What you need to know
- 7.21osmoreceptorA receptor in the hypothalamus that detects changes in the water potential of the blood. in the hypothalamusA region of the brain that controls homeostasis, including body temperature and water balance., the pituitary, and ADHAntidiuretic hormone: released from the posterior pituitary, it makes the collecting duct more permeable to water, so less water is lost in urine. produce negative feedbackA change from the normal level triggers a response that reverses the change, returning conditions to the norm. control of plasmaThe liquid part of blood: water carrying dissolved substances such as glucose, ions, hormones and proteins. concentration and blood volume.
Do not add the renin–angiotensin–aldosterone pathway. Aquaporin is the protein that explains the permeability change; the required idea is the permeability change itself. Diabetes insipidus is enrichment, not a required disease essay.
By the end, you should be able to…
- Trace the path from osmoreceptor to collecting ductThe tube that carries urine through the medulla. ADH controls how much water is reabsorbed from it..
- Predict urine volume when ADH is high or low.
- Show that the response reverses the change in plasma concentration.
Build your understanding
Detectors and the hormone
osmoreceptorA receptor in the hypothalamus that detects changes in the water potential of the blood. in the hypothalamusA region of the brain that controls homeostasis, including body temperature and water balance. detect the concentration of the blood plasmaThe liquid part of blood: water carrying dissolved substances such as glucose, ions, hormones and proteins., which falls in water potentialThe tendency of water to move out of a solution. Pure water is 0; adding solutes makes it negative. Water moves from higher to lower water potential. when too much water has been lost. They cause the posterior pituitary to release more antidiuretic hormone, ADHAntidiuretic hormone: released from the posterior pituitary, it makes the collecting duct more permeable to water, so less water is lost in urine.. ADH travels in the blood to the kidney. It is a peptide hormoneA hormone made of amino acids. It cannot cross the membrane, so it binds to a receptor on the cell surface., so it binds receptors on target cells rather than entering them as a steroid would.
The pituitary does not filter urine. It releases the signal. The kidney is the effector. If you write “the pituitary reabsorbs water”, the organs are swapped.
The collecting duct changes permeability
ADH makes the walls of the distal tubule and the collecting ductThe tube that carries urine through the medulla. ADH controls how much water is reabsorbed from it. more permeable to water. aquaporinA channel protein that lets water move across a membrane quickly. channels are inserted into the membrane. That name explains the permeability. The specification sentence is the permeability itself. Water then leaves the filtrate by osmosisThe net movement of water molecules from a higher water potential to a lower water potential across a partially permeable membrane. into the concentrated medulla built by the loop of HenleThe hairpin loop of the nephron that makes the medulla salty, so water can be reabsorbed from the collecting duct.. The urine becomes smaller in volume and more concentrated. Blood plasma is diluted back toward the set range, and blood volume is supported because that water stays in the body.
When plasma is dilute, or a large volume of water has been drunk, less ADH is released. The ducts stay less permeable, less water is reabsorbed, and a larger volume of dilute urine is produced. Plasma concentration rises back, and excess volume is lost. Both directions are negative feedbackA change from the normal level triggers a response that reverses the change, returning conditions to the norm..
Concentration and volume together
The specification names two controlled variables: plasma concentration and blood volume. They are linked because the water you reabsorb is both a dilution of the plasma and a contribution to volume. A large loss of water in urine lowers volume and, if solutes stay, raises concentration. ADH answers both by keeping water.
Do not explain this with aldosterone or thirst as if they were required 7.21 content. Thirst can accompany the same osmoreceptors, but the hormone in the specification is ADH. Two failures can look similar in real life: too little hormone, or kidneys that do not respond. That distinction is enrichment from the clinical page, not a second mechanism to draw.
Follow the process
Water loss raises the concentration of the plasma → osmoreceptors in the hypothalamus detect that → the posterior pituitary releases more ADH → ADH binds receptors on the collecting-duct cells, and aquaporins are inserted → water leaves the filtrate by osmosis into the salty medulla → a smaller volume of concentrated urine is produced, and the returned water lowers the plasma concentration, so ADH release falls. After a large drink, less ADH is released → the duct stays less permeable → a larger volume of dilute urine leaves. The pituitary releases the hormone. It does not filter the blood.
NOT: the pituitary does not reabsorb the water, and this is not a renin diagram.
- Detector is the hypothalamic osmoreceptor
- ADH comes from the posterior pituitary
- The collecting duct is the effector
- Water return lowers the stimulus
Two traps this diagram blocks
- Do not write that the pituitary reabsorbs water.
- Do not swap the loop of Henle, which builds the gradient, with ADH, which decides if the duct uses it.
From the diagram alone
Question
Explain why urine volume falls when the body is short of water. [4]
Show a model answer
Osmoreceptors cause the posterior pituitary to release more ADH. The collecting duct becomes more permeable, and water is reabsorbed by osmosis down the medullary gradient, so less water is left to form urine.
