Biology – Homeostasis in mammals | e-Consult

Glomerular filtration is the initial step in urine formation, occurring in the glomerulus, which is surrounded by the Bowman's capsule. This process is driven by differences in hydrostatic and osmotic pressure across the glomerular membrane.

Mechanism: Blood enters the glomerulus under high hydrostatic pressure (approximately 60 mmHg). The glomerular capillaries have a highly permeable membrane, the glomerular filtration barrier, which consists of three layers:

• Fenestrated endothelium of the glomerular capillaries: Contains pores that allow passage of most solutes.
• Basement membrane: A negatively charged layer that repels negatively charged proteins.
• Podocytes: Specialized epithelial cells with foot processes that interdigitate, creating filtration slits. These slits further restrict the passage of larger molecules.

This structure allows water, ions (e.g., Na+, K+, Cl-), glucose, amino acids, urea, and small proteins to pass through the glomerular membrane and into the Bowman's capsule. Larger molecules, such as blood cells and large proteins, are retained in the blood.

Factors influencing permeability:

• Glomerular Capillary Hydrostatic Pressure: Higher pressure increases filtration rate.
• Capsular Hydrostatic Pressure: Higher pressure opposes filtration, reducing filtration rate.
• Glomerular Capillary Oncotic Pressure: Higher pressure opposes filtration, reducing filtration rate.
• Glomerular Membrane Area: A larger surface area increases filtration rate.

The fluid that enters the Bowman's capsule is called glomerular filtrate. It is essentially plasma without the large proteins and blood cells.