Excretion in Humans
Senior 2 · Biology
Excretion in Humans
Lesson 3 of 4
22 min
Let's begin
Your kidneys filter around 180 dm³ of fluid out of your blood every day, enough to fill nine 20-litre jerrycans. Yet you produce only about 1.5 dm³ of urine. The kidney's secret is that it first filters out almost everything small, then takes back what the body needs.
Worked example
If 180 dm³ of filtrate is formed in a day and 1.5 dm³ of urine is produced, what percentage of the filtrate is reabsorbed?
Solution: Volume reabsorbed = 180 − 1.5 = 178.5 dm³. Percentage = 178.5 ÷ 180 × 100 ≈ 99.2%. Over 99% of the fluid filtered is returned to the blood.
Urine is made in three stages.
Blood pressure in the glomerulus is high because the efferent arteriole is narrower than the afferent arteriole. This pressure forces fluid out through tiny pores in the capillary walls and the lining of Bowman's capsule, which act together as a very fine filter. This is ultrafiltration (filtration under pressure).
The fluid in Bowman's capsule is called the glomerular filtrate. It is driven by blood pressure, so the kidney cells use no energy for this stage.
As the filtrate flows along the tubule, the substances the body needs are taken back into the blood in the surrounding capillaries.
It is selective because useful substances are taken back while urea is mostly left in the tubule.
Some substances are actively moved from the blood into the tubule, for example certain drugs and excess hydrogen and potassium ions. This helps control the pH and salt balance of the blood.
The fluid that reaches the pelvis, now called urine, is water with urea, salts and other wastes dissolved in it.
| Substance (typical grams per 100 cm³) | Blood plasma | Glomerular filtrate | Urine |
|---|---|---|---|
| Proteins | 8.0 | 0 | 0 |
| Glucose | 0.1 | 0.1 | 0 |
| Urea | 0.03 | 0.03 | 2.0 |
| Salts | 0.72 | 0.72 | 1.5 |
Worked example
Use the table to explain three facts.
Doctors test urine for clues. Glucose in the urine can be a sign of diabetes: the blood glucose is so high that the tubules cannot reabsorb it all. Protein in the urine suggests the glomeruli are damaged and letting large molecules through.
Tip
Filter everything small → Reabsorb what is useful → Secrete extra waste → what is left is urine.
In table questions, compare two columns and name the stage responsible: "absent from filtrate → too big to filter"; "absent from urine → reabsorbed".
Name two substances that pass into Bowman's capsule and two that do not.
Solution: Pass: any two of water, glucose, amino acids, salts, urea. Do not pass: blood cells and plasma proteins.
Why do the cells of the proximal convoluted tubule contain many mitochondria?
Solution: They reabsorb glucose, amino acids and salts by active transport, which needs a lot of energy from respiration. Mitochondria are where aerobic respiration releases that energy.
A person forms 150 dm³ of filtrate in a day and produces 1.5 dm³ of urine. What percentage of the filtrate is reabsorbed?
Solution: (150 − 1.5) ÷ 150 × 100 = 99%.
A urine test shows glucose. Suggest a cause and explain it.
Solution: Diabetes. Blood glucose is so high that more glucose is filtered than the tubule cells can reabsorb, so some is left in the urine.
Remember
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