pH scale — why is stomach acid pH 1-2 and blood pH 7.4

pH stands for “power of Hydrogen.” It measures how acidic or basic a solution is, on a scale from 0 to 14:

• pH < 7 → Acidic (more $\text{H}^+$ ions)
• pH = 7 → Neutral (pure water)
• pH > 7 → Basic/Alkaline (more $\text{OH}^-$ ions)

The lower the pH, the stronger the acid. Each step on the pH scale represents a 10-fold change in $\text{H}^+$ ion concentration. So pH 1 is 10 times more acidic than pH 2.

The stomach secretes hydrochloric acid (HCl) as part of gastric juice. This extremely low pH serves two purposes:

1. Kills bacteria in the food we eat — most germs cannot survive in such strong acid
2. Activates pepsin — the enzyme that digests proteins works only in an acidic environment

The stomach lining has a protective mucus layer that prevents the acid from digesting the stomach itself.

Blood is slightly basic (pH 7.35 to 7.45). This precise pH is essential because:

• Enzymes in the body work only within a very narrow pH range
• Haemoglobin’s ability to carry oxygen depends on blood pH
• Even a small change (below 7.0 or above 7.8) can be life-threatening

The body maintains this pH using buffer systems — chemicals that resist pH changes by absorbing excess $\text{H}^+$ or $\text{OH}^-$ ions.

• Stomach: If acid production increases beyond normal, it causes acidity (heartburn). Antacids like Milk of Magnesia ($\text{Mg(OH)}_2$) are used to neutralise the excess acid.
• Blood: If blood pH drops below 7.35, it’s called acidosis (can cause coma). If it rises above 7.45, it’s called alkalosis (can cause muscle spasms). Both are medical emergencies.