So these are the only two ways by which some given substance can end up in the filtrate and ultimately released in our urine. Now, as I mentioned earlier, different substances have different values for the renal clearance. So let's begin by supposing we have some hypothetical substance. Let's call this substance X and let's discuss how it can actually move within our nephron and what can happen to it within our nephron. And let's study what the renal clearance value is of this substance. So I've outlined four different cases, but you should note that there are more cases than this.