Anatomy and Physiology of the Glomerular Filtration Membrane

These fenestrations range in diameter from approximately $0.07$ to $0.1$ micrometers. While these pores are large enough to allow almost all components of blood plasma to pass through, they are small enough to prevent the filtration of larger formed elements, specifically blood cells and platelets.

Exam Tip: If a question asks which part of the filtration membrane prevents the loss of red blood cells into the urine, the answer is the fenestrations of the glomerular endothelial cells.

Layer 2: The Basal Lamina

Once substances pass through the endothelial fenestrations, they encounter the middle layer: the basal lamina of the glomerulus. This layer is composed of a complex glycoprotein matrix containing a mesh of collagen fibers.

The basal lamina serves two critical functions:

1. Physical Sieve: It acts as a fine mesh that prevents the filtration of larger proteins.
2. Electrical Barrier: The glycoproteins within this matrix are negatively charged. Because most plasma proteins (such as albumin) are also negatively charged, the basal lamina uses electrostatic repulsion to push these proteins away, preventing them from passing through.

Layer 3: Podocytes and the Slit Membrane

The most superficial layer of the filtration membrane is formed by specialized cells called podocytes. These cells are part of the visceral layer of Bowman's capsule and possess long, finger-like extensions known as pedicels.

These pedicels wrap around the glomerular capillaries, much like fingers interlocking around a tube. The narrow gaps between these interdigitating pedicels are called filtration slits. To ensure only the smallest molecules pass, these slits are covered by a very thin, tightly woven structure called the slit membrane.

The slit membrane is the final and most restrictive barrier. It only permits the passage of extremely small molecules, typically those smaller than $0.006$ to $0.007$ micrometers. This includes:

• Water
• Glucose
• Ions (such as $Na^+$ and $K^+$)
• Amino acids
• Very small plasma proteins

Summary of Filtration Selectivity

The glomerular filtration membrane uses both size and electrical charge to ensure that blood plasma is filtered accurately.

By combining these three layers, the kidney ensures that essential components—like blood cells and large proteins—remain in the circulatory system, while metabolic wastes and excess ions are efficiently moved into the nephron for excretion.