How does your kidney decide what to filter out?

Your kidneys don't selectively filter at first - they use pressure to force nearly everything out of your blood (except large proteins and blood cells) into the tubules. This initial filtrate contains water, salts, glucose, urea, creatinine, and other small molecules. The selectivity happens afterward, in the tubules, where useful substances like glucose, amino acids, and most water and salts get actively reabsorbed back into the blood. Waste products like urea and creatinine are poorly reabsorbed and stay in the filtrate to become urine. Some substances are also actively secreted from blood into the tubules. So the kidney's strategy is filter everything small first, then reclaim what the body needs.