The stomach is an important organ of the digestive system that is able to expand significantly to hold food and liquids that are swallowed. It also muscular and churns food into a substance called chyme after secreting digestive juices to aid in their breakdown. While it is important for keeping us healthy, it can also cause many diseases. The stomach is a fascinating organ and has been studied for many years.
The stomach is located in the upper left abdominal cavity. It has three major sections and two passageways in and out. The fundus is in the top region, the body is the center region, and the antrum is the lower region. The passageway into the stomach is the cardia and it is attached to esophagus. The passageway out of the stomach is the pylorus and it is connected to the duodenum.  The fundus is the upper section of the stomach; it is hollow and typically used for expansion. The cardia, a narrow tube that broadens into the fundus. Inside this narrow tube is the esophageal sphincter. The esophageal sphincter is muscle tissue that holds food and acid. The largest section of the stomach is the body; it is in the middle of the stomach. The cardia gets rid of its food and acid by filtering it into the body of the stomach. Below the body is the antrum; it is the lower portion of the main stomach cavity. After the antrum is the pylorus, it holds the pyloric sphincter and also connects the stomach to the duodenum. 
The stomach is composed of three different layers of tissue. The first is longitudinal, this is the outermost layer. The second deepest layer is the circular layer. The innermost wall of the stomach is made up of the oblique layer. Four layers make up the stomach: the serosa, muscularis, submucosa, and mucosa. The serosa is on the outside of the stomach. It is a thin serous membrane and is composed of connective tissue and simple squamous epithelial tissue. The serosa releases a serous fluid which provides protection to the outside of the stomach when it expands during digestion. The muscularis layer is surrounded by the serosa and surrounds the submucosa. It has three different layers of smooth muscle tissue and they all run in different directions. Tissues then connect the muscularis to the submucosa. The submucosa contains different tissues, blood vessels, and nerves. The mucosa is on the very inside of the stomach lining. It is made up of simple columnar epithelial tissue. This layer holds many gastric glands which is where different digestion enzymes and mucus is made. 
The main functions of the stomach are food storage, digestion, and protection. For example, when food contains harmful organisms (bacteria and other micro-organisms) or chemicals the stomach helps to get rid of them. Alcohol and Aspirin are two exceptions that can enter through the stomach walls and into the bloodstream.
The first part of digestion is mechanical digestion. In this part of digestion the stomach breaks down food into a liquid form called chyme. The second is chemical digestion. During this process large molecules are being converted into smaller ones. Through these two processes the body gets its essential needs from the foods that humans eat.
Before the body can start the digestion process it needs to create what is called gastric juice. Your stomach is always secreting this liquid, but contains most of it right after eating. It helps to regulate the stomach's functions. The human body secretes about two liters a day of this gastric juice. It is made up of water, mineral salts, hydrochloric acid, and mucus. This liquid mixed with food helps the stomach to promote digestion. The water in this chemical mixture helps to liquefy the food so that it can be more easily digested. The mucus in the juice also helps protect the stomach from being eaten by the other acids that it contains. 
There are many different stomach-related illnesses. These different stomach illnesses are known as Gastropathy. Some of the most common diseases include; gastritis, gastroparesis, gastric cancer, and gastroenteritis. These are just a few. Below I am going to go over these sickness and their effect on the stomach. 
Gastritis occurs when the balance is lost between the mucous lining and stomach acid. This results in the swelling of the stomach lining. If this gets out of hand it can be life threatening. The stomach acid can make a hole in the stomach. Symptoms are stomach pain and bloody stool. Gastritis is often referred to, but commonly misdiagnosed. 
Gastroparesis is diagnosed as blockage in the stomach that appears to have no reason to be blocked. Food is stopped in the stomach before it can move onto the small intestine. It is a common disease and can be a long-term problem. It is most often caused by diabetes, but also can be caused by strokes and sometimes Parkinson's. 
Gastric cancer is also called gastric carcinoma. It is hereditary and can also come from smoking or drinking. It is more prevalent in men over fifty. There are no guarantees of survival depending on the severity of the cancer. Chemotherapy and radiation are not extremely successful, but depending on the case may be necessary. Surgery is also a common treatment. 
Gastroenteritis is what is commonly referred to as the "stomach flu". It is often viral. While it does not seem extremely serious it can result in death if not treated correctly. It causes swelling in the stomach and small intestine. 
The word stomach comes from the Greek word stoma, which means mouth.  Many people who studied the stomach in ancient times were correct with their predictions of the stomach's purpose and function. They were aware that it was extremely necessary in human life. Some anatomists concluded that the stomach could think for itself and that is why people are aware when they are hungry. Throughout the eleventh century Avicenna made the connection from the stomach to the diet of a person. He realized that diet and activity directly affected feelings in the stomach and the overall health of someone. As more dissections began to occur throughout the eleventh and twelfth the anatomy became more clear. It was decided that the stomach was a cold and dry organ that was the shape of a bean about the size of the average humans fist. It also increased knowledge on the stomach's part in digestion.
Many anatomists throughout the fourteenth to the seventeenth centuries thought that the spirit and the body were separate. They thought that body organs that did not have a spirit were not necessary to the body. This theory was quickly disproved. Leonardo da Vinci began to make the conclusion that stomach was not only correlated to digestion, but also to respiration.  Throughout time, with technology and experiments, many different ancient theories have been disproved. We know more than they did then, and the future will know more than we do. The stomach is extremely important in the health of humans and is responsible for many different processes.
A video about what parts of digestion occur in the stomach
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