OCR A Level Biology A: The kidney, osmoregulation and renal failure (H420) – Revision Notes

These notes condense learning outcomes 5.1.2(c) to 5.1.2(f) of the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025. For full explanations read the study guide, then test yourself with the practice set. Course pages: the OCR A Level Biology hub and the printable checklist. To find weak spots first, use the free diagnostics.

Key terms

Term	Meaning
Nephron	Functional unit of the kidney: renal corpuscle, PCT, loop of Henle, DCT, drained by a collecting duct
Ultrafiltration	Pressure filtration of blood plasma from the glomerulus into Bowman’s capsule
Selective reabsorption	Return of useful substances from the filtrate to the blood, mainly in the PCT
GFR	Glomerular filtration rate: volume of filtrate formed per minute by all nephrons
Osmoreceptor	Cell in the hypothalamus that detects the water potential of the blood
ADH	Antidiuretic hormone, released from the posterior pituitary gland; raises collecting duct permeability to water
Haemodialysis	Removal of waste from blood through a partially permeable membrane in a machine
Monoclonal antibody	Antibody from one clone of cells, specific to one antigen

Structure at a glance

• Kidney: capsule, cortex (corpuscles, PCT, DCT), medulla (loops, collecting ducts), renal pelvis, ureter. Renal artery in, renal vein out.
• Afferent arteriole wider than efferent, which raises the pressure in the glomerulus.
• Filtration barrier: capillary endothelium with gaps, basement membrane, podocytes of Bowman’s capsule.
• PCT cells: microvilli, many mitochondria, tight junctions.
• Medulla: tubes run lengthways. Cortex: round corpuscles and many circular tubule sections.

Nephron regions and jobs

Region	Job
Renal corpuscle (cortex)	Ultrafiltration of plasma
PCT (cortex)	Reabsorbs glucose, amino acids, most ions and much of the water
Loop of Henle (medulla)	Ascending limb pumps ions out but does not let water follow, so the medulla gets a very low water potential; descending limb loses water
DCT (cortex)	Adjusts ions and pH
Collecting duct	Water reabsorbed in response to ADH, down the gradient set up by the loop

Longer loops of Henle give a steeper gradient, so animals of dry places can make more concentrated urine. Urine is water, urea, creatinine and excess ions, with no glucose or protein normally.

Method in steps: ultrafiltration

1. Wide afferent and narrower efferent arteriole give high hydrostatic pressure in the glomerulus.
2. Water and small solutes pass through the endothelium gaps, the basement membrane and the podocyte slits.
3. Blood cells and most proteins stay in the blood.
4. Filtrate contains water, glucose, amino acids, urea and ions.

Net outward pressure = capillary hydrostatic pressure − capsule pressure − effect of plasma proteins.

Method in steps: reabsorption in the PCT

1. Sodium-potassium pump moves sodium ions out of the cell into the blood side.
2. Sodium ions diffuse in from the filtrate through co-transporters, bringing glucose or amino acids with them.
3. Glucose and amino acids exit the cell by facilitated diffusion.
4. Water follows by osmosis.

If the filtered load exceeds the carrier capacity, glucose is left in the filtrate and appears in the urine.

Method in steps: ADH response to a low blood water potential

1. Osmoreceptors in the hypothalamus detect the low water potential.
2. The posterior pituitary releases more ADH.
3. ADH binds to receptors on collecting duct cells.
4. More aquaporins are placed in the cell-surface membrane, so permeability to water rises.
5. More water is reabsorbed by osmosis, down the water potential gradient set up by the loop of Henle.
6. Small volume of concentrated urine; blood water potential rises, which switches the response off (negative feedback).

For a high blood water potential, reverse the steps: less ADH, fewer aquaporins, large volume of dilute urine.

Kidney failure

Effect	Explanation
Lower GFR	Damaged glomeruli filter less plasma
Urea and creatinine rise	Less filtered and excreted
Electrolyte imbalance	Sodium, potassium, phosphate not regulated; high potassium disturbs the heartbeat
Fluid retention	Oedema and higher blood pressure
Protein in urine	Damaged filtration barrier

Treatments compared

Feature	Haemodialysis	Transplant
What it does	Machine removes waste across a partially permeable membrane	Donor kidney does filtration and reabsorption
Key design	Counter-current flow keeps a gradient; dialysis fluid has no urea and normal glucose and ion levels; anticoagulant; air-bubble trap	Tissue-type matching
Advantages	Available without a donor; can be started quickly	No dialysis timetable; more normal diet and fluid intake
Disadvantages	Time-consuming; blood access site can become infected	Few donors; rejection risk; lifelong immunosuppressant drugs raise infection risk

Urine in diagnosis

• Protein in urine (biuret reagent) can suggest kidney damage; glucose (reducing sugar test) can suggest diabetes.
• Pregnancy test: the strip has mobile monoclonal antibodies to hCG on coloured beads, a test line of immobilised antibodies to hCG, and a control line. Two lines mean hCG is present.
• Steroids and drugs: monoclonal antibodies specific to the substance can be used in immunoassays on urine samples.

