Final answer:
The filtration barrier is made up of capillary endothelial cells, the basement membrane, and podocytes, which work together to filter the blood and form a filtrate in the kidneys.
Step-by-step explanation:
The filtration barrier in the kidney is composed of three main structures: capillary endothelial cells, the basement membrane, and podocytes. The fenestrations in the capillary endothelial cells permit most substances to cross based on size, whereas the negatively charged proteins associated with these pores repel negatively charged substances and allow positively charged substances passage.
The basement membrane primarily restricts the filtration of medium-to-large proteins like globulins. Lastly, podocytes, which form the visceral layer of Bowman's capsule, extend finger-like processes called pedicels that interdigitate to form filtration slits, creating a physical barrier that further filtrates the blood. All three components work together to prevent filtration of blood cells and large proteins, while allowing smaller substances (< 4 nm) and most substances up to 8 nm to pass into the filtrate.