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

This guide covers 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. It teaches kidney and nephron structure, ultrafiltration, reabsorption, ADH, kidney failure and its treatment, urine in diagnosis, and the dissection and histology practicals. The meaning of excretion and the liver (5.1.2(a) and (b)) are separate units.

Use it with the revision notes and the practice set. Course pages: the OCR A Level Biology hub and the printable checklist. Practical skills: Development of practical skills. Not sure where you stand? Try the free diagnostics.

Outcomes covered

Outcome	You must be able to
5.1.2(c)(i)	Describe kidney and nephron structure and blood vessels; explain ultrafiltration, selective reabsorption and urine production
5.1.2(c)(ii)	Dissect, examine and draw the external and internal structure of a kidney
5.1.2(c)(iii)	Examine and draw stained sections to show the histology of nephrons
5.1.2(d)	Explain control of blood water potential: osmoreceptors, posterior pituitary, ADH and the collecting duct walls
5.1.2(e)	Explain kidney failure (GFR, electrolyte balance), haemodialysis and transplants
5.1.2(f)	Explain urine tests in diagnosis, including monoclonal antibodies for pregnancy, anabolic steroids and drugs

Gross structure and blood supply

The kidneys are bean-shaped organs at the back of the abdomen, each wrapped in a fibrous capsule. A longitudinal section shows three regions:

• The cortex, a darker outer layer containing the renal corpuscles, the proximal and distal convoluted tubules and the blood capillaries around them.
• The medulla, a paler inner region made of triangular pyramids that contain the loops of Henle and the collecting ducts.
• The renal pelvis, a funnel-shaped space where urine collects before it leaves down the ureter. Ureters carry urine to the bladder.

The renal artery brings blood carrying urea to the kidney. The renal vein carries away blood with less urea, adjusted for water and ions.

The nephron

A nephron is the functional unit of the kidney.

Region	Where	Structure	Main job
Renal corpuscle	Cortex	Glomerulus (knot of capillaries) inside the cup-shaped Bowman’s capsule	Ultrafiltration
Proximal convoluted tubule (PCT)	Cortex	Cuboidal cells with microvilli and many mitochondria	Selective reabsorption
Loop of Henle	Medulla	Descending limb then ascending limb	Builds a low water potential in the medulla
Distal convoluted tubule (DCT)	Cortex	Cuboidal cells, fewer microvilli	Adjusts ions and pH
Collecting duct	Cortex to medulla	Walls respond to ADH	Final water reabsorption

The afferent arteriole carries blood into the glomerulus and the efferent arteriole carries it out. The afferent is wider than the efferent. Blood leaving the efferent arteriole enters a second capillary network around the tubules, the peritubular capillaries, which take up reabsorbed substances.

Ultrafiltration

Ultrafiltration happens in the renal corpuscle. The glomerulus has a high hydrostatic pressure: the narrower efferent arteriole slows the exit of blood that the wider afferent arteriole delivers. Fluid and small solutes are forced out through three layers:

1. The fenestrated (gapped) endothelium of the capillary, which holds back blood cells.
2. The basement membrane, a gel of glycoproteins and collagen, which holds back most plasma proteins because its mesh of fibres is too fine for large molecules.
3. The podocytes of the inner wall of Bowman’s capsule, whose finger-like pedicels leave filtration slits.

The filtrate contains water, glucose, amino acids, urea and ions, but not blood cells or most proteins. The plasma proteins left behind lower the water potential of the blood and pull some water back, but the high hydrostatic pressure outweighs this.

Worked example 1: net movement out of the glomerulus. The hydrostatic pressure in the glomerular capillary is 7.4 kPa, the hydrostatic pressure in the capsule is 1.9 kPa and the effect of plasma proteins is equivalent to 3.3 kPa pulling water into the capillary.

Net outward pressure = 7.4 − 1.9 − 3.3 = 2.2 kPa. This is positive, so fluid moves out into the capsule.

Worked example 2: GFR from clearance. The glomerular filtration rate (GFR) is the volume of filtrate formed per minute by all the nephrons. Creatinine is filtered and is neither reabsorbed nor secreted to a significant extent, so its clearance estimates GFR. A person has plasma creatinine 0.012 mg cm⁻³, urine creatinine 0.90 mg cm⁻³ and urine flow 1.5 cm³ min⁻¹.

GFR = (urine concentration × urine flow) ÷ plasma concentration = (0.90 × 1.5) ÷ 0.012 = 112.5 cm³ min⁻¹.

Reabsorbed each minute = 112.5 − 1.5 = 111 cm³, which is 111 ÷ 112.5 × 100 = 98.7% of the filtrate. Per day the filtrate is 112.5 × 60 × 24 = 162 000 cm³ (162 dm³).

Selective reabsorption and the production of urine

Most reabsorption takes place in the proximal convoluted tubule. Its cells have microvilli for surface area, many mitochondria to supply ATP, and tight junctions that stop fluid leaking between cells.

1. Sodium ions are actively transported out of the PCT cell into the blood side by the sodium-potassium pump, which lowers the sodium ion concentration in the cell.
2. Sodium ions then diffuse in from the filtrate through co-transporter proteins, which carry glucose or amino acids in with them against their concentration gradients.
3. Glucose and amino acids pass from the cell into the capillary by facilitated diffusion.
4. Water moves by osmosis from the filtrate to the blood, because the water potential of the blood is now lower.

All the glucose is normally reabsorbed. Each carrier protein can work only at a maximum rate, so if the filtered load of glucose is too high, some stays in the filtrate and appears in the urine.

Worked example 3: glucose and the carrier limit. Take these as supplied: filtrate forms at 0.1125 dm³ min⁻¹, and the glucose carriers have a combined maximum of 280 mg min⁻¹.

