Stomach

The stomach is an organ of the digestive system. It mixes recently swallowed food with acid and digestive enzymes before moving it to the intestines. It is a sac that sits beneath the diaphragm and its main role is to store food to pass on to the small intestine.

Structure
There are three main regions that make up the stomach. The first is the fundus which extends above the oesophagal sphincter. This part of the stomach is located on the left hand side and accumulates the gases produced during the process of digestion. It can also store food for as long as sixty minutes. The second is called the body and makes up the majority of the stomaches volume. The final region is called the antrum, also referred to as the pylorus and this is much narrower than the other two regions. The antrum can contract in order to push food into the small intestine. This is known as gastric emptying, an important process in the process of digestion. Once it has been pushed through the antrum, the food can be emptied into the intestines via the pyloric sphincter.

Although structure is the same, the size of the stomach is variable amongst individuals. It can also change in size depending on the time of day. For instance, it is designed to change size in order to accommodate the amount of food eaten and will become distended when full. When the stomach is empty, it is against the back of the abdomen and is more cylindrical in shape. When the stomach becomes full, the flattened surfaces become convex and it pushes against the anterior wall of the abdomen. The fundus region of the stomach is lifted higher than the hearts position and can push against the diaphragm. In severe distension, the stomach can interact with the heart and thus palpitations are associated at this point. The stomach also moves when breathing. The stomach is pushed down during inhalation and forced upwards when exhalation occurs. It can also be in a different position depending upon age. As a young baby, the stomach is vertical due to the large size of the liver.

Structure of the stomach wall
The stomach wall is made up of four layers. The first is the serous coat which covers the whole of the organ. The next layer is the muscular coat and this is closely connected to the first layer. There are three sets of fibres that make up the muscular coat and are present in varying degrees of adundance. On top, the longitudinal fibres are distributed thinly over the surface of the stomach and more densley at the pylorous. Beneath these are the circular fibres which form a circular ring around the pylorus. The third set of fibres are the oblique fibres and these can be found at the cardiac end of the stomach. They form a thick layer around this end.

The third layer of the stomach is the areolar coat which is responsible for connecting the muscle and mucous layers of the stomach wall. It also provides support for the blood vessels. The fourth layer is called the mucous membrane and it gives the stomach wall a smooth outer layer. It varies in colour depending on the location on the stomach from pink to reddish brown and it also varies in thickness where it tends to be thicker towards the pylorus. The structure of this mucous layer is interesting under the microscope for it takes the form of a honeycomb appearance due to the number of depressions made in the surface. These depressions are called alveoli and at the bottom of these lie the gastric glands. The top layer of the mucous membrane is made up of a single layer of epithelial cells.

Gastric Glands
There are two types of gastric glands in the stomach and they are sometimes referred to as gastric pits. Both are formed from the basement membrane and are tubular in shape. The pylori glands are located closer to the pylorus. They usually are made up of two to three tube lined with columnar cells and they open into a common duct which is found at the bottom of the alvelous. The cardiac glands are found predominantly near the cardiac end of the stomach but are also present over the whole surface of the organ. They resemble a similar system to the pylori glands in the sense that there are three tubes opening into a single duct but the duct is much smaller in this type of gland.

The glands contain a number of different secretory cells such as chief cells, parietal cells and G cells. These cells all play a different role in the stomach due to the product they secrete. Chief cells secrete pepsinogen which can be used to make the enzyme pepsin. The parietal cells secrete hydrochloric acid which is responsible for the acidic environment found within the stomach. This acidity is especially important in terms of defense for it will kill unwanted bacteria before they have the chance to reach the small intestine. It is also important in producing the enzyme pepsin. The parietal cells also release an intrinsic factor which is important in the role of absorbing vitamin B12. The G cells secrete a hormone called gastrin which is secreted into the blood stream unlike the products of the other secretory cells which are released into the lumen of the stomach.

Acid Production
The production of acid is an important mechanism of the stomach as it is key to the break down of food and the protection of the gastric system from unwanted bacteria. The parietal cells are responsible for the release of acid and this is catalysed by the enzyme carbonic anhydrase which causes carbon dioxide to react with water. Pumps found on the apical membrane of the stomach transport the acid into the lumen of the stomach. The process of acid secretion is stimulated by parasympathetic nervous activity and gastrin. Generally, the acid will be secreted when food is present in the stomach. Pepsinogen is also controlled by similar mechanisms and its levels will rise when there is a release of acid in the stomach.

When food is chewed in the mouth, parasympathetic activity will cause the stomach to start secreting acid, pepsinogen and gastrin. The release of gastrin acts as a feedback loop and ensures that the release of acid and pepsinogen is constant. When food enters the stomach, the proteins cause the activation of chemoreceptors and as the stomach distends, the mechanoreceptors are also activated. This furthers the secretion of the stomachs main products. Once food leaves the stomach, these signals are reduced and the level of agastric acidity rises which suppresses the secretion. The low levels of secretion further prompt gastric emptying so chyme leaves the stomach through the antrum and enters the small intestine.

Function
The main role of the stomach is to store food in order to pass it on to the small intestine. In order to do so, it must first change the structure of the food. This is done through the powerful contractions of the stomach wall. This breaks the food down into manageable particles which are then mixed with the gastric juices of the stomach. This is called chyme and can leave the stomach through the pylorus.

Disease
Like many organs, the stomach is subject to a number of diseases. Gastric ulcers can occur throughout the stomach where the stomach wall erodes away causing the wall to bleed and it is extremely painful. This is usually caused by foreign agents such as aspirin which break down the defence of the stomach. Other conditions include stomach cancer and there are a number of different cancers that can arise. These depend on the type of tissue that they originate from and are caused when stomach cells begin to multiply in an uncontrolled manner to cause malignant growths. Although the cause of stomach cancer is unknown, it is believed that the exposure to carcinogens such as nitrates which may be the main reason for malignancies. The bacterium Helicobacter pylori is also responsible for some forms of stomach cancer and it affects the stomach lining. The bacterium can cause sufficient damage to the lining of the stomach by causing inflammation and gastric ulcers. If treated early enough, the survival rate of stomach cancer is high but the life expectancy is poor if the cancer is more advanced.