Our alimentary canal is essentially a long hollow tube that extends all the way from the mouth to the anus. However, this tube is not homogenous all along its length. Different segments of the alimentary canal have developed different anatomical and physiological specializations that enable them to carry out specific functions in the digestion, absorption and waste elimination processes. Some of the key distinct areas of the alimentary canal include the esophagus, stomach, small intestine, large intestine, rectum and anus.
The stomach is an important region of the alimentary canal. This organ is located just after the esophagus and just before the start of the small intestine. The food and beverages we consume through the oral cavity reach the stomach via the esophagus. Within the stomach, the food gets broken down and digested to some extent. The resulting digested pulp is then passed out into the duodenum, which is the first part of the small intestine.
Functions of the Stomach
The main function of the stomach is to partially digest food. The stomach also regulates the amount of partially digested food that enters the small intestine to help with maximizing absorption of nutrients. Digestion of food in the stomach is aided by mechanical, chemical, and enzymatic processes. The food in the stomach is broken down into smaller pieces through mechanical churning of the food as well as through the action of strongly acidic gastric juices.
Certain enzymes that can act within the highly acidic environment of the stomach also aid in the process of digestion. Pepsin and gastric lipase are examples of two such hardy enzymes in the stomach. Apart from digestion, a small amount of absorption of nutrients also takes place in the stomach. The strong gastric acid and digestive enzymes also destroy microorganisms that enter the digestive tract with food. This can help prevent disease.
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Parts of the Stomach
The stomach itself does not have a homogenous structure. Anatomically, the stomach is subdivided into four distinct regions.
The cardia is the first part of the stomach. It is the region where the stomach meets the end of the esophageal tube. This region is also referred to as the Z- line or the esophagogastric junction. The passage of food from the esophagus to the stomach is regulated by the presence of a sphincter in this region of the stomach. This sphincter, known as the cardiac sphincter or the lower esophageal sphincter, prevents the backward flow of stomach contents into the esophagus.
After the stomach begins, the wall of the stomach arches upwards before coming down. This area of the stomach is known as the fundus. It is the uppermost region of the stomach, and lies just under the diaphragm that separates the thoracic and abdominal cavities. The fundus ends in the same plane as the cardia.
The middle part of the stomach is also the largest. This region of the stomach is referred to as the body of the stomach. The body of the stomach extends between the fundus and the pylorus.
The terminal part of the stomach is referred to as the pylorus. The digested contents of the stomach flow through the pylorus before reaching the duodenum. This passage of digested food material from the stomach to the duodenum is regulated by another sphincter, known as the pyloric sphincter. The pylorus itself is divided into two distinct parts: antrum and canal. the pyloric antrum lies adjacent to the body of the stomach, whereas the pyloric canal is located adjacent to the duodenum.
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Areas of the Abdomen
In order to understand the position of the stomach in the abdominal cavity, we need to have some reference points. Medically, such reference points are provided by dividing the entire abdominal area with imaginary vertical and horizontal lines that start and end at distinct anatomical landmarks. A popular way is to divide the entire abdominal area with three horizontal and two vertical imaginary lines.
The topmost horizontal line used to subdivide the abdominal region is known as the transpyloric line of C. Addison. In the middle, this line lies midway between the pubic symphysis and the suprasternal notch. On the left side, this line lies at the level of the pyloric opening of the stomach. On the right side, this line lies at the level of the first lumbar vertebra.
The subcostal line is the middle horizontal line that connects the lowest reaches of the 10th ribs on either side of the abdomen.
The bottom horizontal line is known as the intertubercular line. It connects the tubercles on the two iliac crests.
Apart from the three horizontal lines described above, two distinct imaginary vertical lines (known as mid-Poupart lines) are also used to subdivide the abdominal region. These vertical lines run in a region that is midway between the pubic symphysis and anterior superior spine at each side of the body.
The arrangement of three horizontal and two vertical imaginary lines divides the entire abdominal area into the following nine distinct regions:
- Right hypochondrium
- Left hypochondrium
- Right lumbar region
- Umbilical region
- Left lumbar region
- Right inguinal or right iliac region
- Hypogastric or suprapubic region
- Left inguinal or left iliac region.
Abdominal organs can now be located with reference to each of these nine distinct abdominal areas.
Another popular way is to subdivide the entire abdominal area into four distinct quadrants by drawing one vertical and one horizontal imaginary line. This arrangement results in the following four reference quadrants:
- Right upper quadrant
- Left upper quadrant
- Right lower quadrant
- Left lower quadrant
Anatomical Location of the Stomach
The exact location of the stomach in the abdominal cavity at a particular time varies with certain factors. These factors include the stage of respiration (inspiration or expiration phase), extent of fullness of the stomach, and body position (lying, sitting, or standing). For the sake of consistency, the anatomical location of the stomach is usually described in a supine body position.
In a supine position, the stomach lies mostly in the left upper quadrant, with some parts extending into the right upper quadrant. Using the more specific 9-subdivision classification of the abdominal area, the location of the stomach can be said to span the epigastrium, left hypochondrium, left lumbar and umbilical regions. In the supine position, the location of the different parts of the stomach are as follows:
- Cardia: The cardia lies behind the sixth rib on the left side.
- Fundus: the fundus lies behind and slightly above the sixth rib on the left side.
- Body: The body is located between the fundus and the pylorus.
- Pylorus: The pylorus is located at the junction of umbilical and epigastric regions.
In simpler terms, the stomach is described as being located in the left upper abdomen. Most of it is tucked under the lower left rib cage and extends towards the upper middle abdominal region.