Here's a breakdown of the process:

1. Filtration: Blood enters the glomerulus, a network of capillaries within the nephron. The high pressure in the glomerulus forces water and small solutes (like glucose, amino acids, and ions) out of the blood and into the Bowman's capsule, forming the filtrate.

2. Reabsorption: As the filtrate travels through the proximal tubule, loop of Henle, and distal tubule, essential substances like water, glucose, amino acids, and ions are reabsorbed back into the bloodstream.

3. Secretion: Some waste products and excess substances are actively secreted from the blood into the filtrate as it passes through the tubules.

4. Urine Formation: After reabsorption and secretion, the remaining filtrate, now concentrated and containing waste products, is called urine. It flows into the collecting duct and eventually exits the nephron and the body.

So, while filtrate is not urine, it's the initial fluid that is further processed and modified to become urine.