What is Normal Human pH?

What is Normal Human pH?

An arterial blood gas (ABG) being taken. This is how pH – and therefore acid-base (alklali) balance) – is determined.

Normal human pH refers mainly to the pH of blood, which it has to be kept within a very tight range for the body to work properly.

In other words, pH is how the acidity of the blood is described – the lower the pH, the more acidic is the blood.

This article is about what is normal human blood pH, and how it is controlled.

🔬 Normal human pH

• Normal blood pH = 7.35 – 7.45
  • pH < 7.35 – blood is ‘acidic’ (see below)
  • pH > 7.45 – blood is alkaline.
  • (pH 7 is neutral – i.e. neither acidic or alkaline)

Note. The pH scale is a ‘logarithmic’ scale (see end of article) from 1–14; where pH 1 is the strongest acid, and pH 14 is the strongest alkali – and pH 7 is neutral.

Hence, below 7 blood (or any fluid) is acidic, and above 7 is alkaline. So, in fact, all normal human blood levels above are slightly alkaline.

Even small changes outside this range can affect enzymes, oxygen delivery, and organ function.

Other body fluids have different pH values (for example, stomach acid is very acidic), but blood pH is the most tightly regulated.

⚙️ How the body controls pH

The body uses three main systems to keep pH stable.

1️⃣ Buffer systems (fast – seconds)

Buffers immediately respond to pH changes.

• Main buffer: bicarbonate buffer system
  • Uses carbonic acid (H₂CO₃) and bicarbonate (HCO₃⁻)
  • Works by neutralising excess acids or bases
• Other buffers:
  • Proteins (like haemoglobin)
  • Phosphate buffers

2️⃣ Lungs (minutes)

• The lungs control pH by adjusting carbon dioxide (CO₂) levels
• CO₂ forms acid in the blood
  • Faster breathing → more CO₂ removed → pH rises
  • Slower breathing → CO₂ retained → pH drops

3️⃣ Kidneys (hours to days)

• The kidneys provide long-term control, via:
  • Excreting hydrogen ions (H⁺) in urine
  • Reabsorbing or producing bicarbonate
• Adjust urine pH to help stabilise blood pH

🧠 Why this matters

If pH goes outside the normal range, it is called:

• • Acidosis (pH < 7.35): can cause fatigue, confusion, shortness of breath (Kussmaul’s breathing)
  • Alkalosis (pH > 7.45): can cause muscle twitching, dizziness, abnormal heart rhythms.

A blood pH of 7.2 or less or 7.5 or more is life-threatening, and requires immediate medical attention.

Quick Summary

• Normal blood pH: 7.35–7.45
• Controlled by: buffers (instant), lungs (minutes), kidneys (long-term)

But what is the pH scale?

The pH scale is a (base-10) logarithmic scale used to measure acidity or basicity, where each whole number change represents a tenfold change in hydrogen ion concentration.

It simplifies expressing the vast range of H⁺ ion (acid) concentrations; using smaller numbers (0-14), with lower values being acidic, higher values basic, and 7 being neutral.

This logarithmic nature makes it easier to compare acidity levels – e.g. pH 3 is 10 times more acidic than pH 4, and 100 times more acidic than pH 5.