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

This study guide is aligned to the OxfordAQA International GCSE Biology (9201) specification, Version 5.2, for exams May/June 2018 onwards. It covers section 3.4.3, Control of water and ion content of the body, statements a to g. The qualification is untiered, so all of this content is for every candidate. Content from any part of the specification can be assessed on either of the two written papers.

Related pages: the OxfordAQA Biology hub, the printable checklist, the revision notes and the practice questions. For temperature and blood glucose, see homeostasis: temperature control and blood glucose. To find weak spots first, take one of the free 10-minute diagnostics.

What this unit covers

Statement	What you must be able to do
3.4.3 a	Name the routes by which water leaves the body; state that excess water is removed in urine
3.4.3 b	State that urea and ions are lost in sweat; excess ions leave in urine
3.4.3 c	Describe three jobs of the liver: deamination to urea, detoxification, breaking down old blood cells and storing iron
3.4.3 d	Describe what a healthy kidney filters, reabsorbs and releases
3.4.3 e, f	Explain the ADH response when blood water content is too low and too high
3.4.3 g	Describe ADH control as negative feedback

Section 3.4.1 and 3.4.2 set the scene. Internal conditions that are controlled include the water content and the ion content of the body. Coordination centres include the brain and spinal cord and the pancreas, and the brain contains receptors sensitive to the concentration of water in the blood.

Why water and ion content must be controlled (3.4.3 a, b)

Cells work properly only when the surrounding fluid has the right amount of water and dissolved ions. The body takes in water in drink and food and loses it in several ways. Balance means total water in equals total water out.

Route	What is lost	Notes
Lungs	Water	Lost when you breathe out
Skin	Water, urea and ions	Lost in sweat
Kidneys	Excess water, excess ions, urea	Lost in urine

Only the kidneys adjust the loss to match the need. You cannot decide to breathe out less water, and sweating is mainly a cooling response (see the homeostasis page). The kidney is where water and ion balance is fine-tuned.

Worked example 1: a water budget

On a normal day a person takes in 1300 cm³ in drink, 800 cm³ in food and gains 250 cm³ from respiration. Losses are 380 cm³ from the lungs, 420 cm³ from the skin and 100 cm³ in other ways. The rest leaves in urine.

1. Total in = 1300 + 800 + 250 = 2350 cm³.
2. Losses other than urine = 380 + 420 + 100 = 900 cm³.
3. Urine = 2350 − 900 = 1450 cm³.

On a hot day the same person drinks 1800 cm³, eats the same food, and loses 420 cm³ from the lungs, 1650 cm³ in sweat and 100 cm³ in other ways.

1. Total in = 1800 + 800 + 250 = 2850 cm³.
2. Other losses = 420 + 1650 + 100 = 2170 cm³.
3. Urine = 2850 − 2170 = 680 cm³.
4. Percentage change = (680 − 1450) ÷ 1450 × 100 = −53.1%, a decrease of about 53%.

Explanation: much more water left in sweat, so the kidneys released less water, giving a smaller volume of more concentrated urine. This is the ADH response (below).

The liver (3.4.3 c)

The liver receives blood carrying digested food and other substances from the gut, and does three jobs named in the specification.

Deamination and urea

Amino acids from protein are not stored. Those not needed for making new protein are in excess. In the liver:

1. Excess amino acids are deaminated, which means the nitrogen-containing part is removed. This forms ammonia.
2. Ammonia is poisonous. The liver converts it into urea, which is less harmful.
3. Urea passes into the blood and is excreted. The kidneys remove most of it in urine, and a little is lost in sweat.

Remember the order: amino acids, ammonia, urea. Do not say amino acids turn straight into urea, and do not say the kidneys make urea.

Detoxification

Poisonous substances, such as alcohol and drugs, are detoxified in the liver. This changes them into less harmful substances. The breakdown products are excreted in the urine by the kidneys.

Old blood cells

Old red blood cells are broken down in the liver. The iron from them is stored in the liver, ready for reuse.

Worked example 2: urea figures

A person makes 28 g of urea in a day. Sweat contains 0.6 g of this, and the rest leaves in urine.

