You have roughly a million filtering units per kidney, and together they process your entire blood volume dozens of times a day. They also quietly run your blood pressure, your red blood cell supply, your acid-base balance and your bones. And they can lose most of their function before producing a single symptom you'd notice.
What kidneys actually do
Filtration is the headline, but it's only one of several jobs:
- Filter waste — urea from protein breakdown, creatinine from muscle, drug metabolites, excess electrolytes.
- Balance fluid and electrolytes — sodium, potassium, calcium, phosphate. Getting potassium wrong is dangerous to the heart, which is why kidney function constrains what you can eat and which medicines are safe.
- Control blood pressure — through the renin-angiotensin-aldosterone system, plus direct control of salt and water. The kidneys are a long-term blood pressure thermostat; that's why kidney disease and hypertension feed each other.
- Make erythropoietin (EPO) — the hormone that tells bone marrow to build red blood cells. Failing kidneys mean anemia, which is why people with advanced kidney disease feel exhausted.
- Activate vitamin D — converting it to its active form for calcium absorption. Impaired kidneys weaken bone over time.
- Regulate acid-base — excreting acid and regenerating bicarbonate to keep blood pH in a narrow band.
The nephron: how filtration works
Each nephron has a glomerulus — a tangled ball of capillaries that filters under pressure — connected to a tubule that reclaims what the body needs and sends the rest to the bladder.
The glomerulus filters small molecules but holds back cells and most protein. The tubule then does enormous reabsorption work: of the volume filtered, well over 99% is taken back. That's why urine output (~1–2 L/day) is tiny compared with the volume actually processed.
This is also why "filtering" is a misleading word. Kidneys aren't a sieve; they're a selective recycling plant with a chemical lab attached.
Reading the numbers: eGFR, creatinine, albumin
- Creatinine — a muscle waste product cleared by the kidneys. Higher blood levels mean lower clearance, but it varies with muscle mass, age, sex and diet, so it's a raw input, not a verdict.
- eGFR (estimated glomerular filtration rate) — calculated from creatinine plus age and sex. It's the standard measure of kidney function, expressed in mL/min/1.73m². Above ~90 is normal. Below 60 for more than three months is the usual threshold for chronic kidney disease.
- UACR (urine albumin-to-creatinine ratio) — detects protein leaking into urine, which signals damage to the filtration barrier. This often becomes abnormal before eGFR drops, which is why both are measured.
A one-off abnormal eGFR isn't a diagnosis. Dehydration, a high-protein meal, intense exercise and some medications can shift it temporarily. Persistence over three months is what makes it chronic kidney disease.
Why kidney disease is so quiet
The kidneys have enormous reserve. You can lose a substantial fraction of nephrons and still have near-normal blood tests, because remaining nephrons compensate by filtering harder. Symptoms tend to appear late:
- Foamy urine (protein) or visible blood
- Swelling in ankles, feet or around the eyes
- Fatigue and shortness of breath (anemia, fluid)
- Reduced urine output
- Persistent nausea, metallic taste, itching in advanced stages
By the time several of these cluster, significant function is gone. This asymmetry is the whole argument for screening people with risk factors.
Who should be checked regularly
- Diabetes or prediabetes — the single leading cause of CKD
- High blood pressure — both cause and consequence
- Cardiovascular disease — they share the same vascular risk terrain
- Family history of kidney disease or polycystic kidney disease
- Age over 60
- Long-term use of NSAIDs (ibuprofen, naproxen), or certain other medications
- Prior acute kidney injury — one episode raises long-term risk substantially
Screening is unglamorous: a blood creatinine for eGFR, a urine sample for albumin. Both are cheap, and abnormal results prompt a repeat in three months.
Protecting them
- Control blood pressure and blood sugar — these two dominate outcomes
- Don't overuse NSAIDs — regular high-dose use reduces blood flow to the glomeruli
- Stay hydrated, especially during illness, heat, or heavy exercise
- Moderate sodium — it raises blood pressure and increases filtration load
- Don't smoke — smoking accelerates kidney decline independently
- Review medications if you have reduced function; several common drugs need dose adjustment
- Newer drug classes (SGLT2 inhibitors, and ACE inhibitors or ARBs where appropriate) substantially slow progression in the right patients — a major shift in kidney care
The takeaway
Your kidneys are doing chemistry you never think about, roughly sixty times your blood volume a day. They fail quietly, and late symptoms mean late diagnosis. A blood test and a urine test catch trouble while it's still manageable — particularly if you have diabetes, hypertension, or a family history.
This article is educational and not a substitute for medical advice. Speak with a clinician about your own results and medications.