Urine Production
Biology: AQA GCSE Higher, topic 5.3.13
Kidneys - Urine Production
Each kidney contains millions of very small structures called nephrons. There are two stages in the process of urine production. These stages happen in every nephron.
Ultrafiltration
- The blood is filtered and all water, urea and salts move into the nephron 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 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.
Test yourself on Urine Production
Why do blood cells and proteins stay in the blood during ultrafiltration?
Why is the second stage of urine production called selective reabsorption?
Why does urine contain urea, but not blood cells or proteins?
If selective reabsorption returned more water into the blood than usual, what effect would this have on the volume of urine produced?
Why is a second stage of selective reabsorption needed after ultrafiltration?