Ultrafiltration and Selective Reabsorption

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.75 — Ultrafiltration in the Bowman's capsule and the composition of the filtrate (bold B statement — Paper 2 only)
• 2.76 — How water is reabsorbed from the collecting duct (bold B statement — Paper 2 only)
• 2.77 — Why selective reabsorption of glucose occurs at the proximal convoluted tubule (bold B statement — Paper 2 only)

Specification section 2(i) Excretion — also taught in Excretion: Getting Rid of What You Made, The Kidney and the Nephron, ADH and the Water Balance of the Blood.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

The previous lesson established WHAT the kidney does: filter almost everything, then reclaim what is worth keeping. This one is about how each half of that actually works — and both halves turn out to be driven by something you have already met, one by pressure and one by active transport.

Ultrafiltration — filtering by size, under pressure

1. Blood enters the glomerulus through a wide vessel and leaves through a narrower one. Fluid arriving faster than it can leave has nowhere to go, so the pressure inside rises sharply.
2. That pressure forces fluid out through the capillary wall and the basement membrane surrounding it, into Bowman's capsule.
3. The basement membrane is the actual sieve, and it discriminates by SIZE and nothing else.
4. SMALL molecules are forced through: water, glucose, amino acids, urea and mineral ions.
5. LARGE ones cannot pass: blood cells and plasma proteins stay in the blood.
6. So the filtrate is simply blood plasma minus its proteins and cells. Two consequences follow, and both are examined constantly: glucose IS in the filtrate, and protein is NOT.

The word 'ultra' is doing real work there. This is not filtering in the sense of a coffee filter catching grounds — it is separation at the scale of individual molecules, sorting glucose from protein purely by how big they are. Nothing recognises anything. A molecule's fate at this stage depends on its size and on nothing else whatever.

Selective reabsorption of glucose — and why it must cost energy

1. The filtrate now contains all the glucose that was in the blood, and the body cannot afford to lose any of it — glucose is the fuel for respiration in every cell.
2. So ALL of it is reabsorbed, in the proximal convoluted tubule, back into the blood.
3. Here is the problem. At the start, glucose is at a similar concentration on both sides. As reabsorption proceeds, the concentration in the tubule falls BELOW that in the blood.
4. From that point on, diffusion would carry glucose the wrong way. Only active transport can move it against its concentration gradient.
5. So carrier proteins in the tubule wall bind each glucose molecule and carry it across, using energy released by respiration — which is why these cells are packed with mitochondria, and why their surface is folded into microvilli to give a large surface area for the carriers to sit in.

Those carrier proteins matter more than they look. There is a fixed number of them, so there is a maximum rate at which glucose can be reclaimed. Normally that capacity is comfortably more than enough. But if the blood glucose concentration rises far above normal, more glucose is filtered than the carriers can handle — they become saturated, working flat out and still unable to keep up — and the surplus simply travels on and appears in the urine. Glucose in the urine is not a kidney fault; it is the sign that the kidney has been asked to do more than any fixed number of carriers could.

Water reabsorption — and where osmoregulation is decided

1. Reabsorbing all that glucose and all those ions leaves the blood more concentrated than the fluid in the tubule.
2. So water follows, by osmosis, from the tubule back into the blood — no energy required, because it is moving down a water concentration gradient.
3. Most of this happens early on, but the crucial part happens at the END, in the collecting duct.
4. There, the amount of water reabsorbed is VARIABLE. The wall can be made more or less permeable to water depending on what the body needs.
5. Reabsorb a lot and you produce a small volume of concentrated urine; reabsorb little and you produce a large volume of dilute urine. That single adjustable step is how the body controls its own water content, and the next lesson is about what controls it.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Ultrafiltration

Filtration at a molecular scale, in which high blood pressure in the glomerulus forces small molecules out through the basement membrane into Bowman's capsule, while blood cells and plasma proteins are too large to pass.

Learn this definition · Selective reabsorption

The return to the blood of useful substances from the filtrate, notably all of the glucose, which is reabsorbed at the proximal convoluted tubule by active transport using carrier proteins and energy from respiration.

Blood plasma, filtrate and urine — the table examiners build questions from

Ultrafiltration — the sequence to write

1. Blood enters the glomerulus through a wide vessel and leaves through a narrower one, so the pressure inside rises.
2. This high pressure forces fluid out through the basement membrane into Bowman's capsule.
3. Small molecules — water, glucose, amino acids, urea and ions — are forced through.
4. Blood cells and plasma proteins are too large to pass and remain in the blood.

Why glucose reabsorption needs active transport

1. All the glucose in the blood is filtered into the tubule, and all of it must be recovered.
2. As reabsorption proceeds, the glucose concentration in the tubule falls below that in the blood.
3. Moving glucose from a lower to a higher concentration is against the concentration gradient, so diffusion cannot do it.
4. Carrier proteins in the tubule wall transport the glucose across, using energy released by respiration.
5. The cells therefore contain many mitochondria and have microvilli to increase the surface area available.

