Maintaining water and nitrogen balance in the body (biology only)

SPEC 4.5.3.3 • BIOLOGY ONLY • PAPER 2

Water & Nitrogen Balance

How kidneys filter blood, why urea forms in the liver, what ADH does to the kidney tubules — and the principles behind dialysis.

When Specific Words Win Marks

This topic punishes vague language harder than almost any other on Paper 2. Examiner reports flag the same two failure points year after year:

Failure Point 1 — Substance Fates

When asked what happens to glucose, protein and urea in the kidneys, students confuse the three fates. Mark schemes award separate marks for each — mix any two up and the answer voids.

Failure Point 2 — The Word "Permeability"

Examiner reports consistently note that hardly any students name the permeability of the kidney tubules to water when describing ADH's effect. The mark scheme wants that exact phrase — vague answers like "water is reabsorbed" don't earn the explanation mark.

In the next 10 minutes, you'll know the named processes, the fate of each substance, and the one word ADH questions hinge on.

By the End of This Lesson

Five outcomes — all on Paper 2, all biology-only. The HT-only items are flagged in colour throughout.

Objective 1

Explain the routes of water loss (lungs, skin, kidneys) and the effect of osmotic change on body cells.

Objective 2

Describe how the kidneys produce urine by filtration and selective reabsorption.

Objective 3 — HT only

Explain how excess amino acids are deaminated in the liver to form ammonia, then urea.

Objective 4 — HT only

Describe the effect of ADH on the permeability of the kidney tubules, and how this is controlled by negative feedback.

Objective 5

Describe the basic principles of kidney dialysis, and evaluate dialysis vs transplant.

Three Routes Water Leaves the Body

Only one of these is under control. Knowing which is the foundation for every question on this topic.

RouteWhat's lostControlled?

LungsWater vapour in exhaled breathNo

SkinWater, ions and urea in sweatNo

KidneysExcess water, ions and urea in urineYes

Spec Phrasing

AQA wording: "There is no control over water, ion or urea loss by the lungs or skin." Quote this directly if asked which losses are controlled.

Why Water Balance Matters

Body cells sit in tissue fluid. If the fluid's water concentration changes, water moves into or out of the cells by osmosis — and the cells stop working efficiently.

Spec Outcome

If body cells lose or gain too much water by osmosis they do not function efficiently. This is the exact wording from the spec — use it verbatim.

Animal cells lack a cell wall — they can burst (lyse) when water rushes in. Plant cells don't burst because the wall resists.

Nitrogen Balance — How Urea Is Made

Excess amino acids from digested protein can't be stored. They have to be safely got rid of. This whole pathway is HT only.

Definition — HT

HT onlyDeamination — the process in the liver where excess amino acids have their amino group removed to form ammonia, which is then converted to urea for safe excretion.

Excess amino acids (from diet)

Ammonia (toxic)

Urea (safer)

Two Organs, Two Roles

Liver = makes urea (deamination). Kidney = filters urea out of blood into urine. Mixing these up is one of the most common errors on this topic.

Step 1 — Filtration

The first stage of urine production. Use the named process — "filter" or "sort" loses the mark.

Definition

Filtration (in the kidney) — small molecules (water, ions, glucose, urea) pass out of the blood and into the kidney tubules. Large molecules (proteins, blood cells) are NOT filtered — they're too big to pass through.

What gets filtered out

Water ·

Ions ·

Glucose ·

Urea

What stays in the blood

✗ Proteins · ✗ Blood cells — too large to pass through the filter

Spec Note

AQA explicitly states: "Knowledge of other parts of the urinary system, the structure of the kidney and the structure of a nephron is not required." Don't waste exam time on nephron anatomy — focus on the named processes.

Step 2 — Selective Reabsorption

After filtration, useful substances are taken back into the blood. The word selective matters — not everything is reabsorbed.

Definition

Selective reabsorption — useful substances (glucose, some ions, water) are reabsorbed from the kidney tubules back into the blood. Urea is not reabsorbed — it stays in the tubules and leaves the body in urine.

Reabsorbed (back into blood)

✓ All glucose · ✓ Needed water · ✓ Needed ions

Not reabsorbed (stay in urine)

✗ Urea · ✗ Excess water · ✗ Excess ions

The remaining fluid (urea + excess water + excess ions) becomes urine, which travels to the bladder.

Three Substances, Three Fates

Recent kidney mark schemes award one mark for each row of this table. Memorise it — it's the highest-yield single resource on this topic.

SubstanceFiltered?Reabsorbed?Final fate

GlucoseYesAll reabsorbedStays in blood

ProteinNo — too large—Stays in blood

UreaYesNot reabsorbedExcreted in urine

Ions / waterYesWhat the body needsExcess excreted in urine

Exam Phrasing

Use the named processes "filtered" and "reabsorbed" exactly. Mark schemes do not credit synonyms like "sorted" or "recycled".

ADH — Controlling Water Reabsorption

HT only. Once blood leaves the kidney tubule, ADH decides how much water comes back into the blood.

Definition — HT

HT onlyADH (Antidiuretic Hormone) — released by the pituitary gland when the blood is too concentrated. ADH acts on the kidney tubules to increase their permeability to water, so more water is reabsorbed from the tubules back into the blood.

Memory hook

Anti-diuretic literally means anti-pee. More ADH → less urine (smaller volume, more concentrated). Less ADH → more urine (larger volume, more dilute).

One Word, Most Marks

Examiner reports repeatedly note that hardly any students use the word permeability. Write "ADH increases the permeability of the kidney tubules to water" — not just "ADH reabsorbs water". That phrasing is the mark.

