Hello, everyone! This is Megan from Kenhub, and welcome to another anatomy tutorial. In today's tutorial, we will be looking at the mucosa and the muscle layers of the stomach. While we're discussing these structures, we're gonna be discussing them as they are seen with the naked eye or macroscopically. We're not going to use histological slides or a microscope. So, let's begin with the complete view of the stomach.
Here, we can see the stomach connecting to the esophagus and duodenum. We can also see its internal surface and the folds which are called rugae. In the next illustration, we can see the muscular layers of the stomach but we will cover these later on in the tutorial. So, the stomach is a muscular bag that expands to accommodate ingested food. The stomach connects the esophagus and the small intestine. It's found in the epigastric, umbilical and left hypochondriac regions of the abdomen. The stomach can be subdivided into four main parts: the cardiac part, the fundus, the gastric body and the pyloric part. The structure of the stomach is similar to the rest of the GI tract and is composed of mucosa, submucosa and the muscular layer. I'll talk a bit more about these layers later on in the tutorial.
When we look at the stomach from an anterior view like in this illustration, we can see the esophagus is the most superior structure. It connects the stomach with the upper alimentary tract. The esophagus is a long, fibromuscular tube that connects the pharynx to the stomach. It passes through the superior and inferior-posterior sections of the mediastinum. The esophagus then enters the abdominal cavity through the esophageal hiatus of the diaphragm at approximately the level of the T10 vertebra. Here, we can see the inferior portion of the esophagus that connects to the stomach.
The esophagus is supplied by the esophageal branches of the thoracic aorta. It's also supplied by the bronchial arteries and the ascending branches of the left gastric artery. The venous drainage is provided by small vessels returning blood to the azygos and hemiazygos veins and by the esophageal branches to the left gastric vein. The innervation of the esophagus is complex but it is mainly supplied by the vagus nerves and sympathetic trunks. The terminal end of the esophagus connects to the cardiac part of the stomach forming the esophageal-stomach junction. Here, the stratified squamous epithelium of the esophagus abruptly changes into simple columnar mucus-secreting gastric epithelium of the stomach.
In the next image, we can see the internal structure of the stomach. We can see the rugae that I mentioned earlier and we can see the cardia highlighted in green. The fundus is the most superior dome-like part of the stomach formed by the upper curvature. It lies in the upper left hypochondriac region of the abdomen just beneath the diaphragm. It is filled with gas and is separated from the cardia by the cardiac incisura or the cardiac notch. The cardiac notch is a groove you can see here. The gastric glands and the mucosa of the fundus of the stomach also contain mucous neck cells, parietal cells, chief cells and enteroendocrine cells.
Again, we can see the inner structure of the stomach and we can see the fundus highlighted in green here. The body of the stomach or the gastric body is the largest part of the stomach. We can see it here highlighted in green. It forms the lesser curvature superiorly and the greater curvature inferiorly. This central portion of the stomach is situated between the fundus and cardiac part of the stomach and the pylorus.
The next portion of the stomach I'm going to cover is the pyloric part. The pyloric antrum is the initial section of the pyloric part of the stomach. It is situated near the inferior margin of the stomach and is proximal to the pyloric sphincter. The pyloric part of the stomach starts at the angular notch here. During peristalsis, it may temporarily be partially or completely closed off from the rest of the stomach. The most distal segment of the pyloric part of the stomach is the pyloric canal. The pyloric canal is approximately 2 to 3 centimeters long and is situated between the pyloric antrum and the pyloric orifice. It is the terminal part of the stomach.
In the next illustration, we can see the pyloric sphincter here highlighted in green. It is a band of smooth muscle that controls the flow of partially digested food from the stomach into the small intestine.
Now that we have covered the different parts of the stomach, let's move on and cover some of the layers of this organ. As I've mentioned before, the stomach is composed of three layers. The innermost layer is the mucosa followed by a submucosa and, most externally, a muscular layer. The muscular layer consists of three layers. A longitudinal layer, a circular layer and an oblique muscle layer. The longitudinal layer of the muscular coat is the outermost muscular layer. We can see that these muscle fibers are situated on the greater and lesser curvatures of the stomach.
Now we flit to the longitudinal muscle layer, let's move on to the circular muscle layer. It is the middle muscular layer of the stomach and consists of circular muscle fibers. This layer of the circular muscle is concentric to the longitudinal axis of the stomach and forms the thick muscular layer at the pylorus which we know as the pyloric sphincter. The oblique layer of muscle fibers of the stomach is the innermost layer of the muscular layer and is responsible for creating the motion that churns and breaks down food in the stomach. It performs more forceful contractions of the fundus of the stomach. In this image, we can see the oblique layer highlighted in green.
Before we move on to the next part of this tutorial, I just want to show you that the longitudinal layer of muscle fibers continues into the duodenum, the next part of the GI tract. Here in the illustration, you can see this longitudinal layer of muscle fibers which plays a crucial role in peristaltic movements.
So, now, let's open up the stomach and see some of the internal features of this organ. The first part that we see here is the gastric canal which is found along the lesser curvature of the stomach and the body of the stomach. It is a channel formed by the longitudinal mucosal folds. These gastric folds which I've mentioned in previous slides are also known as gastric rugae. They are formed by the mucosa of the stomach. They provide the stomach with an increased surface area for nutrient absorption. These mucosal folds stretch when food enters the stomach. This means the stomach is able to increase in volume without increasing the pressure inside the stomach.
Another structure we can see when we look at the open stomach is the pyloric orifice. This is located at the end of the pyloric canal and marks the junction between the stomach and the duodenum. The pyloric canal is surrounded by the pyloric sphincter muscle. The pyloric sphincter marks the end of the stomach and the entrance to the duodenum. This sphincter is a strong band of circular muscle that reinforces the end of the stomach.
Before we finish this tutorial, let's look at this structure highlighted in green here. It is not part of the stomach but is closely related to it. It is the superior part of the duodenum or the first part of the duodenum. The duodenum is a C-shaped loop that makes up the proximal part of the small intestine. It is adjacent to the head of the pancreas and is retroperitoneal except for the very first portion of it.
The duodenum is part of the small intestine and connects the stomach to the jejunum. There are four anatomical parts to it: the superior section, the descending section, the horizontal section and the ascending section. In this image, we see the superior section of the duodenum. It contains a functional dilation known as the ampulla. The duodenum receives partially digested food from the stomach and plays a vital role in the chemical digestion of food. This prepares the bolus of food for absorption further on in the small intestine.
Now that you just completed this video tutorial, then it’s time for you to continue your learning experience by testing and also applying your knowledge. There are three ways you can do so here at Kenhub. The first one is by clicking on our “start training” button, the second one is by browsing through our related articles library, and the third one is by checking out our atlas.
Now, good luck everyone, and I will see you next time.