OxfordAQA International GCSE Biology: Water and ion balance: liver, kidneys and ADH (9201) – Revision Notes

These revision notes are aligned to the OxfordAQA International GCSE Biology (9201) specification, Version 5.2, for exams May/June 2018 onwards. They cover section 3.4.3, Control of water and ion content of the body, statements a to g. The qualification is untiered. For full explanations and worked examples, read the study guide.

Also useful: the OxfordAQA Biology hub, the printable checklist, the practice questions and the free 10-minute diagnostics.

Key terms

Term	Meaning
Excretion	Removal of waste products made by the body, such as urea
Deamination	Removal of the nitrogen-containing part of an excess amino acid, forming ammonia
Urea	Waste made in the liver from ammonia; removed in urine and sweat
Detoxification	Changing poisonous substances into less harmful ones
Filtering	Forcing small molecules out of the blood in the kidney
Reabsorption	Taking useful substances back into the blood from the filtered fluid
ADH	Hormone from the pituitary gland that makes the kidneys reabsorb more water
Negative feedback	A change triggers a response that reverses the change

3.4.3 a, b: losses

Route	Water	Urea	Ions
Lungs (breathing out)	Yes	No	No
Skin (sweat)	Yes	Yes	Yes
Kidneys (urine)	Excess	Yes	Excess

• Water balance: water in = water out over time.
• Only the kidney response is adjusted to keep the balance. The loss in the breath and in sweat is not regulated for this purpose.
• More sweating means more water, urea and ions lost, so a smaller volume of more concentrated urine and a need to drink more.

3.4.3 c: the liver

Job	Detail
Deamination	Excess amino acids are deaminated to form ammonia
Making urea	Ammonia is converted into urea for excretion
Detoxification	Poisonous substances are detoxified; breakdown products leave in urine via the kidneys
Old blood cells	Broken down in the liver; the iron is stored

Method in steps for any liver question:

1. Name the substance (amino acid, ammonia, poison, red blood cell).
2. Say what the liver does to it.
3. Say what is made or stored (urea, harmless product, iron).
4. Say where it goes next (blood, then kidneys, then urine).

Chain to learn: amino acid, then ammonia, then urea, then urine.

3.4.3 d: the healthy kidney

Stage	Result
Blood is filtered	Water, glucose, ions and urea enter the filtrate; blood cells and large proteins stay in the blood
All glucose reabsorbed	None in the urine of a healthy kidney
Needed ions reabsorbed	Excess ions are not
As much water as needed reabsorbed	Amount varies with the body’s needs
Released as urine	Urea, excess ions, excess water

Must-know distinction: filtering is not selective, reabsorption is.

3.4.3 e, f, g: ADH and negative feedback

	Blood water too low	Blood water too high
Pituitary gland	More ADH released	Less ADH released
Kidney	More water reabsorbed	Less water reabsorbed
Urine	Small volume, concentrated	Large volume, dilute
Result	Blood water content rises	Blood water content falls

Negative feedback loop (low water):

low water -> more ADH -> more water reabsorbed -> water content rises -> less ADH

Key phrase for full credit: the change is detected, a response follows, and the response reverses the change.

Liver and kidney compared

Question	Liver	Kidney
Makes urea?	Yes, from ammonia	No
Removes urea from the blood?	No	Yes, in urine
Deals with excess amino acids?	Deaminates them	Does not
Deals with poisons?	Detoxifies them	Excretes the breakdown products
Deals with water balance?	No	Yes, by reabsorbing more or less water
Iron from old red blood cells?	Stores it	No

Learn the pairs: the liver makes and changes; the kidney filters, reabsorbs and releases.

Command words and model answers

Explain questions need a cause and an effect joined by “so” or “because”. Describe questions need only what happens. Suggest questions give you an unfamiliar situation, so apply the same rules.

Model answer 1 (Explain why a person who has been sweating heavily produces a small volume of dark urine): Sweating lowers the water content of the blood. The pituitary gland releases more ADH. ADH makes the kidneys reabsorb more water, so less water is lost in the urine, and the urine is concentrated.

Model answer 2 (Describe the role of the liver in dealing with excess protein in the diet): Excess amino acids are deaminated, forming ammonia. The ammonia is converted into urea. The urea is carried in the blood to the kidneys and excreted in the urine.

