Ultrafiltration
Urine is made in three steps along the nephron: ultrafiltration, selective reabsorption and secretion.
1. Ultrafiltration
In the Bowman's capsule
2. Selective reabsorption
Along the tubule
3. Secretion
Into the tubule
Ultrafiltration
The blood in the glomerulus is under high pressure, so water and small dissolved substances — glucose, amino acids, salts, urea — are forced out into the Bowman's capsule. This is ultrafiltration, and the liquid is the filtrate.
What stays behind
Blood cells and large protein molecules are too big to pass, so they stay in the blood.