Urine forms in two main steps. Filtration pushes small substances out of the blood, and reabsorption takes back what the body needs. What is left becomes urine.
This lesson follows explaining negative feedback. Here you see the kidney carrying out the response step of the loop.
What happens at each stage?
- Filtration: in the glomerulus, blood pressure forces water, glucose, amino acids, urea and salts into Bowman’s capsule. Blood cells and proteins are too large and stay in the blood.
- Selective reabsorption: in the proximal convoluted tubule, all glucose and amino acids, and much of the water and salts, return to the blood.
- Water adjustment: in the distal tubule and collecting duct, ADH controls how much more water is reabsorbed.
- Urine: what remains flows to the bladder as water, urea and some salts.
Worked example: tracking four substances
Question: Trace water, glucose, urea and protein through the nephron.
| Substance | Filtered? | Reabsorbed? | In urine? |
|---|---|---|---|
| Protein | No, too large | Not applicable | No |
| Glucose | Yes | All of it | No |
| Urea | Yes | Little | Yes |
| Water | Yes | Most, amount set by ADH | Yes |
Explain each row in a sentence, such as: “Glucose is filtered because it is small, then all of it is reabsorbed in the proximal convoluted tubule, so none is in the urine.”
The table shows the two rules at work. Filtration sorts by size, while reabsorption sorts by need.
The mistake that loses marks
The common slip is to say that glucose “is not filtered” because the body needs it. Filtration happens before the body can “decide” anything. It is a pressure process that only sorts by size.
| Claim | Correct? | Fix |
|---|---|---|
| “Glucose is not filtered, since we need it” | No | Glucose is filtered, then reabsorbed |
| “Protein is reabsorbed” | No | Protein is never filtered, so it stays in the blood |
The fix is to ask two separate questions for each substance: was it small enough to be filtered, and does the body need it back?
A routine for any substance
- Ask whether it is small enough to pass the filter.
- If not, it stays in the blood.
- If yes, ask whether the body needs it.
- If needed, it is reabsorbed. If not, it leaves in urine.
The biological data interpretation trainer offers tables of substances in filtrate and urine that you can trace this way.
Check yourself
An invented urine test finds glucose and protein in a sample. Using the stages, say which stage could explain each, without diagnosing anyone.
Answer
Glucose in urine means reabsorption in the proximal convoluted tubule did not take back all the glucose. Protein in urine means the filter let large molecules through.
This shows you know which stage handles which substance. A real test result belongs with a doctor.
What to study next
Use tables of intake and output in interpreting water-balance data. For the structures involved, see the wider SPM Biology guide.
If you want a teacher to trace substances with you, see online one-to-one Biology tuition.