Why do plasma proteins and blood cells not enter the renal tube?

Plasma proteins and blood cells do not enter the renal tubule primarily due to their size and charge. The glomerular filtration barrier, composed of endothelial cells, a basement membrane, and podocytes, selectively allows small molecules and water to pass while blocking larger proteins and cellular components. Additionally, the negative charge on the surface of the filtration barrier repels negatively charged plasma proteins, further preventing their passage into the renal tubule. This selective filtration is crucial for maintaining proper blood composition and preventing proteinuria.