Excretion, Grade 11: The Kidney Dissection Adds to 36, Not 35, and Two Free Rubrics

The kidney dissection is a formal assessment printed at 35 marks. Its questions add to 36.

We counted them: 3, 1, 1, 5, 2, 1, 2, 3, 8 and 10. That is 36 against a printed [35], and one step in the dissection carries no mark at all.

It gets worse for the marker. Eighteen of those marks, more than half the task, depend on a rubric that is never supplied. Eight for a labelled drawing and ten for the teacher to assess the dissection and the record of it. So we wrote both rubrics, and they are on this page, free.

Where this fits in the curriculum

Subject Life Sciences
Grade 11
Term 3
Topic Topic 9, Excretion in humans
Units Unit 1 excretion in various organs. Unit 2 the urinary system. Unit 3 osmoregulation. Unit 4 diseases of the urinary system
Formal assessment Practical task, dissect a kidney, printed at 35 marks
Marks Our worksheet is 70

Finding 1: the assessed task adds to 36, not 35

Question What is asked Marks
1 Why is there so much fat around each kidney 3
2 Function of the renal capsule 1
3.1 Where the three tubes enter or leave 1
3.2 Can you tell the three tubes apart, and how 5
3.3 Do they look like the diagram 2
4.1 Find the pelvis, cortex and medulla 1
4.2 Push a needle through each tube to see where it leads No mark
4.3 How a sheep or pig kidney differs from a human one 2
5 Where the branches of the blood vessels lead 3
6 Draw and label your longitudinal section 8
7 Teacher assesses the dissection and the drawing 10
Printed total 35
Actual total 36

Decide before the class and tell them. Two honest options:

  • Mark out of 36 and scale to 35. Nobody loses a mark they earned
  • Drop question 3.3 from 2 to 1. It is the weakest question in the task, since it asks only whether the real kidney looks like a drawing

Question 4.2 is the one that should carry a mark and does not. Pushing a probe into the renal artery, the renal vein and the ureter in turn, and seeing where each one comes out, is the single most instructive thing a learner does in the whole dissection. It is also the step that proves they understood what they were looking at.

And question 3.2 is a yes or no question worth five marks

It reads: "Can you tell these three tubes apart? If you can, explain how."

A learner who writes "no" has answered it. Five marks, gone, and nothing in the wording says they were supposed to try.

Set it as a statement instead: "Describe three differences between the renal artery, the renal vein and the ureter, and state which is which on your specimen."

Tube How to tell it
Renal artery Thickest wall, round and firm, stays open when empty, smallest opening
Renal vein Thin wall, collapses flat when empty, widest opening
Ureter Whitish and muscular, leads down and away from the kidney rather than towards the heart, and a probe pushed into it comes out in the pelvis

The two rubrics the task needs, free

These are ours. Use them, change them, print them. What matters is that every learner in the school is marked against the same thing.

Rubric A: the dissection, 10 marks, group mark

Criterion Marks
Fat removed without tearing the renal capsule, kidney intact and correctly oriented 2
Renal artery, renal vein and ureter all located and correctly identified 3
Longitudinal section is a single clean cut, both halves intact, not sawn 2
Cortex, medulla and pelvis all visible in the cut face and correctly pointed out 2
Bench cleared, every blade accounted for, tissue bagged, hands washed 1
Total 10

Rubric B: the labelled drawing, 8 marks, individual mark

Criterion Marks
Pencil, clean single lines, no shading and no sketching 1
Large enough to fill at least half the space given 1
Proportions roughly match the specimen 1
Label lines ruled, not crossing, each touching the structure it names 1
Labels horizontal, printed, outside the drawing 1
At least six structures correctly labelled: cortex, medulla, pelvis, ureter, renal artery, renal vein 2
Title states what it is and that it is a longitudinal section 1
Total 8

Hand both rubrics out before the dissection, not after. A learner who knows the cut has to be single and clean takes more care with the knife, which is a safety gain as well as a marking one.

