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Anatomy and physiology of the stomach
The stomach is a muscular, J-shaped organ in the upper part of the abdomen. It is part of the digestive system, which extends from the mouth to the anus. The size of the stomach varies from person to person, and from meal to meal.
The stomach is part of the digestive system and is connected to the:
- esophagus – a tube-like organ that connects the mouth and throat to the stomach. The area where the esophagus joins the stomach is called the gastroesophageal (GE) junction.
- small intestine (small bowel) – a long tube-like organ that extends from the stomach to the colon (large intestine or large bowel). The first part of the small intestine is called the duodenum, and it is this part that is connected to the stomach.
The stomach is surrounded by a large number of lymph nodeslymph nodesA small, bean-shaped mass of lymphatic tissue along lymph vessels (tubes through which lymph fluid travels in the body). Lymph nodes store lymphocytes (a type of white blood cell that fights germs, foreign substances or cancer cells) and filters bacteria and foreign substances (including cancer cell.
Regions of the stomach
The stomach is divided into 5 regions:
- The cardia is the first part of the stomach below the esophagus. It contains the cardiac sphincter, which is a thin ring of muscle that helps to prevent stomach contents from going back up into the esophagus.
- The fundus is the rounded area that lies to the left of the cardia and below the diaphragmdiaphragmThe thin muscle below the lungs and heart that separates the chest cavity from the abdomen..
- The body is the largest and main part of the stomach. This is where food is mixed and starts to break down.
- The antrum is the lower part of the stomach. The antrum holds the broken-down food until it is ready to be released into the small intestine. It is sometimes called the pyloric antrum.
- The pylorus is the part of the stomach that connects to the small intestine. This region includes the pyloric sphincter, which is a thick ring of muscle that acts as a valve to control the emptying of stomach contents (chyme) into the duodenum (first part of the small intestine). The pyloric sphincter also prevents the contents of the duodenum from going back into the stomach.
Layers of the stomach wall
The stomach is made up of several layers of tissue:
- The mucosa (mucous membrane) is the inner lining of the stomach. When the stomach is empty the mucosa has a ridged appearance. These ridges (rugae) flatten out as the stomach fills with food.
- The next layer that covers the mucosa is the submucosa. It is made up of connective tissue that contains larger blood and lymph vessels, nerve cells and fibres.
- The muscularis propria (or muscularis externa) is the next layer that covers the submucosa. It is the main muscle of the stomach and is made up of 3 layers of muscle.
- The serosa is the fibrous membrane that covers the outside of the stomach. The serosa of the stomach is also called the visceral peritoneumvisceral peritoneumThe membrane that lines the walls of the abdomen and pelvis (parietal peritoneum), and covers and supports most of the abdominal organs (visceral peritoneum)..
The stomach has 3 main functions:
- temporary storage for food, which passes from the esophagus to the stomach where it is held for 2 hours or longer
- mixing and breakdown of food by contraction and relaxation of the muscle layers in the stomach
- digestion of food
The mucosa contains specialized cells and glands that produce hydrochloric acid and digestive enzymes to help digest food. The mucosa in the cardiac and pyloric regions of the stomach release mucus that helps protect the lining of the stomach from the acid produced for digestion. Other specialized cells in the mucosa of the pylorus release the hormonehormoneA substance that regulates specific body functions, such as metabolism, growth and reproduction. gastrin into the blood. Gastrin helps to stimulate the release of acid and enzymes from the mucosa. Gastrin also helps the muscles of the stomach to start contracting.
Food is broken down into a thick, acidic, soupy mixture called chyme. The pyloric sphincter relaxes once chyme formation is complete. Chyme then passes into the duodenum. The duodenum plays a big role in absorption of the food we eat. The stomach does not play a big role in absorption of food. It only absorbs water, alcohol and some drugs.