The kidneys and ureaSpec 4.5.3.3
The kidneys produce urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water. The waste urea and the excess water and ions leave in the urine.
- Blood is filtered in the kidney. Water, glucose, ions and urea pass out of the blood.
- Useful substances are selectively reabsorbed back into the blood: all of the glucose, some ions and some water.
- The rest, including urea, excess ions and excess water, passes out as urine.
You do not need to know the other parts of the urinary system, the structure of the kidney or the structure of a nephron.
| Substance | Blood before filtration | Fluid after filtration | Urine |
|---|---|---|---|
| Glucose | 5 | 5 | 0 |
| Ions | 9 | 9 | 12 |
| Urea | 2 | 2 | 60 |
This kind of table can be shown as a bar chart. The glucose bar drops to zero in urine because all of it is reabsorbed. The urea bar is much higher in urine because water is reabsorbed while urea is not.
Excess amino acids and urea
The digestion of proteins from the diet results in excess amino acids which need to be excreted safely. In the liver, these amino acids are deaminated to form ammonia. Ammonia is toxic and so it is immediately converted to urea for safe excretion.