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: urine volume falls when the body is short of water. Y: more ADHAntidiuretic hormone: released from the posterior pituitary, it makes the collecting duct more permeable to water, so less water is lost in urine. makes the collecting ductThe tube that carries urine through the medulla. ADH controls how much water is reabsorbed from it. permeable, and the medulla made by the loop of HenleThe hairpin loop of the nephron that makes the medulla salty, so water can be reabsorbed from the collecting duct. is concentrated. Z: water is reabsorbed by osmosisThe net movement of water molecules from a higher water potential to a lower water potential across a partially permeable membrane., so the plasmaThe liquid part of blood: water carrying dissolved substances such as glucose, ions, hormones and proteins. is diluted and blood volume is supported. Incomplete: “The pituitary reabsorbs water.” Improved: “The pituitary releases ADH. The kidney is the effector, and the water returns only because the duct has become permeable.”
High ADH, small volume
More permeable duct, more water reabsorbed, concentrated urine.
Name the feedback
The water returned lowers the concentration that triggered ADH, so release falls.
Avoid this misconception
The claim: ADH is released by the kidney and makes you produce more urine when you are dehydrated.
The correction: The posterior pituitary releases ADH. When you are dehydrated, more ADH causes more water reabsorption and a smaller volume of urine.
Put the ADH feedback loop in order
ADH control is negative feedback: the response reverses the change that started it. Order the loop when water is lost, then when too much water is drunk.
Order the steps
What to do
- Pick a chain with Chain, just below these steps: Water lost or Too much water.
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
You have been sweating and not drinking. Put the ADH response in order.
0 of 7 in the right place
ADH is made in the hypothalamus and stored in the posterior pituitary; only the release is shown. Thirst and other controls of blood volume are left out.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Where are the osmoreceptors, and which gland releases ADH?
Show suggested answer
Osmoreceptors are in the hypothalamus. The posterior pituitary releases ADH.
Practice question
Retrieval and explanation · suggested answer
What does ADH do to the collecting duct?
Show suggested answer
It increases permeability to water, so more water is reabsorbed by osmosis.
Practice question
Retrieval and explanation · suggested answer
A person drinks two litres of water quickly. What happens to ADH and urine volume? Assume the kidneys respond.
Show suggested answer
Plasma is diluted, less ADH is released, the ducts are less permeable, and a larger volume of dilute urine is produced.
Practice question
Retrieval and explanation · suggested answer
Why is this negative feedback?
Show suggested answer
A rise in plasma concentration causes water retention, which lowers the concentration again.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Two people produce large volumes of dilute urine. In one, the pituitary releases almost no ADH. In the other, ADH is high but the collecting ducts do not respond. Why do the symptoms match while the 7.21 mechanism differs? Assume no other hormone is involved.
Show a hint
The duct needs both the hormone and a response.
Explore the reasoning
Both fail to reabsorb water, so urine stays dilute and volume stays high. In the first person the signal is missing. In the second the signal is present and the effector does not answer. Measuring ADH, or seeing whether ducts can respond, would separate them. The specification loop assumes a pituitary signal that the kidney can obey. A symptom alone does not tell you which half failed.
Recall this lesson
Lesson recall questions
Question 1 of 5: Name the hormone that controls the water permeability of the collecting ducts.
ADH and osmoregulation
Practice question
Retrieval and explanation · suggested answer
Name the hormone that controls the water permeability of the collecting ducts.
Lesson: 7C 3: Control of the Kidney and Homeostasis
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
A large urine volume can mean the hormone or the response
7C · Control of the Kidney and HomeostasisThe NHS describes diabetes insipidus as a problem with antidiuretic hormone, also called vasopressin. Either there is not enough hormone, or the kidneys do not respond to it.…Specification 7.21Read & use the scienceClose article
Why this matters
The NHS describes diabetes insipidus as a problem with antidiuretic hormone, also called vasopressin. Either there is not enough hormone, or the kidneys do not respond to it. People then pass large volumes of dilute urine and become very thirsty. That is enrichment. It is not a required disease, and it is not a reason to give treatment advice. It does show why “lots of urine” does not, by itself, name the broken step. 7.21: osmoreceptors, pituitary ADH, and kidney water reabsorption in negative feedback.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.21 ADH, hypothalamus and pituitary in water-balance feedback.
Use the science
Why would treating both causes as “the pituitary failed” be inaccurate?
Explore a possible response
One cause is a lack of ADH. The other is a kidney that does not respond even if ADH is present. The NHS page separates those. The feedback diagram only works if both the release and the duct response occur.
Explore the source
Same urine result, two different faults
7C · Control of the Kidney and HomeostasisNIDDK distinguishes central diabetes insipidus, in which too little vasopressin or ADH is produced, from nephrogenic disease, in which the kidneys do not respond properly to it. Both…Specification 7.21Read & use the scienceClose article
Why this matters
NIDDK distinguishes central diabetes insipidus, in which too little vasopressin or ADH is produced, from nephrogenic disease, in which the kidneys do not respond properly to it. Both can cause large volumes of dilute urine. That is a powerful warning about feedback diagrams: the same final measurement can come from a problem with the signal or with the target. In 7.21, osmoreceptors in the hypothalamus, pituitary release of ADH and kidney water reabsorption form one control chain. The public health page explains the clinical categories; it does not let a student diagnose a person from urine volume alone.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 7.21 ADH, hypothalamus and pituitary in water-balance feedback.
Use the science
Why would measuring urine volume alone fail to distinguish low ADH secretion from poor kidney response to ADH?
Explore a possible response
Both mechanisms reduce water reabsorption and can produce a large urine volume. Hormone measurements or response tests are needed to locate the fault.
Explore the source
- Diabetes insipidusUS National Institute of Diabetes and Digestive and Kidney Diseases