Adaptations of the proximal convoluted tubule

• Microvilli on the cell surface → a large surface area for absorption
• Many carrier proteins in the membrane → a high rate of active transport
• Many mitochondria → plenty of energy released by respiration for that active transport
• A dense capillary network alongside → reabsorbed substances are carried away, keeping gradients favourable

Model answer [4 marks]

Explain how ultrafiltration occurs in the Bowman's capsule, and state what the filtrate contains. [4]

Blood enters the glomerulus through a wider vessel than it leaves by, so the pressure inside rises. This high pressure forces fluid out through the basement membrane into Bowman's capsule. Only small molecules pass through: water, glucose, amino acids, urea and mineral ions. Blood cells and plasma proteins are too large and remain in the blood, so the filtrate is plasma without its proteins or cells.

Model answer [4 marks]

Explain why glucose is reabsorbed by active transport rather than by diffusion. [4]

All of the glucose must be reabsorbed, because the body cannot afford to lose its fuel. As reabsorption proceeds the glucose concentration in the tubule falls below that in the blood, so glucose must be moved against its concentration gradient. Diffusion only moves substances down a gradient, so it cannot do this. Carrier proteins therefore transport the glucose across using energy released by respiration.

Mark-losing trap. Glucose IS in the filtrate and is absent from urine only because it is all reabsorbed. It is not too large to be filtered.

Mark-losing trap. Protein is absent from the filtrate because it is too LARGE, not because it is reabsorbed. The two reasons are not interchangeable.

Mark-losing trap. Ultrafiltration is driven by blood pressure. Water reabsorption is osmosis. Glucose reabsorption is active transport. Three different mechanisms.

Mark-losing trap. Glucose in urine means the carriers are saturated, not that the kidney is broken. There is a fixed number of them.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · Identify [1 mark] — Which substance is present in blood plasma but absent from the filtrate in Bowman's capsule?
2. Grade 7 · Explain [2 marks] — Glucose is present in the filtrate but absent from urine. Explain why.
3. Grade 8 · Explain [4 marks] — Explain how ultrafiltration produces the filtrate in Bowman's capsule. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Calculate [4 marks] — In one day 180 dm³ of filtrate is produced, containing urea at 0.30 g/dm³. The person produces 1.5 dm³ of urine containing urea at 20 g/dm³. Calculate the percentage of the filtered urea that is actually excreted, to one decimal place.
5. 9+ · Analyse [5 marks] — A person with untreated diabetes has a very high blood glucose concentration. Tests show glucose in their urine, and they produce an unusually large volume of urine and are constantly thirsty. Their kidneys are structurally normal. Select every statement that belongs in a full-mark explanation.

The people behind this science

Two ways into the same idea — the one who found the tubule before anyone knew what it did, and the one who explained why water follows without being pushed. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Marcello Malpighi — the one who found the tubule before anyone knew what it did

Malpighi traced the fine tubules running from each of the knots of vessels he had found, and could describe their course precisely without any way of discovering what passed along them. This lesson is the answer to a question he could pose but not settle — and it is worth knowing that the structure was mapped almost two centuries before the mechanism was understood.

• “What did you imagine was travelling along those coiled tubes?”
• “How did you trace a tubule's path without losing it?”
• “Why do you think the tubules are coiled rather than straight?”
• “What would you have needed in order to find out what they did?”

Albert Einstein — the one who explained why water follows without being pushed

Reabsorbing glucose and ions costs the kidney energy, but the water that follows them costs nothing at all — it moves by osmosis, driven only by random molecular motion and a concentration difference. Einstein's 1905 work put that random motion on a mathematical footing and explained why a difference in concentration produces a reliable net flow with nothing doing any pushing.

• “Why does water move on its own when the solute concentration changes?”
• “How can random motion produce a flow in one particular direction?”
• “Does moving water back into the blood cost the body any energy?”
• “What decides how fast water crosses a membrane like this?”

Carry on through the course

• Previous lesson: The Kidney and the Nephron
• Next lesson: ADH and the Water Balance of the Blood
• Levels of Organisation — From organelle to organism — the five-step ladder that every later topic in biology is built on.
• Comparing Cells, and Stem Cells — What plant and animal cells share and where they differ — then why a cell specialising is one of the most powerful ideas in biology.
• Carbohydrates, Proteins and Lipids — The three big food molecules — what they are made of, what they are built from, and how to test for them.
• Enzymes: Catalysts With a Shape — Why a protein with a hole in it speeds up a reaction, and why heat destroys that ability permanently.
• Enzymes and pH — Why acid ruins one enzyme and is the only condition another will work in — and how to find an enzyme's best pH by experiment.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification