pH in the Human Body

Introduction

Your body contains some of the most acidic and most tightly controlled liquids you will ever meet. The fluid in your stomach is strongly acidic, around pH 1 to 2, yet a few centimetres further along the gut the contents are close to neutral or slightly alkaline. Meanwhile your blood stays between pH 7.35 and 7.45 almost all the time. Acid–base chemistry is at work in every stage of digestion and in keeping you alive.

Core explanation

A map of pH in the body. Typical values are:

Location Typical pH --- --- Stomach contents 1–2 (up to about 3 after a meal) Skin surface about 5.5 Saliva about 6.5–7.5 Blood 7.35–7.45 Small intestine about 7–8

Stomach acid. Cells in the stomach lining secrete hydrochloric acid . This strong acid has several jobs: it kills most bacteria swallowed with food, it helps unfold proteins so they are easier to digest, and it provides the acidic conditions needed by the enzyme pepsin , which breaks proteins into smaller pieces. The stomach protects itself with a thick layer of mucus and with hydrogencarbonate ions secreted into that mucus, which neutralise acid close to the stomach wall.

Enzymes and optimum pH. Enzymes are proteins whose shape depends on attractions between charged and polar groups. Changing the H⁺ concentration changes these charges, altering the shape of the active site. Each enzyme therefore has an optimum pH at which it works fastest:

- salivary amylase (starch digestion in the mouth): about pH 7 - pepsin (protein digestion in the stomach): about pH 2 - trypsin and pancreatic enzymes (small intestine): about pH 8

Salivary amylase stops working once food reaches the acidic stomach; pepsin in turn stops working in the small intestine.

Neutralising the stomach contents. When partly digested food leaves the stomach, the pancreas releases a juice rich in hydrogencarbonate ions , and bile from the liver is also alkaline. These neutralise the acid, raising the pH so that intestinal enzymes can work:

HCO₃⁻(aq) + H⁺(aq) → H₂O(l) + CO₂(g)

Antacids. Indigestion and heartburn can occur when too much acid is produced or when stomach acid rises into the oesophagus. Antacids are bases that neutralise excess acid, for example:

Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l)

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Weak, insoluble bases are chosen rather than strong alkalis such as sodium hydroxide, which would be corrosive and dangerous to swallow.

Blood pH. Blood must stay between about 7.35 and 7.45. It is controlled by buffers , mainly the carbonic acid–hydrogencarbonate system, together with the lungs (which remove carbon dioxide) and the kidneys (which remove excess acid or hydrogencarbonate). If blood pH falls below the normal range the condition is called acidosis; if it rises above it, alkalosis. Both disrupt enzymes and nerves and can be life-threatening.

Step-by-step reasoning

To predict whether an enzyme will work in part of the gut:

1. Look up the enzyme's optimum pH. 2. Look up the pH of that region. 3. If they match, the enzyme works well. 4. If they differ greatly, the enzyme's shape changes and it works slowly or not at all.

Visual explanation

Picture the digestive tract as a coloured tube dipped in universal indicator: green-blue in the mouth, bright red in the stomach, then shifting to green and blue-green in the small intestine. Each colour zone has its own enzymes, each tuned to that shade.

Real-world analogy

Enzymes are like specialist workers who each need a certain room temperature to do their job. Move a worker into a room that is far too hot or cold and they stop working. For enzymes, the "room condition" that matters here is pH.

Real-world example

Heartburn remedies sold in pharmacies commonly contain calcium carbonate, magnesium hydroxide or aluminium hydroxide. Some combine a magnesium compound, which tends to have a laxative effect, with an aluminium compound, which tends to be constipating, so the side effects balance out.

Why?

Why does the stomach not digest itself? The mucus layer lining the stomach is a physical barrier, and the hydrogencarbonate ions it contains neutralise acid next to the cells. The lining cells are also replaced rapidly. When this protection fails, stomach ulcers can form.

Common misconception

"Stomach acid is harmful, so the best idea is to neutralise all of it." Stomach acid is essential: it kills microbes and allows pepsin to digest proteins. Antacids are meant only to neutralise excess acid for short-term relief.

Worked example

Question: A patient takes an antacid containing magnesium hydroxide. Write the equation for its reaction with stomach acid and explain why the stomach pH rises.

Reasoning: Magnesium hydroxide is a base; it reacts with hydrochloric acid in a neutralisation reaction, using up H⁺ ions.

Answer: Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l). Hydrogen ions are removed to form water, so their concentration falls and the pH rises.

Quick check

1. What is the normal pH range of human blood? Answer: About 7.35 to 7.45.

Exam focus

Know the pH of stomach acid (1–2) and blood (7.35–7.45), the acid present in the stomach (hydrochloric acid), and equations for antacids neutralising it. Explain optimum pH for enzymes in terms of changes to the shape of the active site.

Advanced insight

The carbonic acid–hydrogencarbonate buffer links breathing to blood chemistry: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. Rapid breathing removes CO₂, shifting the equilibrium to the left and raising blood pH; holding the breath does the opposite. The ratio of hydrogencarbonate to carbonic acid, about 20 to 1, sets the normal blood pH near 7.4.

Summary

The stomach secretes hydrochloric acid, giving pH 1–2, which kills microbes and suits pepsin. Pancreatic hydrogencarbonate neutralises the acid in the small intestine, where enzymes work best near pH 8. Each enzyme has an optimum pH. Antacids are weak bases that neutralise excess acid, and blood pH is held at 7.35–7.45 by buffers, lungs and kidneys.

Practice questions

1. Name the acid in the stomach and give two of its functions. Answer: Hydrochloric acid; it kills bacteria and provides the acidic conditions needed by pepsin to digest proteins. 2. Why does salivary amylase stop working in the stomach? Answer: Its optimum pH is about 7, and the strongly acidic stomach changes the shape of its active site. 3. Write the equation for calcium carbonate neutralising hydrochloric acid. Answer: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g) 4. Why are strong alkalis such as sodium hydroxide never used as antacids? Answer: They are corrosive and would damage the mouth, oesophagus and stomach; mild insoluble bases neutralise acid safely. 5. What ions neutralise stomach acid as food enters the small intestine? Answer: Hydrogencarbonate ions, HCO₃⁻, from pancreatic juice.