• Plasma glucose of 1.0 g dm⁻³ gives 1.0 × 0.1125 = 0.1125 g min⁻¹, or 112.5 mg min⁻¹. This is below 280, so all of it is reabsorbed.
• Plasma glucose of 3.4 g dm⁻³ gives 3.4 × 0.1125 = 0.3825 g min⁻¹, or 382.5 mg min⁻¹. The excess is 382.5 − 280 = 102.5 mg min⁻¹, which is left in the tubule and passes out in the urine.

Unabsorbed glucose holds water in the tubule by osmosis, so urine volume rises.

Urine production. Urine is what is left after filtration and reabsorption: water, urea, ions in excess, creatinine and traces of other waste. Normal urine has no glucose or protein. Some filtered urea diffuses back into the blood.

The loop of Henle and the collecting duct

To make urine more concentrated than blood, the kidney builds a very low water potential in the medulla. Ions are actively pumped out of the ascending limb into the medulla, and that limb is not permeable to water. Water leaves the descending limb by osmosis. The collecting duct then passes through the medulla, so if its walls are permeable, water leaves by osmosis down a water potential gradient. Longer loops give more concentrated urine, which suits animals of dry places.

Control of blood water potential by ADH

This is a negative feedback loop.

1. Osmoreceptors in the hypothalamus detect a fall in the water potential of the blood (too little water, or too much salt).
2. The hypothalamus signals the posterior pituitary gland, which releases antidiuretic hormone (ADH) into the blood.
3. ADH is carried by the blood to the collecting duct, where it attaches to receptors on the wall cells.
4. This leads to more aquaporins (water channels) in the cell-surface membrane, so the walls become more permeable to water.
5. More water leaves the collecting duct by osmosis and enters the blood, leaving a small volume of concentrated urine.

When blood water potential is high, less ADH is released, the walls stay less permeable and a large volume of dilute urine is produced.

Kidney failure and its treatment

Kidney failure means the kidneys can no longer filter the blood properly. Damage to glomeruli lowers the GFR, so less urea and creatinine are removed and these build up in the blood. Failing kidneys also lose control of electrolytes such as sodium, potassium and phosphate. Excess potassium can disturb the heartbeat. Retained water causes oedema and raised blood pressure. Damage to the filtration barrier lets protein leak into urine.

Haemodialysis uses a machine outside the body:

• Blood is taken from a vein or fistula, passes through the dialyser and returns to the body. An anticoagulant stops clotting and a trap removes air bubbles.
• In the dialyser, the blood flows past a partially permeable membrane. Dialysis fluid flows on the other side in the opposite direction (counter-current), so a concentration gradient is kept along the whole length.
• The dialysis fluid has no urea and is set at the normal plasma concentrations of glucose and ions. Urea and excess ions diffuse out of the blood, while glucose and needed ions are not lost.

Worked example 4: urea removal. Blood enters the dialyser at 300 cm³ min⁻¹ with urea at 30 mmol dm⁻³ and leaves with 12 mmol dm⁻³. Urea removed per minute = 0.300 dm³ × (30 − 12) = 5.4 mmol. Over a 4 hour session (240 min) this is 5.4 × 240 = 1 296 mmol, a 60% fall in urea concentration as blood passes through the dialyser.

A transplant gives a donor kidney, from a living or deceased donor, which restores filtration and the hormone roles of the kidney. Tissue types are matched as closely as possible, otherwise the immune system attacks the new organ. The recipient takes immunosuppressant drugs, which raise the risk of infection. A transplant frees the person from a dialysis timetable, but donors are in short supply.

Urine in diagnosis

Urine carries excretory products and other molecules, so it can be tested without taking blood. Protein (biuret reagent) and glucose (reducing sugar test) in urine may point to kidney damage or diabetes.

Pregnancy tests. The hormone hCG appears in the urine of a pregnant woman. In the test strip, urine moves along by capillary action. It first meets mobile monoclonal antibodies, specific to hCG, attached to coloured beads. If hCG is present it binds to them. A test line contains immobilised antibodies to hCG, which catch the hCG-antibody-bead complex and show a colour. A control line catches the beads whether or not hCG is present and shows the test has worked.

Anabolic steroids and drugs. Monoclonal antibodies specific to a steroid or drug can be used in an immunoassay on urine samples, so athletes can be screened. The antibody binds only that molecule.

Dissection and histology practical work

Kidney dissection (PAG1 and PAG2). Use a pig or sheep kidney. Wear gloves and eye protection, cut on a dissecting board with a sharp scalpel, cutting away from your body, and disinfect surfaces and wash hands afterwards. Examine the outside (capsule, renal artery, renal vein, ureter), then cut a longitudinal section and find the cortex, medulla and renal pelvis.

Drawing rules. Use sharp pencil, continuous lines, no shading. Draw large, with ruled label lines that do not cross, a title and a magnification.

Magnification = image size ÷ actual size.

Worked example 5: dissection drawing. The cortex is 6 mm thick in the kidney and 18 mm thick on your drawing. Magnification = 18 ÷ 6 = ×3.

Worked example 6: micrograph. A glomerulus measures 80 mm on a micrograph with magnification ×400. Actual size = 80 ÷ 400 = 0.20 mm = 200 µm.

Stained cortex shows rounded renal corpuscles and many circular tubule sections; the PCT has a brush border and the DCT has fewer microvilli. In the medulla the tubes run lengthways. Draw tissue plans at low power and cells at high power.

Common errors

• Saying ADH “makes the kidney reabsorb water” without naming the collecting duct and aquaporins.
• Confusing the position of osmoreceptors (hypothalamus) with the release site (posterior pituitary).
• Describing dialysis fluid as glucose-free, which would drain glucose from the blood.
• Calling the filtrate urine.

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).