✅ In the filtrate
Water, glucose, urea and ions (such as sodium ions). All of these are small enough to pass out of the blood.
Excretion » Ultrafiltration and Glucose Reabsorption
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 2.75, 2.77
- How blood is filtered under pressure in the Bowman's capsule (ultrafiltration)
- What is in the glomerular filtrate and what stays in the blood
- Why all the glucose is taken back at the proximal convoluted tubule
- How blood, filtrate and urine compare
Note: this lesson is Paper 2 only. It will not be asked on Paper 1.
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Unlock This CourseUltrafiltration: squeezing the blood
In the lesson Structure of a Nephron you met the Bowman's capsule and the glomerulus, the ball of capillaries inside it. Now let us see what happens there. This is where the kidney makes the very first liquid that will become urine.
Key terms:
- Ultrafiltration: filtering the blood under high pressure, so that small molecules are forced out of the capillaries into the Bowman's capsule.
- Glomerular filtrate: the liquid that is forced out of the blood and into the Bowman's capsule.
- Selective reabsorption: taking useful substances back into the blood from the filtrate.
Blood reaches the glomerulus from the renal artery. The blood pressure inside the glomerulus is high. This is partly because the blood vessel taking blood away from the glomerulus is narrower than the one bringing it in, so blood is squeezed in a small space.
The high pressure pushes water and small molecules out through the thin walls of the capillaries. They pass into the Bowman's capsule. Large molecules and blood cells are too big to fit through, so they stay in the blood.
Think of a sieve
Imagine pouring a bowl of soup through a tea strainer while pressing down hard. The thin liquid is forced through and the lumps stay behind. The glomerulus works like a pressure sieve. It sorts by size, not by whether a substance is useful or waste.
What is in the glomerular filtrate?
The filtrate contains the small molecules that can squeeze through. You must know four things in the filtrate.
❌ Stays in the blood
Blood cells (red cells, white cells and platelets) and large proteins. They are too big to pass through.
Notice that the filtrate is not urine yet. It still holds useful things such as glucose and a lot of water. These need to be taken back, which is the job of the next part of the nephron.
Selective reabsorption of glucose
From the Bowman's capsule, the filtrate flows into the proximal convoluted tubule. Here the body takes back the substances it needs. This is called selective reabsorption, because it only takes back what is useful.
The spec asks you to understand why all of the glucose is reabsorbed here. The reason is simple.
- Glucose is a useful molecule. Cells use it in respiration to release energy.
- The body cannot afford to lose it, so all of it is taken back into the blood.
- This is why the urine of a healthy person contains no glucose.
Glucose is reabsorbed by active transport. As you saw in the lesson on active transport, this moves substances against a concentration gradient using energy from respiration. This is needed because the glucose is taken back even when its concentration in the blood is already high. The cells lining this tubule have many mitochondria to supply the energy.
Why test urine for glucose?
If the blood glucose level is far too high, the tubule cannot take back all of the glucose. Some is left in the urine. Doctors can test urine for glucose as one sign that something may be wrong.
Blood, filtrate and urine compared
These three boxes show how the liquids differ. Compare what is in each box.
🩸 Blood
Blood cells, large proteins, glucose, urea, water and ions.
🔄 Glomerular filtrate
Water, glucose, urea and ions. No blood cells and no large proteins.
🚽 Urine (healthy)
Water, urea and ions. No glucose, no blood cells and no large proteins.
Two things stand out. Blood cells and large proteins are missing from the filtrate, because of ultrafiltration. Glucose is in the filtrate but missing from the urine, because of selective reabsorption. How much water and how many ions end up in the urine is the topic of the next lesson, Water Reabsorption, ADH and Urine.
Worked example
Question: A sample of liquid from the Bowman's capsule contains glucose. A sample of urine from the same healthy person does not. Explain why.
Answer: Glucose is small, so it is forced out of the blood into the Bowman's capsule by high pressure. Glucose is useful to the body, so all of it is taken back into the blood at the proximal convoluted tubule by active transport. None is left to pass into the urine.
Common mistakes
Saying the kidney filters out only waste. Ultrafiltration sorts by size, so useful glucose is filtered out too and must be taken back later. Another slip is saying blood cells are in the filtrate. They are too big. Finally, do not say glucose is reabsorbed by diffusion. It is taken back by active transport.
Exam-style question
(a) Name the structure in which ultrafiltration happens. [1 mark]
(b) Explain how ultrafiltration forces small molecules out of the blood. [2 marks]
(c) Name two substances found in the glomerular filtrate. [2 marks]
(d) Explain why a healthy person's urine contains no glucose. [2 marks]
Model answer
(a) The Bowman's capsule (with the glomerulus). (1)
(b) The blood pressure in the glomerulus is high. (1) This forces water and small molecules through the capillary walls into the Bowman's capsule. (1)
(c) Any two from: water, glucose, urea, ions. (2)
(d) All the glucose is useful to the body, so it is reabsorbed (1) into the blood at the proximal convoluted tubule by active transport. (1)
Exam tip
When asked why glucose is not in urine, say two things: it is reabsorbed, and it is reabsorbed because it is useful. Name the proximal convoluted tubule and active transport for full marks.