How the nephron works

The nephron, which is one of the kidney's many filtering units, does its job in two steps, and to understand the order is to understand the whole thing. First the glomerulus, the filter, strains the blood. Then the tubule, the tube that follows, sorts through what was strained and returns to the blood whatever the body still needs, while letting the waste go.

The filter works by size. Its walls are so thin that they let small molecules, waste and fluid, almost all of it water, pass into the tubule; the big stuff, such as proteins and blood cells, does not fit and stays inside the blood vessel. Then, as the fluid runs along the tubule, almost all the water is recovered along with minerals and nutrients. What is left at the end, fluid and waste, is urine. It is like straining a whole pot at once and then fishing out by hand what was actually worth keeping: filtering is the fast part, picking is the fine work.