What is the function of the glomerulus?

Function of the Glomerulus

The glomerulus is the primary filtration unit of the kidney that produces an ultrafiltrate of plasma through a high-pressure capillary system, allowing for waste removal and maintaining sodium/volume homeostasis. 1

Structural Components and Filtration Mechanism

• The glomerulus consists of a specialized capillary bed with unique structural features that enable filtration of blood to form urine 2
• It contains afferent and efferent arterioles whose resistance determines glomerular hemodynamics, controlling glomerular filtration rate (GFR) and renal blood flow 1
• The glomerular filtration barrier consists of three main components:
  • Fenestrated endothelial cells (without diaphragms)
  • Glomerular basement membrane
  • Epithelial cells (podocytes) with filtration slits 3

Physiological Function

• The glomerulus maintains a 60-40 mmHg pressure gradient across its capillary bed, enabling the production of an ultrafiltrate measured as GFR 1
• Total glomerular filtration rate is the product of single-nephron GFR and total nephron number (approximately 1 million nephrons per kidney) 2
• The total length of capillaries in a single glomerulus is 0.95 cm, with all glomeruli combined providing approximately 19 km of capillary length 4
• The total filtration surface area of all glomeruli is about 516.1 cm² 4

Filtration Barrier Properties

• The basement membrane serves as the main filtration barrier, acting as the primary filter for molecules in the size range of plasma proteins (32,000 to 125,000 molecular weight) 5
• The endothelium functions as a valve that controls access to the filter through its fenestrations 5
• The epithelium (podocytes) acts as a monitor that can recover proteins that leak through the filter 5
• The mesangium helps recondition and unclog the filter by incorporating and disposing of filtration residues 5

Autoregulation of Filtration

• The glomerulus can maintain relatively constant GFR across a broad range of blood pressures through autoregulation 2
• The afferent arteriole constricts when blood pressure is high to prevent transmission of systemic pressure to the glomerulus 2
• Conversely, when blood pressure falls, the afferent arteriole dilates to stabilize GFR or minimize its reduction 2
• This autoregulatory mechanism is critical for protecting the glomerulus from pressure-related damage 2

Clinical Significance

• Disordered autoregulation occurs in conditions like diabetes mellitus, reduced renal mass, and proteinuric kidney disease, leading to transmission of systemic pressure to the glomerulus 2
• In chronic kidney disease, as functioning nephrons decrease, hemodynamic stress on surviving glomeruli increases, raising intraglomerular pressure 2
• Glomerular hemodynamic changes initially increase single-nephron GFR to maintain global GFR, but sustained changes can accelerate kidney function loss 2
• Medications that alter glomerular hemodynamics, such as renin-angiotensin system blockers, have shown effectiveness in preserving long-term kidney health 1

Age-Related Changes

• With aging, renal mass shrinks and renal blood flow decreases, accompanied by a gradual loss of functioning nephrons 2
• This loss of renal cortical mass reflects a decline in renal function, decreased glomerular filtration, and reduced tubular function 2
• Renal function typically decreases by about 1% per year beyond age 30-40, potentially declining by 40% by age 70 2

Understanding the glomerular structure and function is essential for comprehending kidney physiology and the pathophysiology of kidney diseases, particularly those affecting filtration and leading to proteinuria or reduced GFR.