Tissue and drawing reminders

• Stained cortex: renal corpuscles with a tuft of capillaries inside a capsule, and PCT sections with a brush border, which stain more densely than DCT sections.
• Stained medulla: long tubes cut lengthways, collecting ducts with pale cuboidal cells.
• Plan drawings at low power show regions only; cell drawings at high power show the cells of a few tubules.

Practical reminders

• Dissection (PAG1, PAG2): gloves, sharp scalpel, cut away from the body, disinfect afterwards; examine outside first, then a longitudinal section.
• Drawing: sharp pencil, continuous lines, no shading, ruled label lines, title, magnification.
• Magnification = image size ÷ actual size. Convert units first: 1 mm = 1 000 µm.

Worked reminder: GFR. Plasma creatinine 0.015 mg cm⁻³, urine creatinine 1.8 mg cm⁻³, urine flow 1.0 cm³ min⁻¹. GFR = (1.8 × 1.0) ÷ 0.015 = 120 cm³ min⁻¹.

Worked reminder: percentage change. Urine flow falls from 1.5 to 0.6 cm³ min⁻¹. Change = (0.6 − 1.5) ÷ 1.5 × 100 = −60%.

Worked reminder: magnification. A tubule on a micrograph is 54 mm wide and is 0.15 mm in life. Magnification = 54 ÷ 0.15 = ×360.

Model sentences to learn

• “Glucose is found in the filtrate but not in the urine because it is reabsorbed by co-transport in the PCT.”
• “ADH makes the collecting duct walls more permeable to water, so more water is reabsorbed by osmosis and the urine is more concentrated.”
• “Dialysis fluid flows in the opposite direction to the blood, so a concentration gradient for urea is kept along the whole membrane.”
• “A donor kidney must be tissue-matched, otherwise the recipient’s immune system attacks it.”

Must-know distinctions

• Ultrafiltration (pressure, no selection except size) versus reabsorption (carriers, selective).
• Filtrate (in the capsule, contains glucose) versus urine (no glucose normally).
• Where blood water potential is detected (osmoreceptors in the hypothalamus) versus where ADH is released (posterior pituitary).
• Haemodialysis (outside the body) versus transplant (inside the body).
• A test line (hCG captured) versus a control line (beads captured whatever the sample).

Quick self-test

1. Name the two regions of the kidney that can be seen in a longitudinal section, apart from the pelvis.
2. Why is the afferent arteriole wider than the efferent arteriole?
3. List the three layers that filtrate crosses in the renal corpuscle.
4. Name two features of PCT cells that suit them to reabsorption.
5. What is the effect of ADH on the collecting duct?
6. A hot day makes blood water potential fall. State the stimulus detector and the gland that releases ADH.
7. A kidney patient has plasma creatinine 0.050 mg cm⁻³, urine creatinine 0.75 mg cm⁻³ and urine flow 1.2 cm³ min⁻¹. Calculate GFR.
8. State two reasons why dialysis fluid has no urea but does contain glucose.
9. State two risks of a kidney transplant.
10. What does the control line in a pregnancy test show?

Answers

1. Cortex and medulla.
2. It raises the hydrostatic pressure in the glomerulus, which drives filtration.
3. Capillary endothelium, basement membrane, podocytes.
4. Microvilli (surface area) and many mitochondria (ATP for active transport); also tight junctions.
5. It makes the walls more permeable to water, by adding aquaporins, so more water is reabsorbed.
6. Osmoreceptors in the hypothalamus; the posterior pituitary gland.
7. (0.75 × 1.2) ÷ 0.050 = 18 cm³ min⁻¹.
8. No urea, so urea diffuses out of the blood; glucose at the normal plasma level, so glucose is not lost from the blood.
9. Rejection by the immune system; infection because of immunosuppressant drugs (also surgical risk).
10. That the beads reached the line and the test worked, whether or not hCG is present.

Where marks are usually lost

• Writing “the kidney filters” without naming the glomerulus and Bowman’s capsule.
• Saying carriers select in filtration; it depends on pressure and the size of molecules.
• Leaving out the aquaporins or the collecting duct in an ADH answer.
• Mixing up detection (hypothalamus) with release (posterior pituitary).
• Forgetting units, or not converting mm to µm in magnification.
• Describing dialysis fluid as pure water.
• Giving only advantages for a treatment when asked to compare.
• Calling a control line a “negative result”.
• Describing a drawing without a magnification, or with shading and crossing label lines.
• Giving a GFR without units.

Official syllabus

OCR Advanced GCE in Biology A (H420), specification Version 4.1 (April 2026), first assessment 2025, published by OCR. Section 5.1.2, outcomes (c) to (f).