Required Practical — Kidney Dissection and Data Analysis
Spec mapping: AQA 7402 Required Practical 5 — dissection of an animal or plant gas-exchange or mass-transport system; this lesson applies RP5 to the mammalian kidney, integrating gross anatomy with the nephron biology developed in lesson 1 and the filtrate composition data developed across the course (refer to the official AQA specification document for exact wording).
The AQA A-Level Biology specification lists twelve Required Practicals (RPs) — laboratory investigations that every candidate must perform and on which Paper 3 assesses experimental skills. Required Practical 5 is the dissection of an animal or plant gas-exchange or mass-transport system. Although the specification leaves the choice of organ open, the mammalian kidney is the canonical option for the Section 3.6 (homeostasis) topic, because the dissection reveals the gross architecture (cortex, medulla, pelvis) that underlies the microscopic nephron biology developed in lesson 1. This lesson develops the practical protocol, the data-analysis skills required, and the synoptic links to the rest of the course. It is written so that a candidate who has performed the dissection in class can revise it for the practical-skills components of Paper 3 and beyond; for any candidate who has not, it can serve as the theoretical preparation expected in advance of the lab.
Key Definition: A dissection is the systematic surgical opening of a biological specimen to expose internal anatomy for study. Mass-transport systems in the AQA sense are organs whose primary role is the bulk movement of substances — circulation, respiration, digestion, excretion. The kidney is a mass-transport system in the excretory and homeostatic sense.
Aims of the Practical
By the end of the dissection, candidates should be able to:
- Identify gross structures of the mammalian kidney externally: renal capsule, renal artery, renal vein, ureter, hilum.
- Identify regions on a longitudinal section: cortex (outer, granular), medulla with renal pyramids (inner, striated), renal pelvis (funnel into ureter), renal columns (cortical tissue between pyramids).
- Correlate gross anatomy with the nephron biology developed in lesson 1: where in the gross structure are glomeruli, loops of Henle, collecting ducts?
- Practise dissection technique: scalpel handling, longitudinal section, structure identification, hazard awareness.
- Analyse filtrate composition data (typically provided by the examiner) and reason about the functional state of each nephron region.
Workflow Overview
flowchart TB
Prep[Risk assessment + PPE + sharps brief] --> Ext[External examination<br/>identify capsule, artery, vein, ureter, hilum]
Ext --> Cut[Longitudinal section<br/>convex border through hilum]
Cut --> Int[Internal identification<br/>cortex, medulla, pyramids, pelvis]
Int --> Map[Correlate gross regions<br/>with nephron components]
Map --> Data[Analyse filtrate composition<br/>plasma → filtrate → PCT → urine]
Data --> Err[Discuss sources of error<br/>and mitigations]
Err --> Write[Write up: drawing, labels, analysis]
Equipment and Materials
A typical RP5 kidney dissection lab requires:
- Specimen: a fresh or chilled (not frozen — ice damages tissue) sheep or pig kidney. Sourced from a butcher with appropriate provenance; one kidney serves 2–3 students. Lamb kidney is too small for clear identification; whole pig kidney is ideal. Frozen specimens lose tissue detail and should be avoided; formalin-preserved specimens are an option but with strict ventilation requirements (formalin is carcinogenic).
- Dissecting board: white plastic or wax-lined tray with sufficient surface area.
- Scalpel and spare blades: sharps protocol requires the teacher to supervise blade changes and dispose in a designated sharps bin.
- Dissecting scissors: blunt-nose for safety; sharp-nose for fine cuts.
- Forceps: at least two pairs, fine and coarse.
- Mounted needles: for tracing ureter and pulling tissue layers.
- Hand lens or stereo microscope: optional but useful for examining cortical surface and the cut face.
- Disposable gloves: nitrile, fitted, replaced if punctured.
- Apron or lab coat: protective layer over uniform.
- Paper towels and waste container: a sealable biohazard bag for tissue disposal; the school's clinical waste contractor handles collection.
Hand-hygiene before and after the practical is mandatory regardless of glove use.
Safety Considerations
Kidney dissection carries modest but real biological and physical risks. The school's risk assessment (a written document covering this practical specifically) must be in place and read by the supervising teacher in advance.
- Biological hazard: animal tissue may carry zoonotic organisms (Salmonella, Campylobacter, occasional bovine prions in cattle though regulated). UK kidneys sourced from licensed abattoirs are inspected and considered Biosafety Level 1; gloves and hand-washing are sufficient.
