Which part of the kidney is primarily responsible for filtration?

The cortex of the kidney is primarily responsible for filtration due to its structure and the presence of nephrons, which are the functional units of the kidney. The renal cortex contains the glomeruli, which are tiny clusters of blood vessels where the filtration process initiates. This is where blood is first filtered to remove waste products, excess substances, and toxins while retaining essential components like red blood cells and proteins.

In contrast, while the nephron itself is critically involved in the overall filtration and reabsorption processes, it is the glomeruli located within the cortex that perform the initial filtration. The medulla, while it plays a key role in concentrating urine, does not directly engage in the filtration of blood nor does the renal pelvis, which is mainly involved in the collection of urine that drains from the kidneys to the ureters. Therefore, the cortex is the correct answer as it encompasses the necessary structures and functions for filtration.