What you drink changes what comes out in your urine — and the difference is measurable. Your kidneys clean your blood by adjusting urine volume and makeup based on what you take in. Tap water passes through quickly and produces pale urine. A salt solution delays water release and makes urine darker. When you drink coffee, caffeine acts directly on the kidneys and speeds things up. By measuring pH, color, and specific gravity of urine samples after each fluid, you can see how the kidneys fine-tune waste removal to keep your blood balanced.