Your cells constantly make waste that would poison you if it built up. Excretion is the removal of these toxic waste products of metabolism. Two main wastes stand out: carbon dioxide from respiration (removed by the lungs) and urea (removed by the kidneys). Your two kidneys filter your entire blood supply around 300 times a day.
Why this matters The kidney is the body's water- and waste-balancing machine. Understanding it explains dialysis, transplants and why kidney failure is life-threatening — and the filtration-versus-reabsorption story is a classic Extended-tier exam question.
Where urea comes from
- We eat more protein than we can store. Excess amino acids cannot be stored, so they are broken down in the liver.
- This process, deamination, removes the nitrogen part and forms urea.
- Urea is carried in the blood to the kidneys, which remove it and make urine.
The kidney and the nephron
Each kidney contains about a million tiny filtering units called nephrons. They work in two key steps:
| Step | What happens | Where |
|---|---|---|
| Filtration | High blood pressure forces small molecules (water, glucose, salts, urea) out of the blood into the tubule | Glomerulus & capsule |
| Reabsorption | Useful substances taken back into the blood: ALL glucose, needed water and salts | Tubule |
- Filtration — blood enters a knot of capillaries (the glomerulus) at high pressure. Small molecules are squeezed out into the capsule. Big things — blood cells and proteins — stay in the blood because they are too large.
- Selective reabsorption — as the filtrate flows along the tubule, all the glucose, the water the body needs, and useful salts are reabsorbed into the surrounding capillaries. This leaves urea, excess water and excess salts.
- The remaining liquid is urine: water, urea and salts. It drains to the bladder to be released.
When kidneys fail: dialysis vs transplant
| Dialysis | Transplant | |
|---|---|---|
| What it is | Machine filters the blood, several sessions a week | A donor kidney is put into the patient |
| Drawbacks | Time-consuming; controlled diet; lifelong | Risk of rejection; immunosuppressant drugs; donor needed |
| Advantage | Available without a donor | A permanent solution; more freedom |
Worked example. “Glucose is filtered out of the blood but is not found in the urine of a healthy person. Explain why (2 marks).” (1) Glucose is small enough to be filtered out at the glomerulus into the tubule; (2) it is all reabsorbed back into the blood by selective reabsorption along the tubule, so none is left to appear in the urine.
Common exam mistake Don't say the kidney “makes” urea — the liver makes urea by deamination; the kidney only removes it. And proteins/blood cells stay in the blood because they are too large to be filtered, not because they are reabsorbed.
Memory hook “Filter everything small, take back what you need.” Sugar (glucose) is precious — it is always 100% reabsorbed in a healthy person, which is why glucose in urine can be a sign of diabetes.