acid, destroys potentially harmful bacteria found in food as well as alters the pH level in the stomach. In doing so, it creates a more acidic environment that enables the second substance, protease, to do its job. Proteases are enzymes that break down protein in ingested foods by severing the bonds in the chains of amino acids, known as polypeptides, that constitute the protein.
Physical processes of digestion also take place in the body of the stomach. Contractions by smooth muscle in the wall of the stomach called the muscularis externa churn the food, acid, and enzymes around like clothing in a washing machine. This serves to better mix the food with the digestive substances as well as reduce its volume by releasing gases that accrue in the upper region of the stomach in a section known as the fundus. The fundus is separated from the body by an anatomical line extending laterally from the cardia to the opposite side of the stomach that is referred to as the antrum cardiacum.
Once the partially digested mass of food, now known as chyme, has been thoroughly mixed in the body of the stomach, it makes its way through the pyloric sphincter. An opening at the bottom of the stomach, the pyloric sphincter separates the contents of the stomach from the small intestine until they have been properly digested. When this process is complete, the chyme is released from the body of the stomach via the sphincter, entering the first section of small intestine known as the duodenum where the next phase of digestion begins.