What process primarily occurs in the kidneys to facilitate filtration?

The primary process that facilitates filtration in the kidneys occurs through passive diffusion in the glomerular capillaries. The glomerulus, a network of tiny blood vessels within the nephron, is designed to filter blood based on size and charge, allowing small molecules, ions, and water to pass through, while retaining larger molecules like proteins and blood cells. This process hinges on the pressure differences within the glomerulus and the surrounding Bowman's capsule, allowing for the efficient removal of waste products and excess substances from the bloodstream without the need for energy expenditure typical of active transport mechanisms.

The other choices involve processes that occur at different stages of urine formation but do not define the primary mechanism of filtration as effectively as passive diffusion in the glomeruli. Active transport happens later in the nephron during reabsorption, osmosis plays a crucial role in water reabsorption, and filtration via nephron tubules refers to processes that are influenced by initial glomerular filtration but are not the primary action of the kidneys in filtering blood.