- Sharps: scalpels are the principal cause of dissection injuries. Always cut away from yourself and others, anchor the specimen firmly before cutting, replace blunt blades promptly, and dispose of used blades in a yellow sharps bin.
- Allergic reactions: rare but possible. Anyone with a known animal-tissue allergy should not perform the dissection.
- Religious and ethical considerations: some candidates may have religious or ethical objections to handling pig (or other) tissue. Alternative options include observation of a virtual dissection (numerous educational videos exist) or examination of preserved specimens — both produce learning outcomes acceptable for RP5 assessment.
Method — Step by Step
External examination
- Place the specimen on the dissecting board with the hilum (the concave indentation where vessels enter) facing you. The kidney is bean-shaped, dark red-brown, with the renal artery, renal vein and ureter emerging from the hilum.
- Identify the renal capsule — a thin, tough fibrous covering. Pinch it with forceps and peel a small region to expose the underlying cortex surface.
- Trace the renal artery (smaller diameter, thicker wall) and renal vein (larger diameter, thinner wall) into the hilum. The ureter is paler, narrower, and emerges below the vessels.
- Note the perinephric fat (often removed by butcher but sometimes retained) — protective adipose tissue cushioning the kidney in vivo.
Longitudinal section
- Orient the kidney with the hilum to your left (or right, by hand preference) and the convex outer border to your right. Anchor the specimen with one hand (gloved, not the hand holding the scalpel).
- Make a single firm longitudinal cut through the kidney, passing through the hilum from the convex outer border. The cut should bisect the kidney, exposing the full cut face from cortex through medulla to pelvis.
- Open the two halves like a book. The cut face reveals the internal architecture.
Internal identification
- Cortex: outer ~5–8 mm rim, lighter colour, with a granular texture from the many glomeruli and convoluted tubules. Identify the cortex as a continuous outer layer.
- Medulla: inner zone, darker and visibly striated. Note that the medulla is organised into renal pyramids — cone-shaped regions with their bases on the cortex and apices pointing inwards toward the pelvis. Pig kidneys typically show 10–15 pyramids; sheep kidneys somewhat fewer.
- Renal columns: cortical tissue extending between adjacent pyramids — extensions of cortex into the medullary zone.
- Renal pelvis: the funnel-shaped cavity collecting urine from the pyramids. Calyces (major and minor) cup each papilla (apex of each pyramid). The pelvis narrows into the ureter.
- Ureter: a thick-walled muscular tube emerging from the pelvis at the hilum. Trace the lumen with a mounted needle if useful.
Correlation with the nephron
The microscopic nephron (lesson 1) maps onto the gross regions:
- Glomeruli and Bowman's capsules — in the cortex.
- Proximal and distal convoluted tubules — in the cortex.
- Loops of Henle — descend from cortex into medulla and return; the loop's depth into medulla determines how much it contributes to the medullary concentration gradient (juxtamedullary nephrons have long loops; cortical nephrons have short loops).
- Collecting ducts — descend through the medullary pyramids, draining at the papilla into the calyx.
- Vasa recta — long capillary loops accompanying the loops of Henle, preserving the medullary gradient by countercurrent exchange.
A skilled candidate should be able to point at a cut kidney and indicate where each nephron component sits architecturally. This synthesis between gross and microscopic anatomy is the central learning outcome of the practical.
Data Analysis — Filtrate Composition Tables
AQA Paper 3 frequently presents candidates with tables of solute concentrations at different points in the nephron and asks for interpretation. The standard layout is reproduced below (typical adult values; concentrations in mmol dm⁻³ or g dm⁻³ depending on solute).
| Solute | Plasma | Glomerular filtrate (Bowman's capsule) | End of PCT | Final urine |
|---|
| Sodium (Na⁺) | 142 mmol dm⁻³ | ~142 | ~142 | ~150 (variable) |
| Glucose | 5 mmol dm⁻³ | ~5 | ~0 | ~0 |
| Urea | 5 mmol dm⁻³ | ~5 | ~7 (mild concentration) | ~400 (heavily concentrated) |
| Protein (albumin) | 40 g dm⁻³ | ~0 | ~0 | ~0 |
| Water | (solvent) | ~99% | ~67% reabsorbed | ~99% reabsorbed |
Interpretation skills
- Glucose in plasma and filtrate is identical (~5 mmol dm⁻³), reflecting free filtration. By the end of the PCT it is ~zero — complete reabsorption by SGLT2/SGLT1 + GLUT2 (synoptic with lesson 1, glucose handling). In diabetes (lesson 2), plasma glucose rises above ~10 mmol dm⁻³, exceeding the renal threshold for SGLT2 saturation; some glucose escapes reabsorption and appears in urine (glucosuria).
