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AQA GCSE Biology · 8461
AQA 8461 · 4.5.3.3 Water and Nitrogen Balance Check the specification (PDF) (opens in a new tab)
Explain the effect of osmotic changes on cells.
Describe the loss of water, ions, and urea via lungs, skin, and kidneys.
Explain the formation of urea from excess amino acids (HT only).
Describe the function of kidneys in filtration and selective reabsorption.
Explain the control of water levels by ADH and negative feedback (HT only).
Describe kidney dialysis and organ transplants.
Cells work efficiently only when their surroundings remain within narrow limits. The kidneys help maintain these conditions by controlling how much water and some dissolved ions remain in the blood. They also remove urea. They do this by filtering the blood and then selectively reabsorbing substances that the body needs.
Filtration alone cannot produce the final urine because useful small molecules are filtered alongside wastes. Selective reabsorption is therefore needed to recover useful substances.
Blood flows through the kidneys under pressure. Small molecules leave the blood and form the filtrate. These include:
Blood cells and large molecules such as proteins are too large to cross the normal filtration barrier, so they remain in the blood. At this stage, the filtrate is not yet urine because it still contains useful substances.
As the filtrate passes along the kidney tubules, substances needed by the body are returned to the blood. In a healthy kidney:
Urea is a waste product and is not usefully recovered. Water, ions and urea that are not reabsorbed form urine. The amounts of water and ions excreted can vary because the kidneys adjust their reabsorption.
Filtration places small molecules into the filtrate; selective reabsorption then returns useful substances to the blood. At Higher Tier, ADH changes kidney-tubule permeability and therefore the amount of water reabsorbed.
The overall sequence is:
blood → filtration → filtrate containing small molecules → selective reabsorption → urine containing substances not reabsorbed
Detailed knowledge of the other parts of the urinary system, the internal structure of the kidney or the structure of a nephron is not required for this AQA topic.
AQA requires you to translate tables and bar charts involving glucose, ions and urea before and after filtration. Begin by checking what the two stages mean. Blood before filtration, newly formed filtrate and final urine are not interchangeable.
The following dataset is deliberately simple. It shows relative concentrations in equal-volume samples before filtration and in newly formed filtrate after filtration:
| Substance | Before filtration / arbitrary units | Newly formed filtrate / arbitrary units |
|---|---|---|
| Glucose | 6 | 6 |
| Ions | 9 | 9 |
| Urea | 3 | 3 |
A numerical table translated into a grouped bar chart. The paired bars reproduce the table values exactly.
To construct the grouped bar chart:
In this illustration, each small dissolved substance can cross the filtration barrier, so its paired bars are equal. This does not mean final urine has the same composition as blood: selective reabsorption happens after filtration. Glucose is then reabsorbed, while some ions and water are reabsorbed according to the body’s needs and urea remains for excretion.
When translating a bar chart back into a table, copy the category names, read each bar against the scale, record the units in the heading and keep the two stages in separate columns.
The water content of the blood can change through drinking, sweating and other water losses. If too much water is lost, the blood becomes too concentrated. The pituitary gland then releases more ADH into the blood.
ADH travels to the kidneys and increases the permeability of the kidney tubules to water. More water can therefore pass from the tubules back into the blood. Less water remains in the urine, so a smaller volume of more concentrated urine is produced.
If the blood contains too much water, less ADH is released. The kidney tubules become less permeable to water, so less water is reabsorbed. A larger volume of more dilute urine is produced.
Negative feedback opposes the original change:
The reverse response occurs when the blood contains too much water. In both directions, the response reduces the deviation rather than making it larger.
Blood too concentrated:
more ADH → tubules more permeable → more water reabsorbed → small volume of concentrated urine
Blood too dilute:
less ADH → tubules less permeable → less water reabsorbed → large volume of dilute urine
This is negative feedback because the response opposes the original change and returns the blood towards its normal water level.
Use reabsorbed, not simply “absorbed”: a substance is first filtered out of the blood and then taken back into it.
Do not describe filtration as removing only wastes. Small useful substances, including glucose, water and ions, enter the filtrate too; selective reabsorption then returns what the body needs.
When interpreting kidney data, distinguish amount from concentration and identify whether the data refer to blood, newly formed filtrate or final urine.
For a bar chart, transfer every numerical value exactly, include units, use a linear scale and label the categories clearly.
In an ADH question, link each step: blood too concentrated → more ADH → tubules more permeable → more water reabsorbed → smaller volume of more concentrated urine.
Remember that ADH control is Higher Tier only for this specification.
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Last updated
Filtration
The removal of small molecules from the blood to form a fluid called filtrate.
Selective reabsorption
The return of particular useful substances from the kidney tubules to the blood according to the body’s needs.
Filtrate
The fluid formed when small molecules are filtered out of the blood; useful substances can still be reabsorbed from it.
ADH
A hormone released by the pituitary gland that increases the permeability of kidney tubules to water.
Negative feedback
A control process in which a departure from the normal level causes responses that oppose the change and restore conditions towards normal.
Urine
The liquid containing water, ions and urea that remains after filtration and selective reabsorption and is excreted from the body.
Put your knowledge into practice — try past paper questions for Biology
Filtration
The removal of small molecules from the blood to form a fluid called filtrate.
Selective reabsorption
The return of particular useful substances from the kidney tubules to the blood according to the body’s needs.
Filtrate
The fluid formed when small molecules are filtered out of the blood; useful substances can still be reabsorbed from it.
ADH
A hormone released by the pituitary gland that increases the permeability of kidney tubules to water.
Negative feedback
A control process in which a departure from the normal level causes responses that oppose the change and restore conditions towards normal.
Urine
The liquid containing water, ions and urea that remains after filtration and selective reabsorption and is excreted from the body.