What Is Kidney Disease? Acute Kidney Injury vs. Chronic Kidney Disease

What Do the Kidneys Do?

Adults have two kidneys, each about the size of a fist, located on either side of the spine below the ribs. Each kidney contains roughly one million "nephrons," each made of a glomerulus (the filter) and a tubule (which reabsorbs and fine-tunes). Together the kidneys filter about 180 liters of blood a day, producing only 1–2 liters of urine.

Key functions:

When the kidneys fail, these functions break down one by one: waste accumulates (uremia), fluid overload, high potassium, metabolic acidosis, hypertension, anemia and bone disease.

Two Major Types of Kidney Disease

Medicine divides kidney dysfunction into two categories based on how fast it develops and how long it lasts.

Acute Kidney Injury (AKI)

Definition: a sudden decline in kidney function over hours to days. Under KDIGO criteria, any one of the following qualifies:

• Serum creatinine rises ≥ 0.3 mg/dL within 48 hours
• Creatinine rises to ≥ 1.5 × baseline within 7 days
• Urine output < 0.5 mL/kg/hour for more than 6 hours

Common causes, grouped by location:

1. Prerenal — reduced blood flow to the kidneys: dehydration, major bleeding, heart failure, septic shock
2. Intrinsic — damage within the kidney: acute tubular necrosis, acute glomerulonephritis, drug toxicity (contrast dye, certain antibiotics, NSAIDs), rhabdomyolysis
3. Postrenal — urinary obstruction: stones, enlarged prostate, tumors

Key point: if the cause is found and removed early, kidney function can often recover partially or fully. Severe AKI may need temporary dialysis, and a history of AKI raises the long-term risk of developing CKD.

Chronic Kidney Disease (CKD)

Definition (KDIGO 2024): abnormalities of kidney structure or function persisting for more than 3 months, with implications for health. Any one of the following qualifies:

• Glomerular filtration rate (GFR) < 60 mL/min/1.73 m²
• Markers of kidney damage: albuminuria (ACR ≥ 30 mg/g), abnormal urine sediment, electrolyte disorders, imaging or biopsy abnormalities, or a history of kidney transplant

Key point: progression is slow and early stages are nearly symptom-free. Once nephrons scar, they do not regenerate — the goal of treatment is to slow progression, not to cure.

Acute vs. Chronic at a Glance

The two can overlap. A CKD patient who becomes dehydrated or takes a nephrotoxic drug can easily suffer an acute injury on top of chronic disease ("acute-on-chronic") — a scenario kidney patients should actively avoid.

Why Are There No Symptoms Early On?

The kidneys have enormous reserve capacity. Even with half their function gone, the remaining nephrons work harder and blood tests may still hover near normal. By the time swelling, fatigue, nausea or increased nighttime urination appear, function has often dropped below 30%.

That is why regular screening — blood creatinine to estimate GFR, and a urine albumin test — matters so much for high-risk groups: people with diabetes, hypertension, cardiovascular disease, a family history, long-term medication use, or age over 65.

The Situation in Taiwan

• CKD prevalence is about 12% — an estimated 2 million people — yet fewer than one in ten know they have it
• More than 90,000 people receive dialysis; Taiwan's per-capita dialysis prevalence has long been among the highest in the world
• The good news: according to the 2023 Taiwan Renal Data System Report, new dialysis starts have declined since their 2019 peak, and the average age at dialysis initiation has risen from about 58 to 67.8, reflecting effective early intervention

Key Takeaways

1. Kidneys do far more than make urine — they manage electrolytes, acid–base balance, blood pressure, red-cell production and bone health
2. AKI is fast and often reversible; CKD is slow and irreversible but can be slowed
3. CKD is defined by abnormalities lasting more than 3 months
4. Early CKD has no symptoms; high-risk individuals need regular blood and urine tests

References

• KDIGO. 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int. 2024.
• KDIGO. Clinical Practice Guideline for Acute Kidney Injury. 2012.
• NIDDK (US National Institute of Diabetes and Digestive and Kidney Diseases). Your Kidneys & How They Work.
• Taiwan Society of Nephrology. 2023 Taiwan Renal Data System Annual Report.
• Taiwan Health Promotion Administration. CKD prevention resources.

This article summarizes publicly available medical information for educational purposes only and is not medical advice. For any questions about your health, illness or physical care, please seek help through professional medical channels and consult your physician.