Why do blood proteins normally remain in the blood during glomerular filtration?

During glomerular filtration, blood proteins are prevented from entering the urine primarily due to the physical and electrostatic properties of the glomerular capillary wall.1

The restriction of these proteins occurs through two main mechanisms:

• Size Restriction: Larger protein molecules are physically blocked from passing through the filtration barrier, which includes the endothelial cells, the basement membrane, and the slit diaphragms between podocytes.1
• Electrostatic Repulsion: The basement membrane contains a negative electrostatic charge due to its content of polyanionic glycopeptides. This negative charge acts as a restrictive force that repels negatively charged macromolecules, such as albumin.1

When the integrity of the glomerulus is compromised by disease or lack of oxygen, these barriers can become more permeable, leading to the abnormal presence of plasma proteins in the urine.1