Urine Production
Biology: Cambridge International GCSE Supplement, topic 13.1.6
Kidneys - Urine Production
Each kidney contains millions of very small structures called tubules. There are two stages in the process of urine production. These stages happen in every kidney tubule.
Ultrafiltration
- The blood capillaries form a knotted cluster called a glomerulus in every tubule.
- The blood is filtered and all water, urea and salts move into the tubule.
- Blood cells and proteins remain in the blood as they are too big to move across the capillary walls.
Selective reabsorption
- Useful substances, including glucose and some water, are reabsorbed from the tubule back into the bloodstream.
- This leaves all urea, excess salts and excess water in the tubule.
- This mixture forms urine, which is collected by the kidneys and travels to the bladder to be stored and then excreted.