Finding 2: ultrafiltration without the pressure that causes it

The summary says: "Ultrafiltration takes place from the glomerulus to the Bowman's capsule, into which useful substances and wastes enter."

That describes what happens and not one word of why, and the why is the mark.

What is missing Why it matters
The efferent arteriole is narrower than the afferent arteriole This is the whole cause. More blood arrives than can leave, so pressure builds up inside the glomerulus and forces fluid out through the capillary wall
What does not pass through Blood cells and large plasma proteins stay in the blood, because they are too big for the filter. This is what makes it ultrafiltration rather than ordinary filtration

The summary does mention the afferent and efferent arterioles, in the bullet before. It just never says that one is narrower than the other, which is the only reason either of them is worth naming.

Test it on yourself. If a question asks why ultrafiltration happens, "because of high pressure" scores one mark. "Because the efferent arteriole is narrower than the afferent arteriole, so blood leaving is restricted and pressure in the glomerulus rises" scores the lot.

Finding 3: urine does not contain ammonia, and that is the point of urea

"The urine contains urea, uric acid, ammonia and other toxic substances."

Ammonia is exactly what urine is designed not to contain.

Step What happens
1 Breaking down excess amino acids in the liver produces ammonia
2 Ammonia is extremely toxic, even in small amounts, and very soluble
3 So the liver immediately converts it to urea, which is far less toxic and can be carried safely in the blood
4 The kidneys remove the urea

Listing ammonia as a thing urine contains removes the reason urea exists at all. A learner who has read that bullet cannot answer "why is nitrogenous waste excreted as urea rather than ammonia", which is a standard question.

There is a small, real exception and it is worth knowing: the tubules do secrete a little ammonium, as part of controlling the pH of the blood. The same summary mentions pH control in the distal tubule two bullets earlier. That is a completely different thing from ammonia being a nitrogenous waste product, and it is not what the bullet means.

Finding 4: bilharzia caused by bilharzia worms

"Bilharzia is cause by parasitic bilharzia worms, which reproduce in the blood vessels of the bladder."

That sentence names no organism, explains no life cycle, and tells a learner nothing about how a person catches it or avoids it. In a country where bilharzia is endemic across the eastern and northern provinces, that is the one disease in this topic that should be taught properly.

Stage What happens
1 An infected person passes eggs in urine. The urinary form is caused by Schistosoma haematobium, a blood fluke
2 Eggs reaching fresh water hatch into a free-swimming larva
3 The larva must find a particular freshwater snail. No snail, no bilharzia
4 Inside the snail it multiplies, then leaves as a second larval stage
5 That larva burrows straight through the skin of anyone in the water. You do not have to drink it
6 The adult flukes mature and live in the blood vessels around the bladder, and the eggs break through into the urine

The snail is the fact that changes behaviour. Still or slow-moving fresh water in an endemic area is the risk. Chlorinated pools, sea water and tap water are not. Blood in the urine after swimming in a dam is the sign to take to a clinic, and it is treatable.

Finding 5: the kidney defined without the part the next bullet needs

"The kidneys consist of the cortex and medulla, where urine is produced in the nephrons."

The very next bullet: "The pelvis collects the urine from all the nephrons."

So the pelvis is not part of the kidney, and yet it is inside the kidney doing the collecting. A kidney has three regions: cortex, medulla and pelvis, and the assessed dissection asks learners to find all three.

This is the fourth time in this grade that a summary has defined a system and left out a part the next sentence depends on.

Topic What was defined What was left out
Micro-organisms Micro-organisms as unicellular The multicellular protists and fungi named two bullets later
Animal nutrition A balanced diet, five components Water and fibre
Gaseous exchange The breathing system as lungs and air passages The diaphragm, which the next bullet says drives breathing
Excretion The kidney as cortex and medulla The pelvis, which the next bullet says collects the urine

Treat every definition in a summary as a list that may be short. Read the next three bullets and check that nothing in them is missing from the definition above. It takes ten seconds and it has found something four times.

