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3.5.8Exchange and Transport

Required Practical- Dissecting Mass Transport Systems — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 39 of 48 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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3.5.8 Required Practical- Dissecting Mass Transport Systems — AQA A Level Biology 7402 notes: Describe how to dissect a mammalian heart (and examine blood vessels), record observations accurately, and relate what is seen to function.This is one of the required practicals for AQA …

  • magnification = size of drawing ÷ actual size
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Tendinous cords
  • Atrioventricular valve

Section 1 of 9Purpose of the practical

This is one of the required practicals for AQA A Level Biology.

Purpose of the practical
A mammalian heart from a sheep, pig or cow, supplied from an abattoir or butcher, is dissected to observe the structure of the chambers, valves and vessels,
  • and to link it to function.
Blood vessels, either part of the specimen or a prepared slide, can be examined to show the differences between an artery and vein.
The skills assessed include safe working, making accurate observations, and scientific drawing.

Section 2 of 9Method

1
Wear a lab coat, eye protection and disposable gloves. Cover cuts and work on a disinfected board.
2
Look at the outside of the intact heart.
  • Identify the right and left sides (the left feels firmer, with a thicker wall), the atria with their ear-like auricles, the ventricles,
    • and the coronary vessels running over the surface.
  • Identify the aorta, the pulmonary artery, the vena cava and the pulmonary veins, and use a probe to check the openings.
3
Cut through the left ventricle wall from the apex towards the aorta with a sharp scalpel, cutting away from your hand. Open it out.
4
Observe and compare the thickness of the walls of the left and right ventricles.
5
Identify the atrioventricular valves, their flaps, the tendinous cords and the papillary muscles.
  • Locate the semilunar valves at the base of the aorta.
6
Cut a transverse section of an artery and a vein and compare them.
7
Draw labelled diagrams and measure the thickness of the walls with a ruler.
8
Dispose of the tissue in the correct bin, disinfect all surfaces and wash hands.

Section 3 of 9Observations and what they show

ObservationExplanation
Left ventricle wall about two to three times thicker than the rightPumps blood at high pressure around the body; right ventricle pumps to nearby lungs
Atria thin-walledOnly push blood a short distance into the ventricles
Tendinous cords attached to AV valvesStop the valves turning inside out when the ventricles contract
Septum between the two sidesPrevents the mixing of oxygenated and deoxygenated blood
Artery wall thick, lumen round; vein wall thin, lumen largerArtery withstands high pressure; vein carries low-pressure blood

Section 4 of 9Safety, ethics and risks

Safety, ethics and risks
Hazards: sharp blades (cuts), pathogens in raw tissue (infection), and disinfectant (irritant).
Control measures: cut away from the body, use a sharp blade for controlled cuts, wear gloves, cover cuts, wash hands, and disinfect surfaces.
Ethics: use tissue from animals already slaughtered for food, treat the material with respect, and dispose of it properly.

A risk assessment requires you to link each hazard, its risk and the precaution; for example, "the scalpel is sharp and may cause a cut, so cut away from the hand and onto the dissection board".

Section 5 of 9Drawing, magnification and evaluation

A biological drawing should be large, in pencil, with clear continuous lines and no shading, accurately proportioned, and labelled with straight lines, with no arrowheads.

Drawing, magnification and evaluation
If a scale is needed, calculate magnification = size of drawing ÷ actual size.
For example, a drawing of the heart 18 cm across of a heart that is 9 cm across has a magnification of ×2.

Limitations

  • the preserved heart is not beating, and the valves are not under pressure, so you cannot see them working;
    • vessels may have been cut short or damaged so that identification is harder.
  • Improvements include using fresh specimens, a camera to record the stages, and pumping water through the vessels to demonstrate the valves.

Section 6 of 9Preparing for the written paper

Questions on required practicals often test the method indirectly.

1
For example, you might be asked why the heart should be cut from the apex towards the aorta,
  • which is to avoid damaging valves and to reveal the full length of the ventricle and the valve at its exit.
2
You may be asked how to identify the left side,
  • which is by the thicker, firmer wall of the left ventricle and its connection with the aorta, the thickest-walled vessel.
3
You may also be asked to suggest how to compare the elasticity of an artery with that of a vein: take strips of equal width from each, hang equal masses on them and compare the extension,
  • or stretch them and see how quickly they return to their original length.
4
The artery stretches less under a load but recoils strongly.

Remember to plan a table before starting, with headings and units, such as vessel type, wall thickness / mm, and lumen diameter / mm.

Preparing for the written paper
Take repeated measurements at different points of the same section and calculate means.
Express thickness relative to the overall diameter if the vessels are of different sizes,
  • since this ratio gives a fairer comparison than the raw thickness.

Section 7 of 9Exam-style questions

  1. Describe how you would show that the wall of the left ventricle is thicker than that of the right ventricle in a dissection. [3]3 marks

  2. Explain why the left ventricle wall is thicker. [2]2 marks

  3. A drawing of a heart is 15 cm long and the actual heart is 10 cm long. Calculate the magnification. [2]2 marks

Section 8 of 9Key terms

TermMeaning
Tendinous cordsFibrous cords that stop AV valve flaps turning inside out.
SeptumWall separating the two sides of the heart.
Atrioventricular valveValve between atrium and ventricle.
Semilunar valvePocket valve at the base of the aorta or pulmonary artery.
MagnificationSize of image divided by actual size.
Risk assessmentIdentification of hazards and how to reduce harm.

Section 9 of 9Exam technique

1
Cutting towards the hand holding the specimen.
2
Not mentioning any safety or hygiene precautions in a method.
3
Reversing left and right when viewing the heart; the heart is described as in the body of the animal.
4
Calling the tendinous cords muscle that opens the valves.
5
Using shading and sketchy lines in biological drawings.

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