How does your kidney decide what to filter from blood?

Your kidneys don't selectively filter at the initial stage - the glomerulus acts more like a size-based sieve, allowing almost everything smaller than large proteins to pass through into the filtrate, including water, salts, glucose, urea, and small molecules. Blood cells and large proteins like albumin are too big and stay in the blood. The real selectivity happens afterward in the tubules, where the kidney decides what to reabsorb back into the blood. Useful substances like glucose, amino acids, and most water are actively reclaimed. Waste products like urea and excess ions are left behind or actively secreted into the filtrate to be excreted as urine. Hormones like aldosterone and ADH regulate how much sodium and water get reabsorbed based on the body's needs.