RP05 - Dissecting an exchange or transport system
A real heart shows what a flat diagram can't: how thin a valve is, how much thicker one ventricle is, how the main arteries cross. You will dissect safely, read each structure as evidence of function, draw what you see, and transfer the method to lungs, gills, insects and plants.
What Required Practical 5 asks you to do
Required Practical 5 allows many specimens, so you learn one in depth and an approach that works for the others.
You dissect an animal or plant gas exchange system, a mass transport system, or an organ within one of these systems. AQA's practical handbook uses a sheep's heart as its worked example, so this lesson does too. The same approach transfers to lungs, gills, insect tracheae and plant tissues.
Dissection
The careful opening and examination of a biological specimen so that visible structures can be identified and related to function.
Cutting is only the means. The aim is to observe real anatomy in three dimensions and use it as evidence of how the system works. In a heart, that means linking chambers, valves, wall thickness and blood vessels to the heart's role as a double pump.
The practical also builds three apparatus and technique skills:
- AT e: producing a scientific drawing from observation, with annotations
- AT h: using organisms safely and ethically
- AT j: using dissection instruments safely
How to dissect a heart safely
Each step here protects either you or a structure you still need, which is why the order matters.
Apparatus and safety
For the AQA heart example, the usual apparatus is below. This method also uses a blunt probe, and a ruler or calipers.
- sheep's heart, dissecting tray and board
- dissecting instruments such as scissors, scalpel and mounted needle
- pins and labels, and a magnifying lens if needed
- beaker of disinfectant, disinfectant spray and paper towels
- access to a camera or paper for a biological drawing
Follow your centre's risk assessment, which may add precautions such as gloves or eye protection.
| Hazard | Precaution | How it reduces the risk |
|---|---|---|
| Microorganisms on raw tissue | Wear a lab coat, cover cuts to skin, wash hands thoroughly afterwards | Keeps microorganisms off skin and out of cuts, so infection is less likely |
| Sharp instruments | Make small controlled cuts, with tools directed away from your fingers | A slipping blade is less likely to reach your hand |
| Contaminated surfaces | Keep the specimen on the tray; disinfect instruments and surfaces afterwards | Contains contamination, then removes it |
Orient and test
- Inspect the outside on a stable tray, finding the pointed apex and the coronary arteries. Don't assume your own left is the heart's left: orient it by vessel connections and the much thicker left ventricular wall, which makes the left side feel firmer when you press gently.
- Gently insert a blunt probe into each vessel opening to find the chamber it joins, never forcing it through a wall. The aorta joins the left ventricle, the pulmonary artery the right ventricle, the vena cava the right atrium and the pulmonary vein the left atrium. Arteries have relatively thick resilient walls; veins are thinner and collapse more easily. Connections are stronger evidence than position or colour, so record an identification as uncertain if a trimmed vessel can't be traced.
- If the risk assessment permits, investigate the valves before cutting, because a slit heart lets the water out. Over the tray, run a gentle stream of water into the top of the aorta or pulmonary artery and watch for the valve closing. Then squeeze the heart gently: the valves open and the water comes out.
Open and compare the chambers
- Following your teacher's demonstration, cut down each side of the heart along the outer wall, opening the atrium and ventricle on that side. Use small controlled scissor cuts, with the lower blade in view and tools directed away from your fingers. Leave the septum intact, cut away from valve cusps, tendinous cords (chordae tendineae) and their attachments, and inspect what lies beneath before extending a cut.
- Trace each atrium through its atrioventricular opening into the ventricle, and each ventricle into its outgoing artery. Push the handle of a dissecting needle up behind the atrioventricular valves and you should notice that the aorta and pulmonary artery cross over.
- Identify the thin valve flaps, cord-like chordae tendineae and their muscular papillary attachments. Hold a cord over a dissecting needle and lift the heart by it to feel its strength. Semi-lunar valves at the artery bases have pocket-like cusps and no such cords.
- Label what you identify with flags made from pins and labels, and record the route and valve response before disturbing the next structure.
- Compare the ventricular walls at corresponding locations, excluding the septum, surrounding fat, papillary muscles and crushed or torn tissue. Measure perpendicular to the wall without compressing it. Wall thickness varies through the heart, so position is a variable to control: state it, repeat matched measurements and calculate a mean.
- Clean up: disinfect instruments, clean surfaces, and put the specimen and disposable materials in the designated biological-waste bag.
Why the valves open and close
In the valve test, water in the artery presses back towards the ventricle. Pressure is now higher on the artery side, so the valve pockets fill and the cusps are pushed together. A brief backflow closes these valves the same way in a living heart.
Squeezing raises pressure in the ventricle above that in the artery, so the valve opens, which models ventricular systole.
Watch out
Don't write that gravity shuts the valve. The water's weight supplies the pressure here, but the cusps close because pressure is higher on the artery side, as they do in a living heart whichever way up the animal is.
Chordae tendineae tether the atrioventricular valves so that ventricular pressure can't turn them inside out; they do not actively pull the valve open.
A damaged or decomposing valve may fail to seal, which does not show that it failed in the living animal.
A full explanation of a valve closing gives the pressure difference, then what the cusps do, then what that means for blood flow.
What each heart structure tells you
Each structure you uncover is evidence of how the heart works, once you link what you see to what it does.
Continue this lesson.
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