What are nephrons and their function in the kidneys?

What Are Nephrons and Their Function in the Kidneys?

Nephrons are the basic structural and functional units of the kidney, with each kidney containing over 1 million of these microscopic filtering units that work together to maintain the body's internal environment through filtration, reabsorption, and secretion. 1, 2

Structural Components of the Nephron

The nephron consists of two main anatomical parts that work in sequence 3, 4:

• The glomerulus: This is the initial filtering component where blood plasma is filtered free of cells and large proteins, producing an ultrafiltrate composed of smaller circulating elements 1
• The tubule: This highly specialized structure collects the glomerular filtrate and modifies it through reabsorption (removing substances from tubular fluid) and secretion (adding substances to tubular fluid) to produce the final urine 3, 1

Core Functions of Nephrons

Filtration Process

The glomerulus filters blood based on size and charge, creating an ultrafiltrate that enters the tubular system 1. Total GFR (glomerular filtration rate) equals the number of nephrons multiplied by single nephron GFR, making nephron number a critical determinant of overall kidney function 5.

Reabsorption and Secretion

The tubular segments are highly specialized at different points along their length 1, 4:

• Proximal tubule: Primary site for reabsorption of filtered substances 3
• Loop of Henle: Contains the thick ascending limb (TAL) where specialized transport occurs 5
• Distal tubule and collecting duct: Fine-tune electrolyte balance and final urine composition 3

Physiological Roles Maintained by Nephrons

Nephrons collectively perform several vital functions 3, 1, 6:

• Waste elimination: Remove toxins produced by cellular metabolism and xenobiotic substances 3
• Fluid and electrolyte homeostasis: Regulate water balance and concentrations of sodium, potassium, calcium, phosphorus, and other ions 3, 2
• Acid-base balance: Maintain proper hydrogen ion concentration 1, 2
• Blood pressure regulation: Through sodium handling and the renin-angiotensin system 3, 6
• Hormone production: Kidneys produce erythropoietin (red blood cell production), calcitriol (calcium metabolism), and renin (blood pressure control) 3, 1

Clinical Significance of Nephron Number

Nephron endowment varies widely in the human population and has major implications for kidney disease susceptibility 6:

• Women typically have smaller kidneys and fewer total nephrons than men, yet paradoxically often have higher uromodulin levels (a marker of tubular function) 5
• Permanent loss of nephron number points toward chronic kidney disease and is not reversible, while reduced single nephron GFR may relate to altered hemodynamics and can potentially be reversed 5
• Progressive nephron loss leads to increased hemodynamic stress on surviving nephrons, with compensatory increases in single nephron GFR that can ultimately accelerate further kidney damage 5

Nephron Adaptation and Compensation

When nephron mass decreases, surviving nephrons undergo functional changes 5:

• Intact nephrons increase their individual production of proteins like uromodulin as a protective mechanism 5
• Efferent arteriolar vasoconstriction coupled with afferent arteriolar dilation increases intraglomerular pressure to maintain GFR despite fewer functioning nephrons 5
• These compensatory hemodynamic changes, while initially beneficial, can lead to glomerular injury and accelerated kidney function loss if sustained 5

Nephron Function Assessment

Multiple modalities can evaluate nephron function 5:

• Glomerular function: Measured by eGFR, measured GFR, and creatinine/cystatin C clearance 5
• Tubular function: Assessed through urine sodium avidity, fractional excretion of sodium, and tubular biomarkers 5
• Nephron mass: Evaluated indirectly through kidney imaging, proteinuria/albuminuria presence, and kidney biopsy when indicated 5
• Nuclear medicine studies: Renal scintigraphy assesses the three sequential phases—perfusion, extraction/excretion by nephrons, and drainage—providing functional information that complements anatomic studies 5