When we eat food, it’s first chewed up in the mouth and mixes with saliva to form a mushy mass called a bolus, and then it travels down a muscular tube called the esophagus.
Next, it enters the stomach through the lower esophageal sphincter, which is a ring of muscle surrounding an opening.
The stomach has three main parts: the fundus, the body and the antrum.
The body is further divided into a proximal portion and a distal portion.
The fundus, and proximal body make up the orad region, or top half of the stomach.
The distal body and the antrum make up the caudad region or bottom half of the stomach.
Once the food bolus is partially digested in the stomach, it’s called chyme and it passes through the pyloric sphincter and into the duodenum, which is the first part of the small intestine.
The pancreas is connected to the duodenum through the pancreatic duct, and it secretes many digestive enzymes to further break down food.
All these part play a role in gastric motility which is the contraction of the stomach to break up food and move it into the intestine.
Like other parts of the gastrointestinal tract, the stomach has 4 layers.
Starting from the innermost layer to the outermost layer, these layers are the mucosa, submucosa, muscularis externa, and serosa.
The mucosa and muscularis externa layers are modified in the stomach.
The mucosa contains various glands filled with different cells that secrete the components of gastric juice.
In the body of the stomach, there are parietal cells that secrete hydrochloric acid or HCl, a strong acid that helps to break down protein, and chief cells that secrete pepsinogen, an inactive enzyme.
When pepsinogen is exposed to HCl it activates to becomes pepsin - an enzyme that helps break down proteins.
In the antrum, there are mucous cells which secrete mucus which protects the stomach lining from the acidic environment. There are also G cells which secrete a hormone called gastrin.
Unlike the others secretions, gastrin is not secreted into the stomach lumen but instead, it’s released into the bloodstream where it can reach the basolateral surface of other stomach cells - which is the cell surface that faces the blood.
Gastrin increases the HCl secretion of the parietal cells, it increases pepsinogen secretion by the chief cells, and it also increases the contraction of the stomach muscles.
The next layer is the submucosa is mainly made up of connective tissue, blood vessels, and nerves - which supply the mucosa.
The nerves in the submucosa branch out and connect with one another forming the submucosal plexus of nerves which actually stretch throughout the gastrointestinal tract.
In the stomach, the submucosal plexus regulates gastric secretions of various glands and controls blood flow to the stomach.
Next, there’s the muscularis externa layer of the stomach which contains three layers of smooth muscle that involuntarily contract in waves called peristalsis. These waves of peristalsis help the stomach mix, digest and empty the food that it has received.
The three muscular layers include an outer longitudinal layer, a middle circular layer, and inner oblique layer which is unique to the stomach. This extra layer of inner oblique muscles provide more forceful contraction in the stomach.
In addition, the thickness of the muscular layers increases as you go from the orad region to the caudad region and that helps push food towards and past the pyloric sphincter.