Urine Production
Biology: Edexcel IGCSE, topic 3.4.3
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 capillaries form a tightly knotted cluster, squeezing out small particles from the blood.
- The blood is filtered and all water, urea and salts are removed from the blood.
- 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, some salts and most water, are reabsorbed back into the bloodstream.
- This leaves all urea, excess salts and excess water collected in the nephron.
- This mixture forms urine, which is collected by the kidneys and travels to the bladder to be stored and then excreted.