Control of blood water potential (A-level only)
AQA
A-Level
Osmoregulation is the homeostatic control of the water potential of blood plasma, which matters because body cells are bathed in tissue fluid derived from plasma, and water moves between these compartments by osmosis. If blood becomes too concentrated, water leaves cells by osmosis and they shrink; if too dilute, water enters and animal cells may burst. The controlled variable is water potential, measured in kPa; pure water is 0 kPa and any solution is negative, so more concentrated plasma has a lower, more negative water potential. Osmoreceptors in the hypothalamus monitor this. Sweating, a salty meal or not drinking makes blood water potential fall; water leaves osmoreceptor cells by osmosis and they shrink, triggering ADH release, so the kidney reabsorbs more water and a small volume of concentrated urine is produced.
9
Objectives
8
Exam Tips
13
Pitfalls
14
Key Terms
17
Mark Points
Subtopics in this area
Osmoregulation as control of the water potential of the blood.
The roles of the hypothalamus, posterior pituitary and antidiuretic hormone (ADH) in osmoregulation.
The structure of the nephron and its role in: the formation of glomerular filtrate reabsorption of glucose and water by the proximal convoluted tubule maintaining a gradient of sodium ions in the medulla by the loop of Henle reabsorption of water by the distal convoluted tubule and collecting ducts.
Control of blood water potential (A-level only) Revision Guide
Learning Objectives
What you need to know and understand
- Explain why the water potential of blood plasma must be held within narrow limits, using osmosis into and out of body cells.
- Describe how osmoreceptors in the hypothalamus detect a fall in blood water potential, and trace the response through to the urine produced.
- Predict the change in urine volume and concentration after a salty meal, heavy sweating or drinking a litre of water, and justify each prediction.
- Describe how osmoreceptors in the hypothalamus detect changes in blood water potential by shrinking or swelling.
- Trace the pathway of ADH from its production in the hypothalamus to its secretion by the posterior pituitary.
- Explain how ADH increases the permeability of the distal convoluted tubule and collecting duct to water, resulting in concentrated urine and restored water potential.
- Name the two regions of the nephron that respond to ADH and describe what happens to their permeability to water when ADH binds.
- Explain how the difference in diameter between the afferent and efferent arterioles produces the pressure needed for ultrafiltration.
- Describe how the loop of Henle acts as a counter-current multiplier to build a sodium ion gradient in the medulla, and use it to explain why an animal with a longer loop produces more concentrated urine.
Marking Points
Key points examiners look for in your answers
- Identifying the water potential of the blood or plasma as the factor being controlled, rather than simply the water content.
- Osmoreceptors in the hypothalamus detecting the change in water potential.
- Water leaving or entering the osmoreceptor cells by osmosis so that they shrink or swell.
- Linking the response to the volume and the concentration of urine produced.
- Naming the mechanism as negative feedback, with the response returning water potential to normal.
- Stating that body cells are bathed in tissue fluid, not directly in blood plasma, and that water moves between plasma and tissue fluid by osmosis.
- Osmoreceptors in the hypothalamus detect a decrease in the water potential of the blood.
- Water leaves the osmoreceptors by osmosis, causing them to shrink and stimulate ADH production.
- ADH is produced in the hypothalamus and secreted into the blood by the posterior pituitary gland.
- ADH binds to receptors with a complementary tertiary structure on the cell-surface membranes of the distal convoluted tubule and collecting duct.
- This increases permeability to water by causing the insertion of aquaporins into the membrane.
- More water is reabsorbed into the blood by osmosis down the water potential gradient, producing a smaller volume of concentrated urine.
- High hydrostatic pressure in the glomerulus forces small molecules through the basement membrane to form glomerular filtrate.
- Glucose is reabsorbed in the proximal convoluted tubule by co-transport with sodium ions.
- The ascending limb of the loop of Henle is impermeable to water and actively transports sodium ions into the medulla, lowering its water potential.
- The loop of Henle acts as a counter-current multiplier, maintaining a steep water potential gradient in the medulla.
- Water leaves the distal convoluted tubule and collecting duct by osmosis down a water potential gradient.
Examiner Tips
Expert advice for maximising your marks
- 💡Always give the direction of change: a fall in blood water potential means more ADH, more water reabsorbed and less urine.
- 💡Use kPa and remember pure water is 0, so every solution is negative and lower always means more negative.
- 💡This topic appears in synoptic essays, so practise linking water potential, osmosis, receptor proteins and negative feedback in one paragraph.
- 💡Clearly distinguish between the roles of the hypothalamus (detects water potential changes, produces ADH) and the posterior pituitary (secretes ADH).
- 💡When explaining how ADH increases water reabsorption, explicitly mention the fusion of vesicles containing aquaporins with the cell-surface membrane.
- 💡Learn the sequence of the nephron (Bowman's capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, collecting duct) to accurately locate processes.
- 💡When explaining the loop of Henle, explicitly use the term 'counter-current multiplier' and state that the ascending limb is impermeable to water.
- 💡When explaining ADH action, detail the sequence: aquaporins inserted into the membrane, increased permeability, then more water reabsorbed by osmosis.
Common Mistakes
Pitfalls to avoid in your exam answers
- Writing that water potential increases when the blood becomes more concentrated. Correction: adding solute makes water potential more negative, so it falls.
- Saying the hypothalamus detects the amount of water in the blood rather than the water potential of the blood. Correction: osmoreceptors respond to water potential changes, detected via osmosis.
- Describing osmoreceptors as detecting sodium ions directly, instead of responding to water moving in or out by osmosis. Correction: the receptor cells shrink or swell as water moves by osmosis.
- Calling the control positive feedback, or leaving out that the response is switched off once normal water potential is restored. Correction: this is negative feedback; the response reverses the initial change.
- Confusing osmoregulation with excretion. Correction: removing urea is excretion; controlling water potential is osmoregulation.
- Stating that every cell sits directly in blood plasma. Correction: most body cells are bathed in tissue fluid, which is derived from plasma.
- Stating the pituitary gland produces ADH. Correction: The hypothalamus produces ADH; the posterior pituitary only stores and secretes it.
- Explaining ADH binding using enzyme terminology like 'active site'. Correction: Use receptor terminology, stating the receptor has a specific tertiary structure complementary to the ADH molecule.
- Forgetting to mention osmosis when describing water reabsorption. Correction: Always state that water moves back into the blood by osmosis down a water potential gradient.
- Writing 'sodium' rather than 'sodium ions' when describing the gradient in the medulla; specifying ions is necessary for scientific accuracy.
- Stating glucose is actively transported into the blood; the active step is the sodium-potassium pump, while glucose enters by co-transport.
- Claiming water is pumped out of the collecting duct instead of moving passively by osmosis down a water potential gradient.
- Forgetting to state that the ascending limb of the loop of Henle is impermeable to water, which is crucial for establishing the medullary concentration gradient.