Urine
Applied Science: BTEC Level 3 National Extended Diploma, topic 4.7.4
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.
Water in the Blood
The volume of urine produced depends on the internal conditions. The volume of water in the blood is monitored by the hypothalamus in the brain. This is how the body responds to having too much or too little water in the blood:
Too little water
- The hypothalamus triggers the pituitary gland to release anti-diuretic hormone (ADH).
- This increases the permeability (ability of things to pass through) of the kidney tubules. This means that more water is reabsorbed back into the bloodstream.
- This results in a small volume of concentrated urine.
Too much water
- The hypothalamus triggers the pituitary gland to stop the release of anti-diuretic hormone (ADH).
- This reduces the permeability of the kidney tubules. This means that less water is reabsorbed back into the bloodstream.
- This results in a large volume of dilute urine.
Test yourself on Urine
Which hormone helps to control the volume of water in urine?
What happens when there is too little water in our body?
What is this diagram an example of?