Here's why:

* Glomerular Filtration: The process of filtering blood in the kidneys occurs at the glomerulus. This is a network of capillaries surrounded by a specialized structure called Bowman's capsule.

* Filtration Barrier: The filtration barrier, which determines what passes from the blood into the filtrate, consists of three layers:

* Endothelium of the capillary: The inner layer is made of specialized cells with pores.

* Glomerular basement membrane: This is a thin, acellular layer composed of collagen and other proteins. It is the primary barrier that prevents proteins from passing into the filtrate.

* Podocytes: These are specialized epithelial cells that wrap around the capillaries. Their foot processes form filtration slits, further restricting the passage of large molecules.

Why Proteins are Excluded:

* Size: Proteins are generally larger than the pores in the glomerular basement membrane.

* Charge: The glomerular basement membrane has a negative charge, which repels negatively charged proteins.

While other layers of the filtration barrier play a role, the glomerular basement membrane is the most crucial for preventing protein loss into the urine.