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Edexcel GCSE Biology · 1BI0
Edexcel 1BI0 · 7.20B Check the specification (PDF) (opens in a new tab)
The body needs to keep the water content of its blood within a suitable range. The kidneys help by adjusting how much water returns to the blood and how much leaves in urine. As fluid passes through a kidney's , some of its water can be reabsorbed into the blood. The remaining fluid becomes urine.
The hypothalamus in the brain detects changes in the water content of the blood. When there is too little water, the pituitary gland releases more ADH (antidiuretic hormone) into the bloodstream. ADH acts on the collecting duct, making it more permeable to water. More water can therefore pass out of the duct and be reabsorbed into the blood. Less water is lost, so a smaller volume of more concentrated urine is produced.
When the blood contains too much water, less ADH is released. The collecting duct becomes less permeable to water, so less water is reabsorbed. More water leaves the body as a larger volume of more dilute urine.
Changing ADH release changes collecting-duct permeability, controlling how much water returns to the blood.
These opposite responses help return the blood's water content towards its normal level. This is negative feedback: a fall in water content leads to greater water retention, while a rise leads to greater water loss. ADH does not make water; it controls how much of the water already in the kidney fluid is returned to the blood.
Too little water in blood: more ADH released → collecting duct more permeable → more water reabsorbed → small volume of concentrated urine.
Too much water in blood: less ADH released → collecting duct less permeable → less water reabsorbed → large volume of dilute urine.
The hypothalamus detects the change; the pituitary gland releases ADH. The responses counteract the original change, so this is negative feedback.
Link each change in ADH to collecting-duct permeability, water reabsorption and the volume and concentration of urine.
Do not say that ADH adds water to the blood: it changes how much water is reabsorbed from the fluid that will become urine.
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ADH (antidiuretic hormone)
A hormone released by the pituitary gland that increases the permeability of the kidney's collecting ducts to water.
Collecting duct
A tube in the kidney through which fluid passes as it becomes urine; water can be reabsorbed from it into the blood.
Permeability
The ease with which a substance can pass through a surface.
Reabsorption
The return of a substance from fluid in the kidney to the blood.
Negative feedback
A control process in which a change triggers a response that counteracts that change.
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ADH (antidiuretic hormone)
A hormone released by the pituitary gland that increases the permeability of the kidney's collecting ducts to water.
Collecting duct
A tube in the kidney through which fluid passes as it becomes urine; water can be reabsorbed from it into the blood.
Permeability
The ease with which a substance can pass through a surface.
Reabsorption
The return of a substance from fluid in the kidney to the blood.
Negative feedback
A control process in which a change triggers a response that counteracts that change.