In the kidney, the structures that filter the blood are the nephrons. Nephrons are the functional units of the kidney responsible for filtering waste products, excess water, and other substances from the blood to form urine. Each kidney contains millions of nephrons, which consist of a glomerulus and a tubule. The glomerulus acts as a filter, allowing small molecules such as water, electrolytes, and waste products to pass through while retaining larger molecules like proteins and blood cells. The filtered fluid then passes through the tubule, where reabsorption of essential substances and further concentration of urine occur. Ultimately, the nephrons play a crucial role in maintaining the body's fluid balance, electrolyte levels, and waste elimination through the process of filtration, reabsorption, and secretion.