OCR GCSE Biology B (J257) — Human Physiology
Understanding water balance explains why drinking too much pure water can be just as dangerous as severe dehydration. Animal cells do not have a protective cell wall to stop them from changing shape. If the blood's water potential is too high, water enters cells by osmosis, causing them to swell and burst in a process called lysis.
Conversely, if the water potential is too low, water leaves the cells, causing them to shrink and shrivel, known as crenation. To prevent this cellular damage, the body relies on osmoregulation to maintain a perfectly stable internal environment.
The control centre for regulating water is the hypothalamus in the brain. It contains specialised sensory neurons called osmoreceptors that constantly monitor the water potential of the blood plasma.
The mechanism of detection relies directly on osmosis. When blood water potential is low (dehydration), water leaves the osmoreceptors, causing them to shrink. This shrinkage triggers them to send electrical nerve impulses to coordinate a response. If blood water potential is high, the osmoreceptors swell, which inhibits their activity and stops them from sending impulses.
Once the hypothalamus processes these impulses, it coordinates with the pituitary gland. The hypothalamus produces a chemical messenger called Antidiuretic Hormone (ADH), but it is stored and released into the bloodstream by the posterior section of the pituitary gland.
ADH travels through the blood plasma to its target organs: the kidneys. Specifically, ADH acts on the collecting ducts within the kidney tubules. It increases the permeability of these ducts by causing protein channels called aquaporins to be inserted into the cell membranes.
When permeability increases (due to high levels of ADH), more water moves from the tubule filtrate back into the blood by osmosis. This process of water returning to the bloodstream is called . As a result of this increased reabsorption, the body produces a small volume of concentrated (dark) urine, which helps to conserve water and restore blood water potential.
Conversely, if blood water potential is high, the pituitary gland releases less ADH, and the collecting ducts become less permeable. Less water is reabsorbed into the blood and more remains in the tubule filtrate, resulting in the production of a large volume of dilute (pale) urine.
The entire process operates as a negative feedback loop. A change in a physiological factor triggers a response that reverses the direction of that change, restoring the balance to a set point.
Examiners expect a step-by-step causal chain when explaining this response.
| Stage | Scenario A: Dehydration (e.g., Sweating heavily) | Scenario B: Overhydration (e.g., Excessive drinking) |
|---|---|---|
| Stimulus | Blood water potential decreases. | Blood water potential increases. |
| Detection | Osmoreceptors in the hypothalamus shrink. | Osmoreceptors in the hypothalamus swell. |
| Processing & Release | Hypothalamus signals the pituitary gland to release more ADH. | Hypothalamus signals the pituitary gland to release less ADH. |
| Effector Action | Kidney collecting ducts become more permeable. | Kidney collecting ducts become less permeable. |
| Correction | More water is reabsorbed back into the blood by osmosis. | Less water is reabsorbed; more stays in the tubule filtrate. |
| Result | A small volume of concentrated (dark) urine is produced. | A large volume of dilute (pale) urine is produced. |
Students often state that the hypothalamus releases ADH. For high marks, clarify that the hypothalamus produces ADH, but the pituitary gland releases it.
OCR mark schemes strictly require the term 'water potential', so never use the phrase 'water concentration' in your exam answers.
In 'Explain' questions about ADH, to gain full marks you must state that ADH increases the 'permeability of the collecting duct', not just the permeability of the kidneys in general.
Always describe urine using both its volume and concentration (e.g., 'a small volume of concentrated urine') rather than just saying 'more urine' or 'less urine'.
Water potential
A measure of the tendency of water molecules to move; blood water potential decreases as it becomes more concentrated with solutes.
Osmosis
The movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
Lysis
The bursting of an animal cell when it takes in too much water by osmosis.
Crenation
The shrinking and shrivelling of an animal cell when it loses too much water by osmosis.
Osmoregulation
The homeostatic control of water potential and salt levels in the body fluids to maintain a stable internal environment.
Hypothalamus
A region in the brain that acts as the control centre for osmoregulation, processing signals and producing ADH.
Osmoreceptors
Specialised sensory receptors in the hypothalamus that detect changes in the water potential of the blood plasma.
Pituitary gland
A gland in the brain that acts as the release site for Antidiuretic Hormone (ADH) into the bloodstream.
Antidiuretic Hormone (ADH)
A chemical messenger that travels in the blood plasma to increase the permeability of kidney tubules to water.
Kidneys
The target organs of ADH where water reabsorption and urine production take place.
Collecting ducts
The final part of the kidney tubule where the final concentration of urine is determined under the influence of ADH.
Permeability
The ability of a membrane to allow substances, such as water, to pass through it.
Aquaporins
Specialised protein channels inserted into the collecting duct membranes by ADH to facilitate water movement.
Reabsorption
The process by which water moves from the kidney tubule filtrate back into the blood by osmosis.
Negative feedback
A regulatory mechanism where a change in a physiological factor triggers a response that reverses the direction of that change.
Put your knowledge into practice — try past paper questions for Biology B
Water potential
A measure of the tendency of water molecules to move; blood water potential decreases as it becomes more concentrated with solutes.
Osmosis
The movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
Lysis
The bursting of an animal cell when it takes in too much water by osmosis.
Crenation
The shrinking and shrivelling of an animal cell when it loses too much water by osmosis.
Osmoregulation
The homeostatic control of water potential and salt levels in the body fluids to maintain a stable internal environment.
Hypothalamus
A region in the brain that acts as the control centre for osmoregulation, processing signals and producing ADH.
Osmoreceptors
Specialised sensory receptors in the hypothalamus that detect changes in the water potential of the blood plasma.
Pituitary gland
A gland in the brain that acts as the release site for Antidiuretic Hormone (ADH) into the bloodstream.
Antidiuretic Hormone (ADH)
A chemical messenger that travels in the blood plasma to increase the permeability of kidney tubules to water.
Kidneys
The target organs of ADH where water reabsorption and urine production take place.
Collecting ducts
The final part of the kidney tubule where the final concentration of urine is determined under the influence of ADH.
Permeability
The ability of a membrane to allow substances, such as water, to pass through it.
Aquaporins
Specialised protein channels inserted into the collecting duct membranes by ADH to facilitate water movement.
Reabsorption
The process by which water moves from the kidney tubule filtrate back into the blood by osmosis.
Negative feedback
A regulatory mechanism where a change in a physiological factor triggers a response that reverses the direction of that change.