- Urea in plasma and filtrate is identical (~5 mmol dm⁻³), reflecting free filtration. In urine it is ~400 mmol dm⁻³ — an ~80-fold concentration. The concentration arises from selective water reabsorption (water leaves while urea stays) plus urea's contribution to the medullary gradient.
- Protein in plasma is ~40 g dm⁻³ but ~0 in filtrate. The glomerular basement membrane excludes large proteins like albumin (~67 kDa). Detectable urinary protein (proteinuria) indicates glomerular damage — clinically significant.
- Sodium: in filtrate at plasma concentration, but final urine sodium varies enormously (10–150 mmol dm⁻³ depending on dietary intake and aldosterone status — lesson 1).
Calculation skills
A typical exam question might ask: "If 180 dm³ of filtrate is formed per day and final urine output is 1.5 dm³ per day, calculate the percentage of water reabsorbed." Answer: (180 − 1.5) / 180 × 100 = 99.2%.
Or: "Plasma glucose is 5 mmol dm⁻³. Glomerular filtration rate is 125 cm³ min⁻¹. Calculate the glucose filtered per minute." Answer: 5 mmol dm⁻³ × 0.125 dm³ min⁻¹ = 0.625 mmol min⁻¹ ≈ 113 mg min⁻¹.
Practise such calculations — they are foundational paper-3 skills and easily yield marks.
Sources of Error
Like all biological practicals, kidney dissection has characteristic error sources that A* candidates should be able to name and mitigate.
- Specimen variability: kidneys from different animals show variation in pyramid number, capsular fat retention and vascular branching. Group practice over multiple specimens averages out the variation.
- Tissue degradation: post-mortem autolysis softens tissue and obscures cortical–medullary boundaries; minimise time between butcher and dissection.
- Observer differences: students differ in dissection skill and in confidence to identify boundaries. Calibration against a teacher-led demonstration reduces this.
- Cutting inaccuracy: a non-longitudinal cut produces an oblique section in which pyramids and pelvis are hard to identify. Practise the cut on a previous kidney before the assessed one.
- Drawing accuracy: candidates frequently underestimate cortical thickness or misplace the pelvis–calyx boundary; reference labelled photographs alongside the specimen.
A robust account of sources of error and mitigation strategies often yields the extension marks on practical-skills questions.
A-Level Extension — Kleiber's Law and Renal Scaling
A mathematical curiosity that A* candidates can deploy on extension questions. Across mammals from shrews to elephants, basal metabolic rate scales with body mass to the ~3/4 power — an observation paraphrased here from Max Kleiber's 1932 work, not quoted. The same scaling exponent applies to renal mass: kidney mass ≈ 0.7% of body mass, with glomerular filtration rate scaling with body mass to ~0.75 power. The implications:
- Per-glomerulus filtration rate is approximately constant across species; the number of glomeruli scales with body mass.
- Specific metabolic rate (per kg) falls with body size; small mammals have very high mass-specific metabolic rates, requiring proportionately high renal clearance per kg.
- Allometric scaling has clinical implications: drug doses for veterinary use are typically scaled by body surface area (∝ mass^(2/3)) rather than mass directly, reflecting the metabolic-clearance scaling.
Allometry is one of biology's deepest cross-cutting principles. The kidney's scaling parallels the heart's (mass scales ~0.98 with body mass), the brain's (~0.8) and the lung's (~1.0). The differences across organs reflect the distinct functional demands each meets.
A-Level Extension — Comparison with Plant or Insect Mass Transport (RP5 Alternatives)
Although this lesson focuses on the mammalian kidney as RP5 specimen, the AQA specification permits alternative specimens. Brief outlines for orientation:
- Plant gas-exchange system: leaf cross-section (Course 7) — palisade mesophyll, spongy mesophyll, stomata. Microscope work with prepared slides; alternative to dissection per se.
- Plant mass-transport system: stem cross-section showing xylem and phloem (Course 7); root anatomy.
- Insect gas-exchange system: tracheal dissection in larger insects (locust); shows tracheal tubes and spiracles. Smaller and more delicate than kidney dissection.
- Fish gas-exchange system: gill arch dissection from a fresh fish; reveals gill filaments and lamellae arranged for countercurrent exchange.
The mammalian kidney is selected for this lesson because the synoptic link with the rest of the course is the tightest. Candidates who have done a different specimen for their school's RP5 should know the kidney dissection theoretically.
Common Errors and Mark-Loss Patterns