1. Percentage lost in sweat = 0.6 ÷ 28 × 100 = 2.1% (to 2 significant figures).
2. Urea in urine = 28 − 0.6 = 27.4 g.
3. A urine sample holding 12 g of urea in 1.5 dm³ has a concentration of 12 ÷ 1.5 = 8 g per dm³.

Good answers to “why is the amount in sweat small?” say that most urea is removed by the kidneys, which are the main excretory route.

The healthy kidney (3.4.3 d)

The kidney does five things, in this order of logic.

Step	What happens
Filtering	Blood is filtered. Small molecules (water, glucose, ions, urea) are forced out of the blood. Blood cells and large proteins stay in the blood
Glucose	All the glucose is reabsorbed into the blood
Ions	The dissolved ions the body needs are reabsorbed
Water	As much water as the body needs is reabsorbed
Urine	Urea, excess ions and excess water are released as urine

Two ideas matter here. First, filtering is not selective: it removes useful and waste substances together. Selection happens afterwards, by reabsorption. Second, the amount of water and ions reabsorbed changes with need, while glucose is reabsorbed completely in a healthy kidney. Glucose in the urine therefore points to a problem (for example, very high blood glucose).

Worked example 3: how much is reabsorbed?

A kidney filters 108 cm³ of fluid out of the blood each minute and releases 1.2 cm³ of urine each minute.

1. Volume reabsorbed = 108 − 1.2 = 106.8 cm³ per minute.
2. Percentage reabsorbed = 106.8 ÷ 108 × 100 = 98.9%.
3. In one hour the filtrate is 108 × 60 = 6480 cm³ = 6.48 dm³, much more than the volume of fluid you drink. Reabsorption is what stops you losing it all.

ADH and water content (3.4.3 e, f)

ADH is a hormone released into the blood by the pituitary gland. It acts on the kidneys. A hormone travels in the blood, so the effect is slower and longer lasting than a nerve impulse.

Receptors in the brain monitor the concentration of water in the blood. The sequence below is what to write.

	Water content too low	Water content too high
Cause (example)	Sweating, not drinking	Drinking a large volume
Pituitary gland	Releases ADH	Releases less ADH
Kidneys	Reabsorb more water	Reabsorb less water
Urine	Smaller volume, more concentrated	Larger volume, more dilute
Blood water content	Rises towards normal	Falls towards normal

Concentrated urine has more dissolved substance (urea and ions) per cm³. Dilute urine has less. The total amount of urea excreted is not changed much by ADH: only the water changes.

Worked example 4: interpreting data

A volunteer stopped drinking for 3 hours. Urine volume fell from 60 cm³ per hour at the start to 22 cm³ per hour at 3 hours. Urine concentration (arbitrary units) rose from 20 to 55.

1. Percentage change in volume = (22 − 60) ÷ 60 × 100 = −63.3%.
2. Concentration rose by a factor of 55 ÷ 20 = 2.75.
3. Explanation: less water in the blood, so the pituitary gland released more ADH; the kidneys reabsorbed more water, so less urine was produced and it was more concentrated.

Link both observations to ADH. An answer that gives only the effect on the urine, without naming the hormone and the kidney, loses credit.

Negative feedback (3.4.3 g)

A negative feedback mechanism reverses a change and returns the condition to its normal level. ADH control follows this loop:

Blood water content too low
   -> receptors detect the change
   -> pituitary gland releases more ADH
   -> kidneys reabsorb more water
   -> blood water content rises to normal
   -> less ADH is released

The same loop runs the other way when water content is too high: less ADH, less water reabsorbed, more dilute urine, water content falls back. In both directions the response opposes the change. This keeps the level close to a set point, with small fluctuations either side.

Common errors

• Writing that the kidneys “make” urea. The liver makes it.
• Saying “ADH makes the kidneys produce less water”. Say it makes the kidneys reabsorb more water.
• Saying the pituitary gland releases ADH when water content is high. It releases less.
• Describing filtering as selective. Reabsorption is selective.
• Saying glucose is excreted in healthy urine. All of it is reabsorbed.
• Describing negative feedback as “the body gets worse”. It means the change is reversed.
• Mixing up concentrated (low volume) and dilute (large volume) urine.

Next steps

Check recall with the revision notes, then attempt the practice questions. For exam technique across the whole course see OxfordAQA IGCSE Biology exam preparation.

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.