Loading…
Loading…
Loading…
OCR GCSE Biology B · J257
OCR J257 Check the specification (PDF) (opens in a new tab)
Cells work properly only when their surroundings remain within a narrow range of conditions. If blood plasma becomes too concentrated, water leaves cells by and they shrink. If it becomes too dilute, water enters cells and they swell; some may burst. keeps the water content and concentration of body fluids within safe limits.
The kidneys contribute by filtering water and urea from the blood into kidney tubules and then reabsorbing as much water as the body requires. The amount reabsorbed is adjusted rather than remaining fixed.
Water balance is controlled through a coordinated sequence:
The hypothalamus therefore acts in both detection and processing. The pituitary releases the hormonal signal, while the kidney tubules are the effectors that change water reabsorption.
ADH acts on collecting ducts to change their permeability to water; the resulting change in water reabsorption opposes the original osmotic challenge.
Water loss during sweating, insufficient drinking or a salty meal can leave the blood with a low water content, so it becomes too concentrated. Osmoreceptors detect this challenge. The hypothalamus causes the pituitary gland to release more ADH.
More ADH makes the collecting ducts more permeable to water. More water therefore moves from the filtrate back into the blood by osmosis. Less water is lost, producing a small volume of concentrated urine. This raises the water content of the blood towards its normal level.
After a person drinks a large quantity of water, the blood may have a high water content and become too dilute. Osmoreceptors detect the change, and the hypothalamus causes the pituitary gland to release less ADH.
The collecting ducts become less permeable to water, so less water is reabsorbed into the blood. More remains in the filtrate and is excreted as a large volume of dilute urine. This lowers the blood’s water content towards its normal level.
In both challenges, the response opposes the original change. Concentrated blood leads to water conservation, whereas dilute blood leads to greater water loss. As the concentration returns towards its normal level, the original stimulus becomes smaller, so the corrective change in ADH release is reduced. This is negative feedback: it limits departure from the normal condition rather than amplifying it.
This material is required for separate science only in this OCR course.
Osmoreceptors in hypothalamus → processing in hypothalamus → pituitary changes ADH release → collecting-duct permeability changes → water reabsorption changes → blood concentration returns towards normal.
The response opposes the osmotic challenge. As normal blood concentration is restored, the stimulus and corrective response decrease.
OCR qualification: separate science only.
For an explanation question, write the whole causal chain: osmotic challenge → change in blood concentration → osmoreceptors in the hypothalamus → altered ADH release by the pituitary gland → altered collecting-duct permeability → altered water reabsorption and urine.
Match the direction of every change to the challenge. Dehydration causes more ADH and more water reabsorption; excess water causes less ADH and less water reabsorption.
Do not say that ADH itself carries water back into the blood. ADH changes the permeability of kidney tubules, allowing more or less water to be reabsorbed by osmosis.
Make negative feedback explicit: the response opposes the original change, and the corrective response decreases as the blood returns towards its normal concentration.
It can see this note and what your specification asks for.
Answers are free with an account.
1 free note left this week
Get more free notesYour answer sets when this note comes back for review, or marks it complete.
Was this note helpful?
Log in to tell us whether it helped, or to report a problem.
Last updated
Osmoregulation
The control of the water content and concentration of body fluids within narrow limits.
Osmoreceptor
A cell that detects changes in the concentration of body fluids.
Hypothalamus
A region of the brain that contains osmoreceptors, processes information about blood concentration and controls the pituitary gland’s release of ADH.
ADH
Antidiuretic hormone; a hormone released into the blood by the pituitary gland that increases the permeability of kidney tubules to water.
Negative feedback
A control mechanism in which a change produces responses that oppose the original change and return a condition towards its normal level.
Osmosis
The movement of water through a partially permeable membrane from a more dilute solution towards a more concentrated solution.
Reabsorption
The movement of a substance from a kidney tubule back into the blood.
Put your knowledge into practice — try past paper questions for Biology B
Osmoregulation
The control of the water content and concentration of body fluids within narrow limits.
Osmoreceptor
A cell that detects changes in the concentration of body fluids.
Hypothalamus
A region of the brain that contains osmoreceptors, processes information about blood concentration and controls the pituitary gland’s release of ADH.
ADH
Antidiuretic hormone; a hormone released into the blood by the pituitary gland that increases the permeability of kidney tubules to water.
Negative feedback
A control mechanism in which a change produces responses that oppose the original change and return a condition towards its normal level.
Osmosis
The movement of water through a partially permeable membrane from a more dilute solution towards a more concentrated solution.
Reabsorption
The movement of a substance from a kidney tubule back into the blood.