ADH & Negative Feedback

HT only. The body uses a feedback loop to keep blood water concentration roughly constant.

The Loop in Two Lines

Concentrated blood → more ADH → permeability up → water reabsorbed → blood restored.

Dilute blood → less ADH → permeability down → water lost in urine → blood restored.

Kidney Dialysis — How It Works

If kidneys fail, urea builds up in the blood (toxic). Dialysis is a machine that does the kidneys' job. The principles are pure diffusion.

Why glucose & ions stay in the blood

The dialysis fluid has the same concentration of glucose and ions as healthy blood — so there's no concentration gradient. Net diffusion = zero. They stay where they are.

Dialysis vs Transplant

The spec asks you to evaluate the two treatments — so know the advantages and disadvantages of each.

Dialysis

Pros: Available now, no donor needed, no immune rejection.

Cons: 3 sessions per week, hours each; controlled diet needed between sessions; long-term costs are high.

Kidney transplant

Pros: Long-term solution, near-normal life, no machine dependence.

Cons: Limited donor supply, surgical risk, lifelong immunosuppressants (immune system would attack the donor kidney).

Key Distinction

Dialysis treats kidney failure (manages symptoms). A transplant replaces the function long-term. Neither "cures" the original disease — but transplant is closest to a permanent solution.

Examiner Alert — The Word "Permeability"

This is the highest-impact alert on the entire topic. It costs students marks every year.

Recurring ADH Failure Pattern

AQA examiner reports consistently observe that hardly any students mention that ADH changes the permeability of the kidney tubules to water. Most write only "water is reabsorbed" — and lose the explanation mark.

For full credit on ADH questions, the mark scheme wants the named mechanism: "ADH increases the permeability of the kidney tubules to water". Without that phrase, the answer is incomplete.

✗ Loses Marks

"ADH makes the kidney reabsorb more water."No mechanism named.

✓ Gets Marks

"ADH increases the permeability of the kidney tubules to water, so more water is reabsorbed into the blood."Mechanism + outcome.

Worked Example — PEE Framework

An AO2 question. Watch how the four marking points map onto the Point–Evidence–Explain chain.

Q: Explain why kidney failure can be treated by dialysis. 4 marks

P

Point. Healthy kidneys filter the blood and remove waste products such as urea.

E

Evidence. When kidneys fail, urea builds up in the blood (toxic) and water/ion balance is disturbed.

P

Point. In dialysis, the patient's blood passes over a partially permeable membrane next to dialysis fluid.

E

Explain. Urea diffuses from the blood into the dialysis fluid (down a concentration gradient); glucose and ions stay in the blood because the dialysis fluid has the same concentration — so the cleaned blood is returned with normal composition.

Why this scores 4/4

One mark per named idea: filter/remove urea · urea builds up when kidneys fail · partially permeable membrane · diffusion + glucose stays. The "so that…" linker at the end is what turns Describe into Explain.

Your Turn

SubstanceIn blood plasmaIn urine

Glucose1.00.0

Protein80.00.0

Urea0.320.0

Use data from the table to explain how the kidney produces this urine composition.4 marks

Plan four marking points before revealing the solution — one for each substance, each using a named process.

P

Point. Protein is 80.0 in blood but 0.0 in urine because protein is not filtered — its molecules are too large to pass through.

E

Evidence. Glucose is 1.0 in blood but 0.0 in urine because, after filtration, all glucose is reabsorbed back into the blood by selective reabsorption.

P

Point. Urea is much higher in urine (20.0) than in blood (0.3) because urea is filtered out of the blood into the kidney tubules.

E

Explain. Urea is not reabsorbed, so it becomes concentrated in the tubules and is excreted in the urine.

Mark-scheme matches: protein not filtered (too large) ✓ · all glucose reabsorbed ✓ · urea filtered ✓ · urea not reabsorbed / excreted ✓. 4 / 4.

Common Errors to Avoid

Four mistakes the mark scheme penalises every year. Memorise the right-hand side.

✗ Wrong

"The kidney makes urea."

✓ Right

Urea is made in the liver ; the kidney filters it out of the blood.

✗ Wrong

"ADH increases urine production."

✓ Right

ADH decreases urine volume — more water is reabsorbed.

✗ Wrong

"Urea is reabsorbed."

✓ Right

Urea is not reabsorbed — it's excreted in urine.

✗ Wrong

"Selective reabsorption removes urea."

✓ Right

Selective reabsorption keeps useful substances (glucose, ions, water); urea is left in the urine.

Summary — Three Takeaways

1. The Kidney's Two Named Processes

Filtration: small molecules (water, ions, glucose, urea) leave the blood; protein and blood cells stay (too large).

Selective reabsorption: all glucose + needed water and ions go back into the blood. Urea is not reabsorbed — it's excreted in urine.

2. ADH — One Word: Permeability HT only

Released by the pituitary when blood is too concentrated.

Increases the permeability of the kidney tubules to water — write this phrase exactly.

Controlled by negative feedback .

3. Nitrogen Balance & Dialysis

Deamination (HT): excess amino acids → ammonia (toxic) → urea — all in the liver .

Dialysis: urea diffuses across a partially permeable membrane into dialysis fluid; glucose and ions stay because their concentrations match.

Transplant: long-term solution but risks rejection — immunosuppressants needed.

Next Lesson

Lesson 5.07 — Hormones in human reproduction: FSH, LH, oestrogen, progesterone, the menstrual cycle, and contraception.