Cambridge O Level Biology · Syllabus 5090 · Excretion
Filtration
What is Filtration?
In the kidney, the movement of small molecules out of the blood in the glomerulus and into the Bowman's capsule, caused by the high pressure of the blood in the glomerulus. Water, glucose, urea and ions are small enough to pass through the capillary wall and the wall of the capsule and enter the capsule as filtrate, while blood cells and large plasma proteins are too large to pass and remain in the blood. Filtration is not selective: it separates by size alone, so useful substances such as glucose are filtered out along with waste, and are recovered afterwards by selective reabsorption.
This definition is part of the Excretion chapter in Cambridge O Level Biology.
Filtration in context
Excretion is the removal from organisms of toxic materials, the waste products of metabolism, and substances in excess of requirements. The two metabolic wastes you must know are carbon dioxide, made by respiration in every cell and excreted by the lungs in expired air, and urea, made in the liver from excess amino acids and removed from the blood by the kidneys in urine. The kidney does this in three steps: filtration at the glomerulus forces small molecules out of the blood, selective reabsorption returns all the glucose, some ions and most of the water to the blood, and what is left — urea, excess water and excess ions — is urine.
Common mistakes with Filtration
- “Filtration happens because molecules diffuse into the Bowman's capsule.” Why wrong Diffusion needs a concentration gradient and no pressure. Filtration is a physical push driven by blood pressure, and it moves everything small enough regardless of concentration. Say instead Blood in the glomerulus is under high pressure, which forces small molecules through the capillary and capsule walls.
- “Selective reabsorption removes useful substances from the blood.” Why wrong The direction is backwards. Reabsorption puts substances back into the blood, having previously lost them at filtration. Say instead Selective reabsorption returns useful substances from the tubule to the blood in the surrounding capillaries.
Questions students ask about Filtration
How does filtration work in the kidney?
Blood in the glomerulus is under high pressure, and that pressure forces small molecules through the capillary wall and the wall of the Bowman's capsule. Water, glucose, urea and ions pass through; blood cells and large plasma proteins are too large and remain in the blood. Filtration separates by size alone, so it is not selective.
What is the difference between filtrate and urine?
Filtrate is the fluid in the Bowman's capsule immediately after filtration; it contains water, glucose, urea and ions. Urine is what remains after selective reabsorption has returned all the glucose, some ions and most water to the blood, so urine contains urea, excess water and excess ions but no glucose.