Final answer:
The kidneys filter blood through a million nephrons that start with a glomerulus, followed by reabsorption and secretion in the renal tubule. Waste, water, and excess substances are turned into urine, which is then stored in the bladder.
Step-by-step explanation:
The kidneys are vital organs responsible for filtering blood and producing urine. This process begins in the functional units of the kidneys, called nephrons. Each kidney has around one million nephrons, and each nephron begins with a structure known as the glomerulus, where blood is filtered. The filter enables small molecules, waste, and water to pass through, forming a pre-urine substance.
Subsequently, this fluid goes through the renal tubule, where several substances are either added or removed by processes known as secretion and reabsorption, according to the body's needs. Glucose, certain salts, and a controlled amount of water are reabsorbed into the bloodstream, while other substances, including urea, excess salts, and additional water, are left in the tubular fluid. Finally, the concentrated liquid, now urine, flows from the collecting ducts into the ureters, which lead to the bladder where urine is stored before urination.
Through this intricate process, the kidneys regulate the composition, volume, and pressure of the blood, and maintain overall fluid and electrolyte balance in the body. By producing urine, they also eliminate waste products and excess substances, ensuring the body's proper homeostasis.