Your Stomach Makes Strong Acid, So Why Doesn't It Dissolve Itself?
― The Guard Is a Slimy Layer 0.2 Millimetres Thick
Your stomach is filled with an acid strong enough to turn a tough lump of meat into mush in a few hours. Yet the stomach that makes this acid stays perfectly fine. It isn't because the stomach has thick walls. The answer lies in a very thin slimy layer that coats the inside of the stomach.
Your stomach growls with hunger. It is almost empty, yet it is already pouring out acid.
That acid is far stronger than lemon juice. If a good amount stayed on your finger, your skin would sting and become sore.
Your stomach pours more than a litre of it into itself every day. Even so, it is fine the next day, and the day after that. Why?
There are only two reasons
The inside of the stomach is coated with a layer of slimy mucus. The stomach wall itself lets alkali seep into this layer. As the acid moves through it, it is neutralised, and by the time it reaches the cells it is close to neutral.
The defence is not perfect. The cells lining the stomach are said to be replaced by new ones within a few days. The strategy is not to block everything, but to replace what gets damaged as soon as it does.
Let's take them one at a time. First, what does that slime actually do?
How far does the acid weaken inside the slime?
Mucus is like a net soaked with water. It is about 0.2 millimetres thick, roughly the width of two hairs. That looks thin, but what matters here is less the thickness than the fact that the water in it does not flow.
The cells of the stomach wall quietly pump bicarbonate ions into this mucus. These are alkaline and work like baking soda. Because the water in the mucus stays still, the alkali stays where it is put. When acid comes in from the outside, it is neutralised where the two meet.
Look at Figure 1. The stomach interior is on the left and the stomach wall is on the right. The acid tries to soak in toward the right, while the alkali seeps out toward the left. As a result of this tug-of-war, the acid's strength drops sharply within the mucus layer alone. The pH inside the stomach is about 1.5, but at the surface of the cells it has been measured at about 7, which is close to neutral.
Damage still happens, so it is rebuilt every day
The mucus layer is not perfect. Food rubbing past wears it away, and when the stomach moves, some of it peels off. So the stomach keeps making mucus as fast as it is worn away. The defence is not a savings account but a daily balancing act.
The cells themselves work the same way. The cells lining the stomach are said to be replaced by new ones within a few days. It is one of the fastest-turnover places in the body. Even if they are a little damaged, by the time those cells are spent, the next ones are already growing from below.
Put the other way round, trouble starts when this renewal cannot keep up. If mucus production drops, or more of it is worn away, the acid has longer to reach the wall. Many stomach problems are thought to come less from acid being too strong and more from the defence having grown thin.
The digestive enzyme the stomach makes cuts up proteins. That means it could cut the stomach itself. So the stomach sends this enzyme out in an inactive form, and it changes into its cutting form only when it meets the strong acid inside the stomach. It is safe where it is made and dangerous only where it is used.
Most of the bacteria that arrive with food are said to be unable to survive acid this strong. So it serves not only digestion but also as a checkpoint that stops them before they get into the body. This is why the stomach uses strong acid even though it needs a separate protective system to do so.
Summary
The stomach doesn't dissolve itself, and not because it is tough. It is because it keeps rebuilding a thin layer that neutralises the acid before it arrives. The defence works through speed, not hardness.
What protects the stomach is not a thick wall, but
a 0.2-millimetre slimy layer that is replaced every day.
There is a place in the mouth that likewise "dissolves every day and recovers every day." See also Teeth are the hardest thing in the body, so why do cavities dissolve them? On the balance between acid and alkali, the same bicarbonate ions also appear in Why can't you hold your breath for even a minute?
- Put the same amount of water into two clear glasses. In one, dissolve a teaspoon of potato starch and stir well until it thickens.
- Gently drop one drop of food colouring or soy sauce into each. Do not stir.
- After 1 minute, 5 minutes and 10 minutes, compare how far the colour has spread.
In the thickened glass, the colour is slow to reach the bottom. The mucus layer does the same thing to acid. In still water, things are no longer carried along by a current. They can only seep through slowly, and that buys time for neutralisation.
For those who want to know more ― terms, formulas and links to textbooksEach part is marked with its level, from middle-school science to university specialist courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Chemistry / Chemistry"
- High school+Advanced high-school material, or textbook sidebar content
- UniversityUniversity specialist courses (physiology, biochemistry), not taught in high school
- ResearchNot yet settled even at university level; topics researchers are still investigating
Middle schoolTerms: this phenomenon has names
- Hydrochloric acid: the acid the stomach produces. It is dissolved in water, split into hydrogen ions and chloride ions.
- Neutralisation: a reaction in which an acid and an alkali meet and cancel out each other's properties. It produces water and a salt.
- Bicarbonate ion: the alkaline ion found in baking soda. The stomach wall pumps it into the mucus.
- Diffusion: the movement by which things spread by themselves, without stirring, from where they are concentrated to where they are dilute.
Middle schoolHigh schoolChecking with formulas: how many times weaker does the acid get in the mucus layer?