Finding 6: the gloves are specified as latex

Latex allergy is real, it is not rare, and it can be serious. Nitrile gloves do the same job, cost the same or less, and nobody reacts to them.

Price In stock
Nitrile, powder free, box of 100 R85,00 150
Latex, powder free, box of 100 R95,00 221

The nitrile is ten rand cheaper and we are recommending it over a line we also sell. That is the whole argument.

Running the dissection safely

The printed safety note says to handle the blade with care and wash your hands afterwards. True, and not enough for raw offal in a classroom.

  1. Buy the kidneys fresh from a butcher on the day, or the day before, and keep them refrigerated. Not frozen and thawed twice
  2. No eating, drinking or phones at the bench. Say it before the gloves go on
  3. Cover any cut on a hand with a plaster before gloving
  4. Cut away from yourself and away from the person opposite. The kidney is slippery and the task asks for one firm clean stroke
  5. Count the blades out and count them back. A disposable scalpel is the safest option because it is counted, used once and binned in a sharps container
  6. Double bag the tissue and bin it the same day. Disinfect the benches and the boards
  7. Wash hands with soap, gloves off last

What the four units cover

Unit What is in it
1. Excretion in various organs Lungs, liver, skin and kidneys, and what each one gets rid of
2. The urinary system Kidney, ureter, bladder, urethra, the nephron, ultrafiltration, reabsorption and tubular secretion
3. Osmoregulation ADH and water balance, aldosterone and salt balance, both as negative feedback
4. Diseases of the urinary system Cystitis, bilharzia, kidney stones, dialysis and transplant

The nephron, in the order the fluid travels

  1. Glomerulus. A ball of capillaries. Ultrafiltration happens here
  2. Bowman's capsule. The cup that catches the filtrate
  3. Proximal tubule. Nearest to the capsule. Most reabsorption of glucose, amino acids and salts
  4. Loop of Henle. Salts are pumped out into the medulla, which makes the medulla salty and lets water be drawn out later
  5. Distal tubule. Fine adjustment, tubular secretion and pH control
  6. Collecting duct. Shared between nephrons, runs down through the salty medulla, and this is where ADH does its work

Proximal means nearest, distal means furthest. If you remember that, you never have to guess which tubule is which.

What this topic costs

Under R15 a group in consumables, plus a kidney from a butcher. This is one of the cheapest formal assessments in the grade.

Item Price In stock Why
Disposable sterile scalpel, no. 11 R5,46 318 The safest option. Counted out, used once, into a sharps bin. No blade changing
Nitrile gloves, box of 100 R85,00 150 About R1,70 a learner, and no latex allergy
Dissecting needles, pack of 5 R92,23 25 This is the probe question 4.2 needs. Reusable for years
Forceps, blunt point R20,00 619 For lifting the capsule and the fat
Dissecting board, 450 x 300 mm R686,26 43 Reusable. A kitchen chopping board also works
Dissecting tray with wax R900,00 9 Better than a board if you want to pin anything
Dissecting set of instruments R425,00 4 Everything in one case. Only four left

The cheapest honest way to run this: one disposable scalpel and one pair of gloves per learner, one needle and one pair of forceps per group, and a kitchen chopping board. Everything except the scalpel and the gloves lasts for years.

More at safety gear.

We do not sell kidneys and we are not going to start. A butcher will sell you sheep or pig kidneys for a few rand each, fresh, which is exactly what this task needs.

What you should be able to do by the end

  • Name the four excretory organs and what each one excretes
  • Explain why nitrogenous waste leaves as urea and not ammonia
  • Name the parts of the urinary system and the three regions of the kidney, pelvis included
  • Describe a nephron in the order the fluid travels
  • Explain ultrafiltration, including the arteriole difference that causes it and what does not pass through
  • Explain selective reabsorption and tubular secretion
  • Work through ADH and aldosterone as negative feedback, in both directions
  • Give the bilharzia life cycle, including the snail, and say how to avoid it
  • Tell a renal artery, a renal vein and a ureter apart on a real specimen

Free worksheet and marking memo

Both free, no sign up, straight to the PDF.

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