How urine forms.

Only a small segment of peritubular capillary is shown.

1. Glomerular filtration Protein-free plasma forced out of glomerular capillaries by blood pressure enters Bowman’s capsule.
2. Tubular reabsorption Essential ions, nutrients, water, and some urea in the filtrate return to the blood. Green arrows indicate reabsorption.
3. Tubular secretion Wastes and excess ions are moved from the blood into the filtrate for elimination in urine. Blue arrows indicate secretion.
4. Hormones that alter permeability of distal and collecting tubules adjust urine concentration.