Hearing that "pH rises from 1.5 to 7" makes it hard to picture how big a difference that is. That is because pH is a special scale in which every step changes the concentration by a factor of 10. Let's go back to the original formula and work out the actual concentrations.
| In symbols | [H⁺] ＝ 10 to the power (－pH) ／ n ＝ c × V |
| In words | Hydrogen ion concentration ＝ 10 to the power of minus pH; amount of acid ＝ concentration × volume |
| Where the formulas come from | The first is simply the definition of pH: pH is the common logarithm of the hydrogen ion concentration, with a minus sign. The second is the definition of concentration, which tells how many moles are in one litre. |
| [H⁺] | Hydrogen ion concentration. Unit: moles per litre |
| pH | A scale of acid strength. The smaller the number, the stronger the acid |
| c | Concentration. Unit: moles per litre |
| V | Volume. Unit: litres |
| n | Amount of substance. Unit: moles |
| pH inside the stomach (when empty) | Said to be about 1.5 |
| pH at the surface of the stomach wall | Said to be about 7.0 |
| 10 to the power −1.5 | Said to be about 0.032 (moles per litre) |
| 10 to the power −7 | 0.0000001 (moles per litre) |
| Amount of gastric juice produced per day | Said to be about 1.5 litres |
| Weight of 1 mole of sodium bicarbonate (baking soda) | 84 grams |
| Ratio of concentration inside the stomach to the wall surface | 0.032 ÷ 0.0000001 ＝ 320000 |
| Amount of acid produced per day (moles) | 0.032 × 1.5 ＝ 0.048 |
| Weight of baking soda that neutralises the same amount (grams) | 0.048 × 84 ＝ 4.032 |
The acid's concentration falls to one 320,000th while crossing the 0.2-millimetre layer. And a whole day's acid can be cancelled by just 4 grams of baking soda, about a teaspoon. For all its strength, the actual amount is surprisingly small.
High schoolHigh school+Why can such a thin layer cut the acid so far?
High schoolNeutralisation happens when acid and alkali meet in the same place. What matters here is that the water in the mucus does not move. If there were a flow, the alkali that seeped out would be washed away at once, and the area near the wall could not be kept neutral. The mucus holds the place for neutralisation in position.
High school+The balance between this "speed of seeping in" and the "speed of neutralising" sets the shape of the pH inside the layer. The seeping is diffusion and the cancelling is a neutralisation reaction, and depending on which is faster, the position where pH rises shifts toward the inside or the outside of the layer. The way the dotted line in Figure 1 rises partway through the layer is the result.
UniversityThe mucus–bicarbonate barrier and the buffer equation
The mechanism just described is called the mucus–bicarbonate barrier in physiology. The bicarbonate ions are made by an enzyme called carbonic anhydrase, which prepares them from carbon dioxide and water in an instant. The pH in the layer is set by the balance between carbonic acid and bicarbonate ions, so the standard way to treat it is the Henderson–Hasselbalch equation. The acid-secreting side uses a transporter called the proton pump, which is the target of many stomach medicines.
📖 Derivations of the equation and further reading: Carbonic anhydrase (Japanese Wikipedia) ／ Henderson–Hasselbalch equation (Japanese Wikipedia)
ResearchWhat is still not clearly understood
- How hard the layer is to measure Mucus is soft, and it shrinks or runs during specimen preparation. How thick it is in a living stomach is reported to vary quite a lot depending on the method of measurement.
- How bacteria get through Certain bacteria survive in this strong acid and live in the stomach. How they move through the mucus net is still being studied, with one leading idea being that they make the mucus itself softer.
- How drugs thin the defence Some painkillers are known to irritate the stomach, but whether the deciding factor is the amount of mucus, the amount of alkali or the blood flow is still being sorted out.
In other words, what this article says is also "an explanation as far as we know today." If your stomach keeps bothering you, please consult a medical institution rather than judging for yourself.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: acids and alkalis, neutralisation | Where acid is cancelled by alkali |
| High school | Basic Chemistry: pH and neutralisation; Chemistry: reaction rates | All of the calculations in "Checking with formulas" |
| High school+ | Chemistry: buffer solutions; Biology: digestion and enzymes | Where pH rises inside the layer |
| University | Physiology and biochemistry (mucus–bicarbonate barrier, transporters) | The section on carbonic anhydrase and the proton pump |
| Research | Gastroenterology and microbiology | Layer thickness, and how bacteria get through |
| ― | Links to daily life | Not thinking of stomach trouble only in terms of acid strength |
- The Nobel Prize in Physiology or Medicine 2005 (Nobel Foundation; discovery of the stomach bacterium and gastritis)
- Allen, A. & Flemström, G., Gastroduodenal mucus bicarbonate barrier: protection against acid and pepsin, American Journal of Physiology - Cell Physiology（2005）
- Hongo Toshinori et al. (eds.), 『標準生理学』 (Standard Physiology), Igaku-Shoin (chapter on the digestive system: gastric juice secretion and protection of the stomach lining)
- Carbonic anhydrase (炭酸脱水酵素, Japanese Wikipedia)
※This article is a popular-science explanation for general readers. The figures given are rough guides to help you understand the mechanism. For decisions about your health or treatment, follow the instructions of a medical professional.