Model answer 3 (State what happens to glucose in the healthy kidney): All of the glucose is reabsorbed into the blood, so none is released in the urine.

Model answer 4 (Explain what is meant by negative feedback): A change in the condition is detected, and a response reverses the change so the condition returns towards its normal level.

Sequences to learn

• Protein in the diet, then amino acids in the blood, then excess amino acids deaminated in the liver, then ammonia, then urea, then blood, then kidneys, then urine.
• Low blood water content, then detected, then pituitary gland, then more ADH in the blood, then kidneys reabsorb more water, then concentrated urine, then blood water content back towards normal.
• High blood water content, then detected, then pituitary gland, then less ADH, then kidneys reabsorb less water, then dilute urine, then blood water content back towards normal.
• Old red blood cell, then broken down in the liver, then iron stored for later use.

Quick check: in each sequence, name the organ or gland that acts at every arrow. If you cannot, revisit the matching section above.

Reading data questions

1. Say what the graph or table shows in one sentence, using figures from it (a start value, an end value, and the change).
2. If urine volume is down and concentration is up, link this to more ADH and more water reabsorbed.
3. If urine volume is up and concentration is down, link this to less ADH and less water reabsorbed.
4. Add the cause: the blood water content was low or high before the change.
5. Check units and the time period before you calculate a rate or a percentage change.

Worked reminder: a volunteer produces 135 cm³ of urine in 90 minutes. Convert 90 minutes to 1.5 hours. Rate = 135 ÷ 1.5 = 90 cm³ per hour.

Worked reminders

Percentage change = (new value − old value) ÷ old value × 100.

Reminder 1: urine volume falls from 1500 cm³ to 900 cm³ in a day. Change = (900 − 1500) ÷ 1500 × 100 = −40%.

Reminder 2: 1200 cm³ of urine in 24 hours is 1200 ÷ 24 = 50 cm³ per hour.

Reminder 3: to convert, 0.9 dm³ × 1000 = 900 cm³.

Reading data: a table or graph showing urine volume down and concentration up points to more ADH. Volume up and concentration down points to less ADH. Always add the link to the blood water content and the kidney.

Quick self-test

1. State two ways in which water leaves the body other than in urine.
2. Name two substances other than water lost in sweat.
3. Put in order: urea, ammonia, amino acids.
4. What happens to the iron from old red blood cells?
5. Which dissolved substance is completely reabsorbed in a healthy kidney?
6. Name the hormone and the gland that releases it when blood water content falls.
7. Describe the effect of more ADH on the urine.
8. A person produces 1200 cm³ of urine in 24 hours. Calculate the mean volume per hour.
9. Urine volume falls from 1500 cm³ to 900 cm³. Calculate the percentage change.
10. Explain why drinking a large volume of water makes the urine more dilute.

Answers

1. In the breath from the lungs, and in sweat from the skin.
2. Urea and ions (salts).
3. Amino acids, then ammonia, then urea.
4. It is stored (in the liver).
5. Glucose.
6. ADH, released by the pituitary gland.
7. The kidneys reabsorb more water, so a smaller volume of more concentrated urine is produced.
8. 1200 ÷ 24 = 50 cm³ per hour.
9. (900 − 1500) ÷ 1500 × 100 = −40% (a 40% decrease).
10. Blood water content rises, so the pituitary gland releases less ADH. The kidneys reabsorb less water, so more water is lost in a larger volume of dilute urine.

Where marks are usually lost

• Saying the kidneys make urea, or that amino acids become urea without ammonia.
• Writing “ADH is made by the kidneys”. It is released by the pituitary gland.
• Writing “less water is produced” instead of “less water is reabsorbed”.
• Forgetting to say that urine is more concentrated, not only smaller in volume.
• Leaving out the link between a data trend and the blood water content.
• Giving a percentage change with the wrong sign or dividing by the new value instead of the old.
• Stating that negative feedback “makes the change bigger”.
• Forgetting units, or mixing cm³ and dm³ in the same calculation.
• Saying glucose is lost in the urine of a healthy person.

Official syllabus

OxfordAQA International GCSE Biology (9201), specification Version 5.2, for exams May/June 2018 onwards, published by OxfordAQA. Section 3.4.3, Control of water and ion content of the body.