AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
The kidneys produce urine by filtration of the blood and selective…
Full specification wording
Maintaining water and nitrogen balance in the body (biology only): The kidneys produce urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water.
Urine is produced in two main stages.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 11 of 17
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Urine is produced in two main stages. First, filtration in the kidney forces small molecules from the blood into the kidney tubule: water, glucose, ions and urea pass through, while blood cells and large proteins do not. Second, selective reabsorption returns useful substances to the blood, including all of the glucose, some ions and some water, while urea and excess ions and water remain to form urine. For example, glucose is normally absent from urine because it is completely reabsorbed, whereas urea is always present because it is not reabsorbed. This process maintains water and nitrogen balance and prevents useful materials being lost.
What earns the marks
- Filtration of blood occurs in the kidneys and forms a filtrate.
- Small molecules such as water, glucose, ions and urea enter the filtrate.
- Blood cells and large proteins are not filtered into the filtrate.
- Selective reabsorption returns useful substances to the blood.
- Glucose is fully reabsorbed, so it is normally absent from urine.
- Some ions and some water are reabsorbed, while urea is not reabsorbed and is excreted in urine.
Common mistakes and how to put them right
Saying all filtered substances are reabsorbed.
Put it right by stating reabsorption is selective.
Stating that glucose is normally present in urine.
Put it right by stating glucose is fully reabsorbed.
Confusing filtration with reabsorption.
Put it right by describing filtration as movement into the tubule and reabsorption as return to the blood.
Exam tips
- Use the two-stage structure: filtration then selective reabsorption.
- Name substances reabsorbed and substances excreted to show selectivity.
- Link the process to water and nitrogen balance in the body.
Try the lesson
The start of the Maintaining water and nitrogen balance in the body (biology only): The kidneys produce urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water. lesson, made from checked teaching notes for this specification point.
Have a go first
How do the kidneys produce urine from the blood?
Listen
Urine is produced in two main stages. First, filtration in the kidney forces small molecules from the blood into the kidney tubule: water, glucose, ions and urea pass through, while blood cells and large proteins do not. Second, selective reabsorption returns useful substances to the blood, including all of the glucose, some ions and some water, while urea and excess ions and water remain to form urine. For example, glucose is normally absent from urine because it is completely reabsorbed, whereas urea is always present because it is not reabsorbed. This process maintains water and nitrogen balance and prevents useful materials being lost. A strong answer earns marks for these points. Filtration of blood occurs in the kidneys and forms a filtrate. Small molecules such as water, glucose, ions and urea enter the filtrate. Blood cells and large proteins are not filtered into the filtrate. Selective reabsorption returns useful substances to the blood. A common slip: Saying all filtered substances are reabsorbed. Instead, stating reabsorption is selective. Exam tip: Use the two-stage structure: filtration then selective reabsorption.
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Key words
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Filtration
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Selective reabsorption
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See every definition
- Filtration
- The process in the kidneys that forces small molecules from the blood into the kidney tubule, while blood cells and large proteins do not pass through.
- Selective reabsorption
- The process that returns useful substances to the blood, including all of the glucose, some ions and some water, while urea and excess ions and water remain to form urine.
Label the diagram
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1Box 1
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3Box 3
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