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Your body isn’t one chemical environment; it’s dozens. In one place, a fluid is acidic enough to dissolve metal. A few centimetres away, another fluid is alkaline enough to neutralise it. Each compartment has a pH suited to the job it does, and your body works hard to keep each one in range.
Here’s a tour, from the most acidic to the most alkaline, with the reason behind each number.
The pH map of the body
| Location | Typical pH | Why |
|---|---|---|
| Stomach (gastric juice) | 1.5–3.5 | kills microbes, unfolds proteins, activates pepsin |
| Inside lysosomes | 4.5–5.0 | digestive enzymes inside cells work best in acid |
| Vagina | 3.8–4.5 | lactic acid from beneficial bacteria discourages infections |
| Skin surface | 4.5–5.5 | the “acid mantle” protects against some microbes |
| Urine | 4.5–8.0 (usually 5.5–6.5) | kidneys excrete excess acid or base |
| Sweat | 4.5–7.0 | varies with sweat rate and region |
| Saliva | 6.2–7.6 | protects teeth; starts starch digestion |
| Inside most cells (cytoplasm) | about 7.2 | optimum for most cellular enzymes |
| Cerebrospinal fluid | about 7.3 | tightly controlled for brain function |
| Blood (arterial) | 7.35–7.45 | kept very stable by buffers, lungs and kidneys |
| Bile | 7.0–8.0 | helps neutralise stomach acid entering the intestine |
| Small intestine | 6–7.4 (rising along its length) | suits pancreatic enzymes |
| Pancreatic juice | 7.8–8.4 | rich in hydrogencarbonate to neutralise stomach acid |
| Inside mitochondria (matrix) | about 7.8 | proton gradient drives ATP production |
The stomach: pH 1.5–3.5
The stomach lining secretes hydrochloric acid at a concentration of roughly 0.1 mol/dm³ when you eat. The acid does three jobs:
- Kills most bacteria and other microbes swallowed with food.
- Denatures (unfolds) proteins, making them easier to digest.
- Activates pepsin, the protein-digesting enzyme, which only works well at low pH.
How does the stomach avoid digesting itself? A thick layer of mucus, kept slightly alkaline by hydrogencarbonate secreted underneath it, protects the lining. When that protection fails, ulcers can form. Read more in stomach acid chemistry.
The small intestine: neutralising the acid
When partly digested food leaves the stomach, it’s strongly acidic. The pancreas releases juice rich in hydrogencarbonate ions (HCO₃⁻), and the liver sends bile. Together they neutralise the acid:
HCO₃⁻ + H⁺ → H₂O + CO₂
This raises the pH to near neutral, protecting the intestinal lining and allowing the pancreatic enzymes (amylase, lipase and proteases like trypsin), which work best around pH 7–8, to do their job.
This handover is a good example of why enzymes have optimum pH values. Pepsin works best near pH 2; trypsin near pH 8. Each enzyme’s shape depends on the charges on its amino acids, which depend on pH.
Blood: pH 7.35–7.45
Blood has one of the most tightly controlled pH values in the body. Below 7.35 is acidosis; above 7.45 is alkalosis. Values below about 6.8 or above about 7.8 are generally life-threatening.
The main defence is the carbonic acid–hydrogencarbonate buffer, supported by haemoglobin and plasma proteins, with the lungs adjusting CO₂ within minutes and the kidneys adjusting hydrogencarbonate over hours and days. Full details are in the blood buffer system.
Inside cells: pH about 7.2
The cytoplasm of most cells sits slightly below blood pH, around 7.2. Cells use membrane pumps to push H⁺ out and phosphate and protein buffers to hold the pH steady. Inside cells, some compartments are deliberately different:
- Lysosomes, the cell’s recycling centres, pump in protons to keep their interior at pH 4.5–5.0. Their digestive enzymes work best there, and if a lysosome leaks, the enzymes are much less active in the neutral cytoplasm, which is a built-in safety feature.
- Mitochondria pump protons out of their inner compartment. The resulting gradient, a difference of roughly 0.5–1 pH unit across the inner membrane together with an electrical potential, drives the enzyme ATP synthase to make ATP, the cell’s energy currency.
Skin: pH 4.5–5.5
Healthy skin is slightly acidic, thanks to fatty acids, lactic acid and amino acids from sweat and the skin’s own metabolism. This “acid mantle” helps:
- discourage growth of some harmful bacteria
- keep the outer layer of skin cells healthy and cohesive
- support the enzymes that maintain the skin barrier
Ordinary soap (pH 9–10) raises skin pH temporarily; healthy skin restores it within hours.
Urine: 4.5–8.0
Urine pH varies more than any other body fluid because the kidneys use it to dump excess acid or base. A high-protein diet tends to make urine more acidic; a diet rich in fruit and vegetables tends to make it more alkaline. This is often misunderstood: changes in urine pH show the kidneys doing their job of keeping blood pH constant, not that the blood itself has changed.
Urine pH matters medically. Some kidney stones form more easily in acidic urine (uric acid stones), others in alkaline urine (some calcium phosphate stones).
Saliva and teeth: pH 6.2–7.6
Saliva is close to neutral and contains hydrogencarbonate and phosphate buffers. After you eat sugar, mouth bacteria produce acids and plaque pH can fall below 5.5, the critical pH at which tooth enamel (a calcium phosphate mineral called hydroxyapatite) starts to dissolve. Saliva gradually neutralises this acid over 20–40 minutes, and enamel can remineralise.
This is why frequent snacking on sugary or acidic foods is harder on teeth than eating the same amount at mealtimes: the mouth spends more total time below pH 5.5.
How the body measures pH
Your body doesn’t just control pH; it constantly measures it. Specialised sensor cells called chemoreceptors in the brainstem and in the carotid and aortic bodies (small clusters of cells near major arteries) detect changes in the H⁺ concentration and CO₂ level of blood and cerebrospinal fluid. When they sense rising acidity, they signal the breathing centre to increase the rate and depth of breathing. This is why the urge to breathe after holding your breath comes mainly from rising CO₂, not falling oxygen.
When body pH goes wrong
| Condition | What happens to pH | Example causes |
|---|---|---|
| Acid reflux | stomach acid reaches the oesophagus (normally near neutral) | weakened valve between stomach and oesophagus |
| Metabolic acidosis | blood pH falls | diabetic ketoacidosis, kidney failure, severe diarrhoea |
| Respiratory acidosis | blood pH falls | lung disease, slow breathing |
| Respiratory alkalosis | blood pH rises | hyperventilation, high altitude |
| Tooth decay | plaque pH below 5.5 for long periods | frequent sugar intake |
| Bacterial vaginosis | vaginal pH rises above 4.5 | loss of lactic-acid-producing bacteria |
Common misconceptions
- “Food changes blood pH.” Food changes urine pH; blood pH stays in its narrow range in healthy people.
- “The whole body has one pH.” Different compartments differ by up to about six pH units.
- “Acidic foods make the body acidic.” Lemon juice is acidic in the glass but is metabolised completely; its effect on blood pH is negligible.
- “Stomach acid is bad.” It’s essential for digestion and protection. Problems arise only when it ends up in the wrong place.
Key takeaways
- pH in the body ranges from about 1.5 in the stomach to about 8.4 in pancreatic juice.
- Each compartment’s pH suits the enzymes and processes working there.
- Blood pH is held at 7.35–7.45 by buffers, the lungs and the kidneys; urine pH varies so blood doesn’t have to.
- Teeth start to dissolve below about pH 5.5.
- For the calculations behind these numbers, see the pH scale explained.
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