The kidney works in two stages: first it filters a large volume of blood plasma by size, then it takes back the useful substances, and what is left becomes urine. The idea matters more than the labels, and it appears in questions about glucose, urea and water.
This skill follows distinguishing excretion from egestion, because urea is the waste substance the kidney removes. It sets up the control ideas in negative feedback.
How does filtration work?
Blood enters each kidney under pressure. The pressure forces water and small molecules out of the blood and into a tubule. This liquid is called the filtrate.
Small molecules pass through: water, glucose, salts (ions), amino acids and urea. Large molecules and cells stay in the blood: proteins and red blood cells are too big for the filter. Filtration is about size, not usefulness, so useful and waste substances both leave the blood at this stage.
How does selective reabsorption work?
As the filtrate flows along the tubule, the body takes back what it needs. All of the glucose is reabsorbed into the blood, together with amino acids, most of the water and some salts. Some of this needs active transport, which uses energy, because the substance moves against its concentration gradient.
Urea is not reabsorbed in the same way, so it stays in the tubule and leaves in the urine. The amount of water and salts reabsorbed can change, which is how the body adjusts urine volume and concentration.
Worked example
A model kidney produces 150 dm³ of filtrate in a day and 1.5 dm³ of urine. These numbers are invented to practise the idea and are not average values for a person. What percentage of the filtrate volume is reabsorbed?
Step 1: find the volume reabsorbed. 150 − 1.5 = 148.5 dm³.
Step 2: divide by the filtrate volume. 148.5 ÷ 150 = 0.99.
Step 3: convert to a percentage. 0.99 × 100 = 99%.
Check: 1.5 is 1% of 150 (150 ÷ 100 = 1.5), so urine is 1% and reabsorbed is the other 99%.
Now suppose the model filtrate contains 5.0 units of glucose in a sample and the model urine contains none. The glucose was filtered, then all of it was selectively reabsorbed, so it never reached the urine.
The mistake to watch for
The common error is to say that the kidney filters out only the waste, as if it selected urea at the start.
Mistaken answer: “Filtration removes urea and keeps glucose in the blood.”
This misses that glucose does enter the filtrate. The filter separates by size, not by usefulness.
The correction is to give both stages. “Small molecules including glucose and urea are filtered out of the blood. Glucose is then selectively reabsorbed, while urea stays in the urine.” That is the sequence markers look for.
Check yourself
Try these, then open each answer.
1. Name one substance that is too large to pass into the filtrate.
Show answer
Protein (or red blood cells). They are too large to pass through the filter and stay in the blood.
2. In a model, 200 dm³ of filtrate gives 2 dm³ of urine. What percentage is reabsorbed?
Show answer
200 − 2 = 198 dm³ reabsorbed. 198 ÷ 200 = 0.99, so 99%. Check: 2 is 1% of 200.
3. Why does selective reabsorption of glucose need energy?
Show answer
Glucose is taken back into the blood by active transport, which can move a substance against its concentration gradient and so needs energy from respiration.
Where this leads next
The kidney is one organ with one job. Next, interpret a negative-feedback loop to see the general control pattern that the body uses everywhere. If the two-stage explanation stays shaky when exam wording changes, our teachers can rebuild it in online one-to-one Biology tuition.