Final answer:
Blood enters the kidney through the renal artery, which is narrower to maintain high pressure required for filtration in the nephrons.
Step-by-step explanation:
The structure through which blood enters the kidney is known as the renal artery, which branches into smaller arteries and ultimately into capillaries supplying the nephrons. As blood passes through the capillaries in the glomerulus of a nephron, blood pressure forces water and dissolved substances across the capillary walls into Bowman's capsule, beginning the filtration process.
The renal artery is narrower than the outgoing vessels, such as the renal vein, which results in a higher pressure necessary for filtration within the kidneys.