Functions of the Kidneys
The kidneys are vital organs that perform multiple essential functions including waste elimination, fluid and electrolyte regulation, acid-base balance maintenance, and hormonal production. 1
Primary Functions
Filtration and Waste Elimination: Kidneys filter blood to remove metabolic waste products and toxins, which are then excreted as urine 1
Fluid Balance Regulation: Kidneys maintain proper fluid balance by controlling water retention and excretion based on the body's needs 1, 2
Electrolyte Homeostasis: Kidneys regulate levels of critical electrolytes including sodium, potassium, calcium, and phosphorus 3
Acid-Base Balance: Kidneys help maintain proper pH by excreting hydrogen ions and reabsorbing bicarbonate 4
Hormonal Functions: Kidneys produce important hormones including:
- Erythropoietin (stimulates red blood cell production)
- Calcitriol (active form of vitamin D)
- Renin (regulates blood pressure through the renin-angiotensin-aldosterone system) 1
Kidney Structure and Function
Nephron: The functional unit of the kidney consisting of:
- Glomerulus: Site of blood filtration
- Tubule: Site of reabsorption and secretion that determines final urine composition 1
Filtration Process: Blood is filtered through the glomerular membrane, creating an ultrafiltrate that enters the tubular system 1
Reabsorption and Secretion: The tubular system selectively reabsorbs essential substances (water, electrolytes, glucose) and secretes additional waste products 1
Kidney Function Parameters
Glomerular Filtration Rate (GFR): The primary measure of kidney function, normally >90 ml/min/1.73m² 5
Classification of Kidney Function: Based on GFR categories:
- G1: Normal kidney function (GFR ≥90 ml/min/1.73m²)
- G2: Mild decrease (GFR 60-89 ml/min/1.73m²)
- G3A: Mild-moderate decrease (GFR 45-59 ml/min/1.73m²)
- G3B: Moderate-severe decrease (GFR 30-44 ml/min/1.73m²)
- G4: Severe decrease (GFR 15-29 ml/min/1.73m²)
- G5: Kidney failure (GFR <15 ml/min/1.73m²) 5
Electrolyte Regulation
Sodium Balance: Kidneys regulate sodium through fractional excretion, which increases as GFR decreases to maintain sodium balance until GFR falls below 15 ml/min 3
Potassium Balance: Kidneys maintain potassium homeostasis through:
- Filtration at the glomerulus
- Reabsorption in the proximal tubule
- Secretion in the distal tubule (regulated by aldosterone) 3
Calcium and Phosphate Regulation: Kidneys activate vitamin D (calcitriol) which enhances intestinal calcium absorption and regulates phosphate excretion 3, 1
Acid-Base Regulation
Bicarbonate Reabsorption: Kidneys reabsorb filtered bicarbonate to maintain blood pH 4
Acid Excretion: Kidneys excrete hydrogen ions and generate new bicarbonate to compensate for metabolic acid production 4
Ammonia Production: Kidneys produce ammonia to buffer excess acid 4
Kidney Function in Disease States
Acute Kidney Injury (AKI): Sudden decline in kidney function characterized by:
- Increase in serum creatinine by 0.3 mg/dL within 48 hours
- Increase in serum creatinine to 1.5 times baseline within 7 days
- Urine output <0.5 mL/kg/hr for 6 hours 5
Chronic Kidney Disease (CKD): Progressive loss of kidney function over time, defined as:
- Abnormalities of kidney structure or function present for >3 months
- GFR <60 ml/min/1.73m² for >3 months 5
Compensatory Mechanisms in Reduced GFR:
- Activation of the renin-angiotensin-aldosterone system
- Increased sodium reabsorption
- Efferent arteriolar constriction to maintain filtration fraction 6
Clinical Implications of Kidney Dysfunction
Electrolyte Disturbances: As GFR declines, disorders of sodium, potassium, calcium, and phosphate become more common 7
Metabolic Acidosis: Occurs with GFR below 20 ml/min due to decreased acid excretion and bicarbonate generation 3
Fluid Overload: Results from impaired sodium and water excretion 3
Uremia: Accumulation of waste products leading to systemic symptoms when kidney function is severely impaired 5