The structure responsible for filtering blood in the kidneys is the nephron. Each kidney contains approximately a million nephrons, which consist of a glomerulus for filtering blood and a tubule for reabsorbing necessary substances and secreting waste. As blood passes through the nephron, it undergoes a process of filtration, reabsorption, and secretion, ultimately leading to the production of urine that is excreted from the body, thereby playing a critical role in maintaining fluid and electrolyte balance.