Explain filtration and selective reabsorption
Describe ultrafiltration in Bowman’s capsule and selective reabsorption in the proximal tubule.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not a practical skills certification.
Before you begin
- A control system detects a change, communicates information and changes an effector response.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where does most glucose reabsorption occur?
What you will learn
- Explain formation of glomerular filtrate by ultrafiltration
- Describe selective reabsorption in the proximal convoluted tubule
Build the explanation
High hydrostatic pressure in glomerular capillaries forces water and small dissolved substances through the filtration barrier into Bowman’s capsule. Blood cells and most large plasma proteins remain in the blood. In the proximal convoluted tubule, useful substances such as all filtered glucose, amino acids, ions and much water are selectively reabsorbed into nearby capillaries. The fluid that remains continues along the nephron.
Apply the limits
Filtration is driven by pressure at the glomerulus; selective reabsorption returns useful materials to blood. Do not say red blood cells or large plasma proteins normally pass into glomerular filtrate.
Pause and explain
A learner says useful glucose is removed from blood by the glomerulus and then destroyed. Correct the pathway.
Worked example
Why is glucose present in filtrate but normally absent from healthy urine?
Show the worked solution
- Glucose is small enough to enter the filtrate.
- It is selectively reabsorbed in the proximal convoluted tubule.
Answer Glucose is filtered and then normally reabsorbed in the proximal convoluted tubule.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Compare ultrafiltration with selective reabsorption.
Give me a hint
Name the relevant structure or signal, then state its effect.
Compare my reasoning
- Ultrafiltration moves water and small solutes from blood into Bowman’s capsule under pressure.
- Selective reabsorption returns useful substances from the tubule to blood.
Ultrafiltration forms filtrate at Bowman’s capsule under glomerular pressure. Selective reabsorption in the proximal convoluted tubule returns useful substances to the blood.
Look for these in your work
- Correct mechanism
- Direction of response explained
A sample of filtrate contains glucose and red blood cells. Which result is expected to be absent in normal ultrafiltration, and why?
Give me a hint
Separate the observed change from the body's response.
Compare my reasoning
- Red blood cells are too large to pass the filtration barrier.
- Glucose is small enough to be filtered and is normally reabsorbed later.
Red blood cells should remain in glomerular blood because they are too large to pass the filtration barrier. Glucose can enter filtrate and is normally reabsorbed in the proximal tubule.
Look for these in your work
- Evidence interpreted
- Control step identified
A learner says useful glucose is removed from blood by the glomerulus and then destroyed. Correct the pathway.
Give me a hint
Apply the mechanism to the new context without claiming more than the evidence shows.
Compare my reasoning
- Glucose is filtered into Bowman’s capsule.
- It is normally selectively reabsorbed into blood in the proximal tubule.
Glucose enters filtrate during ultrafiltration but is normally selectively reabsorbed into nearby blood capillaries in the proximal convoluted tubule; it is not destroyed by filtration.
Look for these in your work
- New context applied
- Limits preserved
Common mistakes
- Confusing filtration with active secretion
- Saying the kidney reabsorbs all filtrate
What can you explain now?
Where does most glucose reabsorption occur?
Compare with the explanation
The proximal convoluted tubule
Filtered glucose is normally selectively reabsorbed there.
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Make it stick
What enters glomerular filtrate, and what is selectively returned to blood?
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