How does the kidney filter out different molecules?

In the blood flowing through the kidneys, you find lots of different substances, like glucose and amino acids (which the body wants to keep and use) and urea (which the body wants to get rid of). The first important process is ultrafiltration. In a leaky network of blood vessels called the glomerulus, the kidney creates very high pressure. This forces small molecules like urea, glucose and amino acids out of the blood vessels through very small holes. They pass into Bowman's capsule, which is the first part of the nephron.

The most important thing here is how big the molecules are. Blood cells are too big to pass through the small holes in the blood vessels, even under pressure. Smaller molecules are the right size to get through under pressure, so they do get filtered out.