What is normal human pH?
Normal human pH = 7.40 (7.35-7.45). A lower number indicates an acidic tendency, a higher number indicates an alkalotic tendency.
To maintain homeostasis, the human body employs many physiological adaptations. One of the most important is maintaining an acid-base balance. As we say, in the absence of disease, the pH of the human body ranges between 7.35 to 7.45, with the average at 7.40.
A pH at this level is ideal for many biological processes, one of the most important being blood oxygenation.
What is the pH scale?
The pH scale ranges from 0 to 14. The readings are based around a pH of 7, which is neutral, like pure water; and,
- A pH below 7 is acidic
- A pH higher than 7 is alkaline (or basic).
This scale might seem small, but each level is 10 times bigger than the next. For example, a pH of 9 is 10 times more alkaline than a pH of 8. A pH of 2 is 10 times more acidic than a pH of 3, and 100 times more acidic than a reading of 4.
Acidosis and alkalosis
A pH below 7.35 is called acidosis (i.e. blood is too acidic), and a pH above 7.45 is alkalosis (blood too alkaline). Due to the importance of sustaining a pH level in the necessary narrow range, the human body contains compensatory mechanisms.
The human body experiences 4 main types of acid-based disorders: metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis. If one of these conditions occurs, the body induces a counterbalance in the form of an opposite condition.
For example, if a patient is experiencing metabolic acidosis (e.g. due to acute kidney injury (AKI)), their body attempts to induce respiratory alkalosis to compensate (by breathing faster; this is called Kussmaul’s breathing). It is rare for the compensation to make the pH completely normal at 7.4.
In the case of the patient with AKI, we might say that an arterial blood gas (ABG; main way of testing pH done in hospitals) shows a ‘metabolic acidosis with a partial respiratory compensation’.
The causes (and treatments) of these 4 disorders is complex and beyond the scope of this article (but explained more in the reference below).
Other resources
A normal human being
Review article on acid-base balance: Hopkins, 2022
