OCR GCSE Biology A: Osmoregulation: water balance, the kidney and ADH (J247) – Revision Notes

These notes condense statements B3.3f to B3.3j of module B3.3 in OCR GCSE (9-1) Gateway Science Biology A (J247), specification Version 4.0 (August 2026), for first assessment in 2018. For full explanations and worked examples, read the study guide. B3.3i and B3.3j are in bold in the specification, so they are tested only in the Higher Tier papers; they are labelled below. Course pages: OCR GCSE Biology hub | checklist | all free 10-minute diagnostics.

Key terms

Term	Meaning
Osmosis	Water moving through a partially permeable membrane, from a dilute region to a more concentrated one
Water potential	Describe as higher (dilute), lower (concentrated) or equal; no calculations needed
Lysis	An animal cell bursting after taking in too much water
Filtrate	Fluid forced out of the blood into the Bowman’s capsule
Reabsorption	Substances moving from the tubule back into the blood
ADH	Hormone that changes how permeable the collecting duct is to water (Higher Tier only)
Negative feedback	A response that undoes the change which set it off
Water balance	Water gained equals water lost

B3.3f: Cells and osmotic changes

Fluid around an animal cell	Water moves	Outcome
Higher water potential than the cell	In	Swells, may burst (lysis)
Equal	No net movement	No change
Lower water potential than the cell	Out	Shrinks

Why animal cells burst: no cell wall to resist swelling. That is why blood and tissue fluid are kept at a steady water potential.

Method for data questions:

1. Find the solution where the cell volume or mass does not change: equal water potential.
2. Solutions with a smaller concentration than that gain water; stronger ones lose water.
3. Percentage change = (new - original) / original x 100.

Small reminder: a cell volume going from 40 to 34 units is (34 - 40) / 40 x 100 = -15%, a 15% fall.

B3.3h: Structure

Urinary system: renal artery (blood in) and renal vein (blood out) -> kidney -> ureter -> bladder -> urethra.

Region of kidney	Contains
Cortex	Bowman’s capsules, convoluted tubules
Medulla	Loops of Henlé, collecting ducts
Pelvis	Collects urine and passes it to the ureter

Order of the tubule: Bowman’s capsule, proximal convoluted tubule, loop of Henlé, collecting duct.

Part	Job
Bowman’s capsule	Filtration under pressure: water, glucose, urea and ions leave the blood; cells and proteins do not
Proximal convoluted tubule	Selective reabsorption: all glucose, much water and many ions go back to the blood
Loop of Henlé	More water reabsorbed; fluid becomes more concentrated
Collecting duct	Final adjustment of water reabsorbed; carries fluid to the pelvis

B3.3g: Water balance

Water gained	Water lost
Drinks	Urine
Food	Sweat
Respiration	Breath, faeces

The kidneys alter the volume and concentration of urine to match what the body needs.

Body state	Urine
Too much water	Large volume, dilute
Too little water	Small volume, concentrated

Method for water balance sums: add all gains, add all non-urine losses, urine = gains - non-urine losses (when balance is kept).

Small reminder: gains 2400 cm³, non-urine losses 1100 cm³, so urine = 2400 - 1100 = 1300 cm³.

B3.3i: ADH and negative feedback (Higher Tier only)

• Released into the blood by the pituitary gland; the hypothalamus detects the water potential of the blood.
• Target: the kidney tubules, mainly the collecting duct.
• ADH raises the permeability of the walls to water, so more water is reabsorbed.

	More ADH	Less ADH
Trigger	Blood too concentrated	Blood too dilute
Permeability	Up	Down
Water reabsorbed	More	Less
Urine	Small volume, concentrated	Large volume, dilute

Feedback chain (write it in this order):

1. Blood water potential falls.
2. Hypothalamus detects it.
3. More ADH is released.
4. Collecting duct more permeable, more water reabsorbed.
5. Blood water potential rises to normal.
6. Less ADH is released.

B3.3j: Challenges (Higher Tier only)

Challenge	Blood becomes	Response
Hot weather, heavy sweating, dehydration	More concentrated	Thirst; more ADH; little concentrated urine
Drinking too much	More dilute	Less ADH; much dilute urine
Very salty food	More concentrated	Thirst; more ADH; water kept, salt lost in small volume of concentrated urine

Thirst is a response of behaviour: it makes you take in water, while the kidneys reduce losses. Both are needed.

Risk of failure: dehydration shrinks cells; very large drinks dilute body fluids so cells swell.

