Normal Kidney Function Explained
What Do the Kidneys Do?
It sounds like a simple question, but the answer is more complex than many people realise.
Most people think the kidneys only remove waste from the body. While this is important, it is just one of seven essential kidney functions. Focusing only on blood creatinine or eGFR misses the bigger picture.
Healthy kidneys support many systems in the body, often without causing any symptoms.
Why Normal Kidney Function Matters
When both kidneys are working normally:
- Blood and urine tests stay within the normal range
- You feel well
- There are no symptoms of chronic kidney disease (CKD)
Early CKD (Stages 1–3B) often causes no symptoms, which is why CKD is known as a ‘silent disease’. Blood and urine tests are essential for early detection.
The 7 Normal Functions of the Kidneys
1. Waste Removal
The kidneys remove waste products from the blood and excrete them in urine. These include urea and creatinine.
When waste removal is normal:
- You feel clear-headed
- There is no itching, nausea, or ‘brain fog’
Normal blood tests:
- Creatinine: 60–120 µmol/L
- GFR: 90–120 mL/min
- 45–60 mL/min is normal for people with a single kidney
- Urea: 3–7 mmol/L
Most people with mildly reduced GFR do not progress to kidney failure.
2. Water and Salt Balance
Healthy kidneys remove excess water and salts, including sodium and potassium.
This prevents:
- Leg swelling (oedema)
- Fluid in the lungs
Normal salts blood tests:
- Sodium (Na): 135–145 mmol/L
- Potassium (K): 3.5–5.3 mmol/L
Potassium may rise in more advanced CKD (CKD4-5).
3. Acid Removal
The kidneys help maintain the body’s acid–base balance.
When this function is normal:
- Appetite is normal
- Muscles and bones function properly
Normal bicarbonate blood test:
-
Bicarbonate: 22–28 mmol/L
Too much acid is more common in advanced CKD.
4. Renin Production (Blood Pressure Control)
The kidneys produce renin, a hormone that helps regulate blood pressure.
Normal kidney function usually means:
-
Blood pressure ≤130/80 mmHg
High blood pressure often causes no symptoms, even when abnormal.
5. Erythropoietin (EPO) Production
The kidneys produce erythropoietin (EPO), which stimulates red blood cell production.
When this function is normal:
- You have good energy levels
- You are not anaemic (Hb < 110 g/L in a woman, < 130 g/L in a man)
Normal haemoglobin (Hb):
- Women: 110–150 g/L
- Men: 130–170 g/L
6. Vitamin D Activation
The kidneys convert vitamin D into its active form, which is essential for bone health.
Normal kidney function helps maintain:
- Strong bones
- Normal calcium and phosphate levels
Normal bone blood tests:
- Calcium: 2.2–2.6 mmol/L
- Phosphate: 0.8–1.4 mmol/L
- Parathyroid hormone (PTH): <5 pmol/L
7. Prostaglandin Production
The kidneys also produce prostaglandins, which have complex effects on:
- Blood flow
- Kidney protection
- Salt handling
There are no routine blood tests or symptoms that directly measure this function.
Other Signs of Normal Kidney Function
Renal Ultrasound
A normal scan shows:
- Two kidneys (or one if born with one)
- Normal shape and structure
- Size around 12 × 6 × 3 cm
- Kidneys under 10 cm are considered small
Urine Albumin–Creatinine Ratio (uACR)
This urine test measures protein leakage.
- Normal uACR: ≤3 mg/mmol
- Higher levels suggest kidney damage, even if blood tests are normal
Summary
Normal kidney function involves much more than a normal creatinine or eGFR.
Healthy kidneys:
- Perform seven vital functions
- Maintain normal blood pressure, blood counts, and bone health
- Keep blood and urine tests within the normal range
- Cause no symptoms (as often does milder CKD; CKD1-3B).
Understanding these functions helps explain why kidney disease can be silent—and why regular testing matters.
