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Why do proteins usually remain in the blood during glomerular filtration?
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renal system
Diagram showing the location of the kidneys in the abdominal cavity and their attachment to major arteries and veins.
Proteins are generally retained in the blood during glomerular filtration due to a combination of size-based restriction and electrostatic repulsion.
The glomerular capillary wall acts as a selective barrier consisting of three layers: the fenestrated endothelium, the basement membrane, and the podocyte layer with its slit diaphragms. The basement membrane, composed of a hydrated gel structure, serves as the primary restriction point for larger molecules.1
Two specific mechanisms prevent the passage of most proteins:
- Size Exclusion: Larger protein molecules are physically blocked by the structural pores of the glomerular wall.1
- Electrostatic Repulsion: The basement membrane contains a negative (anionic) charge due to its content of sialic and aspartic acids. This negative charge repels negatively charged macromolecules, such as albumin, providing an additional barrier to their filtration.1
When the integrity of the glomerulus is compromised by disease or oxygen deprivation, these barriers can become more permeable, leading to the leakage of plasma proteins into the urine.1