What is the hilum of the kidney?

The hilum of the kidney is best defined as a concave notch that serves as the entry and exit point for various structures, including blood vessels, lymphatic vessels, nerves, and the ureter. This anatomical feature is crucial because it allows these important connections to extend from the kidney to the rest of the body. The blood vessels, such as the renal artery and vein, are integral for supplying oxygenated blood to the kidney and draining deoxygenated blood from it, while the ureter transports urine from the kidney to the bladder.

In contrast, the outer layer of the kidney is known as the renal capsule, which serves to protect the kidney, not to facilitate the entrance and exit of vessels. The region for urine production refers to the functional kidney tissue, mainly the renal cortex and renal medulla, where the nephrons filter blood and produce urine. Lastly, while the nephron is indeed the functional unit of the kidney, it does not describe the hilum but rather is the structure responsible for urine formation. Thus, the concave notch that allows for vital connections is accurately captured by identifying the hilum of the kidney.