BB003 – Stomach acid Basics

Maagzuur regulatie simpel
I found some interesting facts about the stomach after rereading the book ‘Medical Physiology’ (Boron & Boulpaep, 2nd edition). Since this is quite a complex topic, I divided it into two parts. The first part is, in my mind, the basic knowledge required to get a general overview.

 

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Figure 1: Protein digestion
In figure 1 I depicted the a schematic version of the anatomy of the stomach and protein digestion.  Food that has been chewed and swallowed can only leave the stomach if it is smaller than 2 mm. Fluids can therefore pass through the stomach almost immediately. Muscles in the stomach wall mix and churn the food, grinding it against itself to reduce the size. Combined with digestive enzymes for carbohydrates, fats and proteins, the food is sufficiently digested in a few hours and can pass into the small intestine.

Digestion of carbohydrates and fats starts in the mouth by mixing in enzymes to break carbohydrate chains (amylases) or fats into fatty acids (lipases). These enzymes remain functional in the stomach, with the addition of enzymes to break proteins down into small amino acid chains (peptidases). The digestive enzymes are made by cells in the lining of the stomach (epithelium). The stomach lining contains many alcoves (crypts) and is covered by a thin layer of gel or slime (mucus).

I use pepsin here as an example of a cool digestive enzyme, a peptidase that plays an important role in protein digestion in the stomach. As shown in figure 1, pepsin breaks proteins down into small pieces, called peptones. Pepsin, unlike other types of peptidases, can break the middle of the peptide chain. This allows pepsin to rapidly break down any kind of protein it encounters.
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Figure 2: Pepsinogen and pepsin
However, pepsin is not made as such. Chief cells in the stomach lining make pepsinogen, a precursor of pepsin, and release it to the stomach content. Pepsinogen only becomes active if the pH is lower than 3.5. On the inside of cells, the pH is around 7, so pepsinogen remains inactive until it moves from the cell to into the stomach itself. Parietal cells produce the acid to lower the pH of the stomach. The acidic environment created by the parietal cells transforms pepsinogen into pepsin. The more acidic the environment, the faster the transformation and the faster pepsin can break down proteins.

 

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Figure 3: Pepsin inactivation
So pepsin in an acidic environment is very nice for digesting proteins in the stomach, but can be dangerous for the cells in the stomach lining. Pepsin is just as good at digesting cellular peptides and a pH of 2 or 3 can cause some severe acid burns. This is what the mucus layer protects from. Mucus is made by mucous cells at the entrance of a crypt, allowing the mucus to cover the crypt entrance and protect the underlying cells. The mucus layer can hardly be penetrated by the acid and and has a high pH. Even though pepsin works really well in an acidic environment, it stops working completely when the pH increases. The protection this alkaline mucus layer produces ensures that the stomach does not digest itself.
A small detail: the chief and parietal cells are located in the middle of the crypt. So the mucus layer lies between the crypt content and the stomach proper. How does the acid and pepsinogen get through the mucus? The book said: it is transported in a pressurized flow through the middle of the crypt. I cannot think of a better explaination myself 😉
Protein digestion by the stomach is pretty simple compared to my previous post. Of course, it gets more complex as you get further into it. There are a number of regulatory systems involved with digestion in general. I will discuss some in the next part, for enthusiasts. So I will cover the basics of stomach acid regulation in the next part. See you there 🙂

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