Final answer:
The nephron filtration barrier selectively filters blood based on size and charge due to its specialized components, such as fenestrations, negatively charged proteins, and the basement membrane, ensuring that larger or negatively charged substances do not pass through while smaller, positively charged compounds do.
Step-by-step explanation:
The nephron filtration barrier is selective to size and charge because it is designed to efficiently filter blood while maintaining essential molecules within the bloodstream. The filtration relies on various components such as fenestrations in capillary endothelial cells, negatively charged proteins, the basement membrane, and podocytes with filtration slits. These structural features ensure that cells and large proteins remain in the bloodstream, while allowing smaller, positively charged substances and necessary solutes to pass through and form a primary filtrate. The role of the mesangial cells in regulating the glomerular filtration rate contributes to the selectivity of this process as well. The resulting filtrate then undergoes reabsorption and secretion in the subsequent segments of the nephron to form urine.