Final answer:
The filtration membrane in the kidneys is composed of fenestrations in capillary endothelial cells, a fused basement membrane, and filtration slits created by podocytes, playing a key role in filtering the blood and forming urine without cells or large proteins.
Step-by-step explanation:
The Filtration Membrane in Kidneys
The structures that make up the filtration membrane in the kidneys are crucial for the process of filtering blood and forming urine. The membrane consists of fenestrations in the capillary endothelial cells, a fused basement membrane, and filtration slits between the interdigitating foot processes of podocytes. The fenestrations prevent filtration of blood cells or large proteins, allowing smaller substances to pass through. Substances smaller than 4 nm can pass through readily, and a majority can pass freely up to 8 nm in size. The proteins associated with the pores have a negative electric charge, which repels negatively charged substances while allowing positively charged ones to pass more readily.
The basement membrane is key in preventing the filtration of medium-to-large proteins such as globulins. Furthermore, mesangial cells, which are also part of the filtration membrane, can contract to regulate the filtration rate of the glomerulus. This combination of structures ensures that the resulting filtrate is free of cells and large proteins and contains a slight predominance of positively charged substances.