Enzymes and cells work only within a narrow pH range, so the body keeps blood pH between 7.35 and 7.45. Metabolism makes acid all the time, and three systems work together to remove it.
Buffers
Buffers act within seconds by binding or releasing hydrogen ions. The main one in blood is bicarbonate, with help from proteins such as hemoglobin.
The lungs
Carbon dioxide forms acid in the blood. Breathing faster blows off CO2 and raises pH, and breathing slower keeps CO2 and lowers pH, with effects within minutes.
The kidneys
The kidneys excrete hydrogen ions and make or reclaim bicarbonate. They are the slowest system, taking hours to days, but the most powerful.
Compensation
When one system causes a problem, the other tries to correct it. A metabolic acidosis makes the patient breathe fast, and a chronic respiratory acidosis makes the kidneys keep bicarbonate.
| Value | Normal | Controlled by |
|---|---|---|
| pH | 7.35 to 7.45 | Balance of CO2 and bicarbonate |
| PaCO2 | 35 to 45 mmHg | Lungs |
| HCO3- | 22 to 26 mmol/L | Kidneys |
At the bedside
Read a blood gas in order: pH first, then PaCO2 for the lungs, then bicarbonate for the kidneys. Capnography gives a breath-by-breath view of the CO2 side.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).