Small reminder: a person of mass 55.0 kg falls to 54.2 kg after a hot run. Loss = 0.8 kg, which is 0.8 / 55.0 x 100 = 1.5% of body mass.

Answer frames to learn

Use these as skeletons and swap in the details of the question.

Explain why a cell shrinks. The fluid has a lower water potential than the cytoplasm. Water moves out of the cell by osmosis, through the partially permeable membrane. The cell loses volume and shrinks.

Describe how the kidney makes concentrated urine (Higher Tier only). The blood is too concentrated. The hypothalamus detects this. More ADH is released. The collecting duct becomes more permeable to water. More water is reabsorbed into the blood. A small volume of concentrated urine is produced.

Explain the response to excess water (Higher Tier only). The blood is more dilute. Less ADH is released. The collecting duct is less permeable. Less water is reabsorbed. A large volume of dilute urine is produced, which removes the extra water.

Reading data in this module

Typical tables give urine volume against time, cell volume against solution concentration, or urine volume against ADH level. Steps:

1. Read the axis labels and units before you look at the numbers.
2. Describe the trend first (rises, falls, levels off), quoting two values with units.
3. Then explain it using water potential, permeability and water reabsorbed.
4. For percentage questions, show the subtraction, the division by the original value and the multiplication by 100.

Small reminder: urine volume falls from 1300 cm³ to 400 cm³ on a hot day. Change = 400 - 1300 = -900, and -900 / 1300 x 100 = -69.2%, a fall of about 69%.

Must-know distinctions

• Filtration (in Bowman’s capsule, by pressure, not selective for useful substances) versus reabsorption (later, selective).
• Concentrated urine (small volume, little water) versus dilute urine (large volume, much water).
• High ADH is a response to a concentrated blood; low ADH is a response to a dilute blood.
• Water potential: higher means more dilute; water moves from higher to lower.
• Cortex (capsules) versus medulla (loops and collecting ducts).

Quick self-test

1. What happens to a red blood cell placed in a fluid with a lower water potential than its cytoplasm?
2. Why does an animal cell burst in pure water?
3. Name the four parts of the kidney tubule in order.
4. Which region of the kidney contains the Bowman’s capsules?
5. A person gains 2400 cm³ of water and loses 1100 cm³ by routes other than urine. How much urine keeps the balance?
6. State two substances forced into the Bowman’s capsule that are not proteins or blood cells.
7. (Higher Tier only) Where is ADH released from, and where does it act?
8. (Higher Tier only) State the effect of ADH on the collecting duct.
9. (Higher Tier only) Describe the urine produced after a very salty meal and give the reason.
10. (Higher Tier only) Explain why drinking a lot of water makes the urine pale and plentiful.
11. A cell volume falls from 40 to 34 units. Calculate the percentage change.

Answers

1. Water leaves by osmosis and the cell shrinks.
2. Water enters by osmosis, and with no cell wall the membrane tears (lysis).
3. Bowman’s capsule, proximal convoluted tubule, loop of Henlé, collecting duct.
4. The cortex.
5. 2400 - 1100 = 1300 cm³.
6. Any two of water, glucose, urea, ions.
7. Released from the pituitary gland into the blood; acts on the kidney tubules, mainly the collecting duct.
8. It increases the permeability of the duct walls to water, so more water is reabsorbed.
9. Small volume and concentrated: the blood is too concentrated, so more ADH is released and more water is reabsorbed; excess salt leaves in the urine.
10. The blood becomes more dilute, less ADH is released, the ducts are less permeable, and less water is reabsorbed.
11. (34 - 40) / 40 x 100 = -15%, a fall of 15%.

Where marks are usually lost

• Saying a cell “bursts” without saying that water enters by osmosis.
• Using vague words such as “weaker” or “stronger” water instead of describing a higher or lower water potential.
• Saying the kidney makes ADH, or that ADH “tells the kidney to keep water” without naming permeability.
• Saying that ADH changes the amount of urine without saying the permeability of the collecting duct changes.
• Describing only one direction of the feedback when the question asks for the response to too much water.
• Missing the stage where the blood returns to normal and ADH release falls.
• Writing the percentage change without a sign or direction, or using the final value as the divisor in place of the starting value.
• Confusing the cortex and the medulla, or placing the loop of Henlé before the proximal convoluted tubule.
• Forgetting thirst when asked for the response to dehydration.

Test yourself next with the practice questions, and use the diagnostics to find weak spots.

Official syllabus

OCR GCSE (9-1) Gateway Science Biology A (J247), specification Version 4.0 (August 2026), for first assessment in 2018, published by OCR.