Hello people, this is my first ever blog post and I must say I am very excited. As previously mentioned this is a revision blog, therefore the anatomy part might be a little dry, but if it helps you then we both win! In future posts I’ll try to throw some humour in (the odd medical pun or a rare post about cats), so that you’ll still want to read about this topic and not gouge your eyes out with a pencil.
Currently I am studying the gastrointestinal (GI) tract, therefore I chose today’s post to be on the stomach, as it is one of the most important structures of the body that performs a vital function. With the aid of my own diagrams I’ll go over the anatomy, histology and physiology, although I might leave physiology for another post as it is quite complex!
What makes a good stomach?
The stomach is essentially a J-shaped enlargement of the GI tract that connects oesophagus to the duodenum (where most of digestion takes place). Duodenum also means “12”, because it is roughly as long as the width of 12 fingers. As the picture below shows, there are different anatomical parts to the stomach that I need to know (who knows, maybe you need to know it too).
The oesophagus feeds into the stomach via the lower oesophageal sphincter (LOS or LES) into the cardia. Just superiorly (situated above) to the left of cardia you’ll hopefully find the fundus. The body is the large central portion of the stomach, and the pyloric part is divided into essentially 3 sections- pyloric antrum, pyloric canal and the pylorus (which contains the pyloric sphincter). I’ll explain the physiology of each part in another post.
The main functions of the stomach are to:
- Mix saliva, food, and gastric juice to form chyme (not the most pleasant of words, I know)
- Serve as a reservoir for food before releasing it into the small intestine
- Secrete gastric juice, which is:
- HCl (hydrochloric acid) – kills microbes, denatures protein as well as provides a very acidic environment (~2pH) so that pepsinogen can be activated
- Pepsinogen – is converted to the enzyme pepsin by the pH of the HCl. Pepsin is important for protein digestion.
- Intrinsic factor – aids with vitamin B12 absorption
- Gastric lipase – triglyceride digestion
- Secrete a hormone called gastrin into the blood (yes, even your stomach is hormonal)
For those who are wondering, what is chyme? It’s basically a pulpy acidic liquid that was once your delicious food but is now mixed with gastric juices. What is also important to notice is that when the stomach is empty, the mucosa (lining) lies in large folds called rugae (no, it’s not the music genre). These folds also help your stomach to expand (probably too much) during Christmas meals.
Layer it out!
I won’t go into too much histological detail, however I’ll mention a few things. In the picture above I show all the different layers of the stomach wall, but what is very interesting (at least for me) is that it has 3 layers of smooth muscle – oblique, circular and longitudinal. These are used in the mechanical digestion of food, which are peristaltic movements.
Peristalsis is prevalent throughout the digestive system, though it is often misunderstood. It’s the progression of coordinated smooth muscle contractions and relaxations that move food along the GI tract. In the stomach, these are called mixing waves and they occur every 15-25 seconds, pushing the food towards the pylorus. As the food reaches the pylorus about 2-3 mL of chyme is pushed out of the pyloric sphincter and the rest is pushed back up to the fundus for further mixing. This is called gastric emptying and it can take 2-4 hours for the stomach to empty, depending on how much and what you ate.
I think this is a long enough post for today, so I’ll spare you the physiology til’ next time!