How do the kidneys filter blood to form urine in the nephron?

Each kidney has millions of tiny filtering units called nephrons, and urine formation happens in three steps. First is filtration. Blood enters a cluster of capillaries called the glomerulus under high pressure, and water, glucose, salts, urea and other small molecules are pushed out into the Bowman's capsule, forming a filtrate. Large items like blood cells and proteins stay in the blood. Second is selective reabsorption. As the filtrate flows through the tubule, useful substances like glucose, most water and needed salts are reabsorbed back into the blood. Third is secretion, where extra wastes and ions are added from the blood into the tubule. What remains, mainly water, urea and excess salts, is urine. It collects in the collecting ducts and flows to the urinary bladder for removal.