ADH and the Water Balance of the Blood

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.78 — The role of ADH in regulating the water content of the blood (bold B statement — Paper 2 only)
• 2.79 — Urine contains water, urea and ions (bold B statement — Paper 2 only)

Specification section 2(i) Excretion — also taught in Excretion: Getting Rid of What You Made, The Kidney and the Nephron, Ultrafiltration and Selective Reabsorption.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

You can drink two litres of water in an afternoon, or sweat two litres out on a hot day, and the concentration of your blood barely shifts. That is not luck. Something is measuring it continuously and correcting it, and the correction happens at one adjustable point — the last stretch of the nephron, where the amount of water taken back can be turned up or down at will.

The loop, when you are short of water

1. You sweat heavily, or eat something very salty, and the water concentration of your blood falls.
2. Receptors in the hypothalamus, in the brain, detect that fall.
3. The pituitary gland releases MORE of the hormone ADH into the blood.
4. ADH travels in the blood to the kidneys, where it makes the walls of the collecting ducts MORE permeable to water.
5. More water is therefore reabsorbed from the filtrate back into the blood, and a small volume of concentrated urine is produced.
6. The water concentration of the blood rises back towards normal — which removes the original stimulus, so less ADH is released. The correction switches itself off.

The same loop, running the other way

1. You drink a large volume of water, and the water concentration of your blood rises.
2. The hypothalamus detects the rise.
3. The pituitary releases LESS ADH.
4. The collecting duct walls become less permeable to water.
5. Less water is reabsorbed, so a large volume of dilute urine is produced.
6. The blood returns towards its normal concentration — and again, the response ends because the thing that triggered it has gone.

That shape has a name, and it is worth taking seriously because the rest of the course is full of it. NEGATIVE FEEDBACK means the response acts to reverse the change that produced it. A fall in blood water triggers a response that raises blood water; a rise triggers a response that lowers it. Nothing has to know what the right answer is in advance — the system is simply built so that any departure from normal generates a push back towards it, and the push fades as normal is approached.

Be precise about what ADH actually changes, because this is where marks go. It does not act on urea, and it does not decide how much waste to excrete. It changes ONE thing: how permeable the collecting duct wall is to water. Everything else follows from that. Concentrated urine and dilute urine contain much the same amount of urea — the difference is how much water it is dissolved in, which is why the volume can swing enormously while the excretion of waste goes on more or less unchanged.

So what is in urine at the end of all this? Three things: water, urea, and mineral ions the body did not need. In a healthy person there is no glucose, because all of it was reabsorbed, and no protein, because it was never filtered in the first place. Finding either in a urine sample is a medical finding rather than a normal result — and knowing which of those two reasons applies to which substance is exactly what the previous lesson was for.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · ADH (antidiuretic hormone)

A hormone released by the pituitary gland which increases the permeability of the collecting duct walls to water, so that more water is reabsorbed into the blood.

Learn this definition · Negative feedback

A control mechanism in which a change from the normal level triggers a response that reverses that change, returning the system towards its normal level.

The two directions of the loop — learn them as a mirrored pair

The ADH response to dehydration — the sequence to write

1. The water concentration of the blood falls.
2. Receptors in the hypothalamus detect the change.
3. The pituitary gland releases more ADH into the blood.
4. ADH makes the collecting duct walls more permeable to water.
5. More water is reabsorbed into the blood, and a small volume of concentrated urine is produced.
6. Blood water concentration returns to normal, so less ADH is released — the response switches itself off.

What urine contains — and what it must not

• WATER — in a variable amount, set by how much ADH was released
• UREA — the nitrogenous waste made by the liver
• MINERAL IONS — those in excess of the body's requirements
• NO glucose in a healthy person — all of it is reabsorbed at the proximal convoluted tubule
• NO protein in a healthy person — it is too large to be filtered at all

Model answer [5 marks]

A person sweats heavily during exercise. Explain how ADH restores the water content of their blood. [5]

Sweating lowers the water concentration of the blood. Receptors in the hypothalamus detect this, and the pituitary gland releases more ADH into the blood. ADH increases the permeability of the collecting duct walls to water, so more water is reabsorbed from the filtrate into the blood. A small volume of concentrated urine is produced. The blood water concentration returns to normal, and less ADH is then released.

