- Osmoregulation maintains water and solute balance.
- Osmoreceptors in the hypothalamus detect blood concentration.
- ADH acts on collecting ducts in the kidney.
- More ADH produces lower-volume, more concentrated urine.
Year 12 Biology
Module 8 · IQ1
⏱ ~45 min
Practice bank · 3 Short Answer
Lesson 4 of 21
Water Balance: ADH, Aldosterone and the Kidney
Water balance is homeostasis for blood concentration. Learn how the brain, hormones and kidneys work together to conserve or remove water and salts.
Today's question: If sweating removes water from the body, how do your brain and kidneys detect the change and reduce water loss in urine?
0/5TASKS
Warm up first
Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.
Worksheets
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.
Lesson map
Water, salts and kidney control
This lesson separates the two main control ideas: ADH changes water reabsorption; aldosterone changes salt reabsorption.
- 1Blood concentration must stay in range.Losing water makes blood more concentrated.
- 2ADH controls water reabsorption.More ADH means more water returns to the blood.
- 3Aldosterone controls sodium reabsorption.Water can then follow sodium by osmosis.
Know what matters
- Aldosterone increases sodium reabsorption in the distal tubule.
- Water follows sodium by osmosis.
- Neural and hormonal coordination can work together.
- How aquaporins change collecting duct permeability.
- Why diabetes insipidus causes large volumes of dilute urine.
- Why dehydration can exceed the system's ability to compensate.
0
Predict first: dehydrated after sport
connect
A student sweats heavily and does not drink enough water. Their blood becomes more concentrated. What should happen to ADH?
1
Vocabulary that unlocks the kidney loop
vocab
Osmoregulation is the control of water and dissolved substances. In this lesson, watch whether the question is asking about water directly or sodium first.
Osmolalityhow concentrated the blood is
ADHhormone that increases water reabsorption
Collecting ductkidney region where ADH changes water movement
Aquaporinwater channel protein
Aldosteronehormone that increases sodium reabsorption
True or false: ADH mainly acts on the collecting duct.
2
The ADH negative feedback loop
apply
When blood becomes too concentrated, ADH increases water reabsorption. This makes urine volume fall and helps dilute the blood back toward normal.
1StimulusBlood water falls
2ReceptorOsmoreceptors detect concentration
3SignalMore ADH released
4EffectorCollecting duct reabsorbs water
5ResponseUrine volume decreases
Sort the ADH loop+7 XP
Put the dehydration response in order.
- Osmoreceptors detect increased blood concentration.
- Collecting ducts become more permeable to water.
- Water loss makes blood more concentrated.
- More water returns to blood and urine volume falls.
- ADH release increases.
3
ADH vs aldosterone
compare
ADH
Acts mainly on collecting ducts. It increases water reabsorption directly by increasing water permeability.
Aldosterone
Acts mainly on the distal tubule. It increases sodium reabsorption; water follows by osmosis.
HSC exam move
When writing about aldosterone, state sodium first. Water movement is the consequence, not the direct hormone action.
4
Choose your route
differentiate
Complete the ADH loop.
Explain how ADH helps maintain water balance.
Compare ADH and aldosterone in kidney control.
5
Exit check
retrieve
Memorise
ADH -> collecting duct -> water reabsorption.
Understand
More ADH reduces water loss in urine.
Apply
Use dehydration or overhydration scenarios to choose ADH direction.
Avoid
Do not say aldosterone directly reabsorbs water.
01
Multiple Choice
+5 XP
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
02
Short Answer, 16 marks
+5 XP
ApplyBand 4(5 marks) 1. Describe the complete homeostatic pathway that restores blood osmolarity to normal when a person becomes dehydrated. Name all five stimulus-response components, identify the hormone involved, and state where it acts in the kidney and how it produces its effect.
AnalyseBand 4–5(5 marks) 2. Compare the ADH and aldosterone pathways. Identify: (a) the stimulus each responds to; (b) the site of action in the kidney; (c) the direct effect on the nephron (what is reabsorbed); (d) how water is ultimately retained.
EvaluateBand 5–6(5 marks) 3. A person's collecting ducts no longer respond to ADH. Predict the effect on urine volume and concentration, explain what happens to blood osmolarity over time, and explain why this is a failure of homeostasis.
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Answers
SA1 (5 marks): Stimulus: blood osmolarity rises above ~295 mOsm/kg due to dehydration [1]. Receptor: osmoreceptors in the hypothalamus detect increased osmolarity, they shrink as water leaves by osmosis [1]. Control centre and hormone: the hypothalamus signals the posterior pituitary, which releases ADH into the bloodstream [1]. Effector and site: ADH acts on the collecting duct, inserting aquaporin water channels and dramatically increasing water permeability [1]. Response: water is reabsorbed through the aquaporins → concentrated, low-volume urine → blood osmolarity falls toward normal. Negative feedback, the response opposes the stimulus and ADH secretion decreases as osmolarity normalises (self-limiting) [1].
SA2 (5 marks): ADH: (a) stimulus = rising blood osmolarity; (b) site = collecting duct; (c) direct effect = aquaporin insertion; (d) water reabsorbed directly through aquaporins by osmosis [2]. Aldosterone: (a) stimulus = low blood pressure/volume (via RAAS); (b) site = distal tubule (DCT); (c) direct effect = increased Na⁺ reabsorption; (d) water follows Na⁺ passively by osmosis, raising blood volume [2]. Key difference: ADH responds to osmolarity and directly increases water permeability; aldosterone responds to blood pressure and causes water retention indirectly by first reabsorbing Na⁺ [1].
SA3 (5 marks): Urine volume becomes high and urine is dilute because ADH can no longer make the collecting ducts more permeable to water, so less water is reabsorbed [2]. Continued water loss makes blood more concentrated, so blood osmolarity rises [1]. This is a homeostatic failure because the hormone signal may still be produced, but the effector cannot carry out the response that reduces the original change [2].
Check what actually stuck
Take the full module quiz
quiz
A full module quiz covering every lesson in this module, not just this one. Set aside a decent block of time and treat it like a real assessment.
Start the module quiz →Race Through Water Balance
Rapid-fire questions on ADH, aldosterone and the kidney. Pool: lessons 1–4.
How did your thinking change?
Return to your Think First predictions and consider the 2003 USARIEM Iraq study findings. Soldiers losing 1.5–2 L/h of water in 50°C conditions showed a 4-fold increase in ADH and urine volume dropping from 60 to 10 mL/h, yet 50% of heat casualties still resulted from osmoregulatory failure. This tells you both that the ADH feedback system works powerfully, and that it has physiological limits beyond which even maximum hormone secretion cannot compensate.
- Q1, maintaining blood volume: The kidney is the organ, specifically via ADH-mediated aquaporin insertion in the collecting duct. In the USARIEM study, this reduced urine output 6-fold (60 → 10 mL/h) despite a 4-fold ADH increase.
- Q2, salty meal and urine: Blood osmolarity rises → more ADH → collecting duct more permeable → more water reabsorbed → urine volume decreases and becomes more concentrated, the same pathway that failed under extreme Iraq heat conditions when water intake was insufficient.
- State the key difference between what ADH responds to (osmolarity) and what aldosterone responds to (blood pressure/sodium concentration).