29.9 : Glomerular Filtration

The filtration membrane in the renal system is a highly specialized structure essential for filtering blood. It consists of glomerular capillaries and podocytes, forming a selective barrier that permits the passage of water and small solutes while restricting most plasma proteins and blood cells.

Components of the Filtration Membrane

The filtration process involves three key layers: the glomerular endothelial cells, the basement membrane, and the podocyte-formed filtration slits.

1. Glomerular Endothelial Cells:
   The endothelial cells lining the glomerular capillaries contain large pores called fenestrations. These fenestrations allow all solutes in blood plasma to pass through while preventing the filtration of blood cells. This ensures that cellular components of blood remain within the circulation.
2. Basement Membrane:
   Situated between the endothelium and podocytes, the basement membrane is composed of fine collagen fibers and negatively charged glycoproteins. The membrane's pores permit the passage of water and small solutes, but the negative charge repels plasma proteins, effectively restricting their filtration.
3. Podocytes and Filtration Slits:
   Podocytes are specialized cells with footlike extensions called pedicels that wrap around the capillaries. The spaces between the pedicels, known as filtration slits, are covered by a thin slit membrane. This membrane permits the passage of molecules smaller than 0.006–0.007 μm in diameter. Substances that can pass include water, glucose, vitamins, amino acids, very small plasma proteins, ammonia, urea, and ions.

Functionality of the Filtration Membrane

The filtration membrane's design ensures selective permeability, allowing the kidneys to efficiently filter blood while retaining essential proteins and cells. Its combination of physical and charge-based filtration mechanisms prevents the loss of large molecules and cellular components, enabling effective waste removal and fluid regulation.