Model answer [3 marks]

Explain why a large volume of dilute urine and a small volume of concentrated urine may contain almost the same mass of urea. [3]

ADH controls only how permeable the collecting duct is to water, so it changes how much water is reabsorbed. It does not change how much urea is excreted. The same mass of urea is therefore dissolved in a different volume of water, which alters the concentration of the urine but not the quantity of waste removed.

Mark-losing trap. ADH changes the PERMEABILITY of the collecting duct to water. It does not pump water and it does not act on urea.

Mark-losing trap. More ADH means LESS urine, not more. The name antidiuretic says so — it acts against the production of urine.

Mark-losing trap. Name the hypothalamus as the detector and the pituitary as the releaser. They are different structures with different jobs.

Mark-losing trap. Negative feedback reverses the change. Saying it 'keeps things the same' describes the result, not the mechanism.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — Which three substances are found in the urine of a healthy person?
2. Grade 7 · Predict [2 marks] — A person drinks two litres of water quickly. What happens to the amount of ADH released, and to their urine?
3. Grade 8 · Explain [5 marks] — A hiker sweats heavily and drinks nothing for several hours. Explain how their body restores the water content of their blood. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Calculate [4 marks] — On a cool day a person produces 2.5 dm³ of urine containing urea at 8.0 g/dm³. On a hot day the same person produces 0.80 dm³ of urine containing urea at 25 g/dm³. Calculate the difference between the masses of urea excreted on the two days.
5. 9+ · Analyse [5 marks] — A patient produces around 15 dm³ of very dilute urine each day and is constantly thirsty, however much they drink. Tests show their kidneys are structurally normal, their blood glucose is normal, and their pituitary gland releases no ADH. Select every statement that belongs in a full-mark explanation.

The people behind this science

Two ways into the same idea — the one who made chemical messengers possible, and the one who named what is being kept constant. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

William Harvey — the one who made chemical messengers possible

ADH is made in one organ and acts on another, several handspans away, with nothing connecting them but blood. That only works if blood genuinely travels in a circuit reaching every organ and returning — which is Harvey's discovery. In a body where blood ebbed and seeped as the ancients believed, chemical messaging of this kind would be impossible.

• “How does something released in the head reach an organ in the abdomen?”
• “What would happen to such a signal if the blood did not travel in a circuit?”
• “How quickly does blood get from one organ to another?”
• “Did you imagine the blood might be carrying anything besides itself?”

Claude Bernard — the one who named what is being kept constant

This lesson is one worked example of Bernard's central claim: that the fluid surrounding our cells is held nearly constant, and that an animal is free of its surroundings only because it carries its own conditions inside it. The water concentration of the blood is exactly the sort of thing he meant. He would also recognise the argument of the last question on this page — he spent his career showing that a function is proved by removing it and watching what fails.

• “Why does it matter so much that the fluid around our cells stays the same?”
• “How would you prove that a condition is genuinely being regulated rather than merely staying steady?”
• “What did you learn by removing a function and seeing what went wrong?”
• “Is an animal that controls its inside really freer than one that does not?”

Carry on through the course

• Previous lesson: Ultrafiltration and Selective Reabsorption
• Next lesson: Homeostasis and the Control of Body Temperature
• Levels of Organisation — From organelle to organism — the five-step ladder that every later topic in biology is built on.
• Comparing Cells, and Stem Cells — What plant and animal cells share and where they differ — then why a cell specialising is one of the most powerful ideas in biology.
• Carbohydrates, Proteins and Lipids — The three big food molecules — what they are made of, what they are built from, and how to test for them.
• Enzymes: Catalysts With a Shape — Why a protein with a hole in it speeds up a reaction, and why heat destroys that ability permanently.
• Enzymes and pH — Why acid ruins one enzyme and is the only condition another will work in — and how to find an enzyme's best pH by experiment.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification