Circulatory and Respiratory Systems, Grade 9: Dissection and Heart Rate
Squeeze both sides of a sheep heart and one of them is obviously harder than the other.
That is the left ventricle, and it is thicker because it pumps blood to your feet and back while the right side only pumps as far as the lungs. No diagram teaches that as well as thirty seconds of holding one.
This page covers how the circulatory and respiratory systems work together, the heart and lung dissection done properly, and the heart rate investigation, including the measurement most classes skip and the one instruction the textbook leaves out.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 9 |
| Term | 1 |
| Strand | Life and living |
| Topic | Topic 4, Circulatory and respiratory systems |
| Status | Skills focus: make a dissection. No formally assessed task |
| Marks | None prescribed. Our worksheet is 40 marks |
How the two systems work together
| Step | What happens |
|---|---|
| 1 | Oxygen is inhaled into the lungs |
| 2 | It diffuses from the alveoli into the blood in the capillaries |
| 3 | Oxygenated blood returns to the left side of the heart |
| 4 | The left ventricle pumps it to the whole body |
| 5 | Oxygen diffuses into the body cells; carbon dioxide diffuses out |
| 6 | Deoxygenated blood returns to the right side of the heart |
| 7 | The right ventricle pumps it to the lungs, and the carbon dioxide is exhaled |
Arteries and veins are defined by direction, not by what they carry. Arteries lead away from the heart, veins lead back. The pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood, which is the exception every exam asks about.
Breathing, in one table
| Inhalation | Exhalation | |
|---|---|---|
| Diaphragm | Contracts, moves down | Relaxes, moves up |
| Ribcage | Moves up and out | Moves down and in |
| Chest volume | Increases | Decreases |
| Pressure inside | Falls below the air outside | Rises above it |
| Air | Flows in | Flows out |
Inhalation is active and exhalation is passive. You use muscles to breathe in. Breathing out is mostly those muscles letting go.
The heart rate investigation
Ten volunteers, each with a partner taking the readings.
- Resting pulse after two minutes sitting still. Count for 20 seconds and multiply by 3
- Two minutes of the same exercise for everyone: step-ups, star jumps or jogging on the spot
- Pulse immediately afterwards
- Pulse every minute until it returns to resting. Record how long that takes
- Plot resting against post-exercise rates as a bar graph
Do not use your thumb
Your thumb has its own pulse. Use two fingers on the wrist, on the thumb side. A learner who uses their thumb may be counting their own heartbeat mixed with their partner's, and it is the commonest reason for a reading that makes no sense.
The measurement worth taking
| Typical for a teenager | |
|---|---|
| Resting | 60 to 100 bpm |
| After two minutes of exercise | 120 to 180 bpm |
| Recovery to resting | 1 to 5 minutes |
Everybody already knows that exercise raises your pulse. The number that carries information is how long it takes to come back down: a faster recovery suggests better cardiovascular fitness.
Discuss it as a class pattern, not as a ranking. Nobody's number needs a name attached to it, and any learner with asthma, a heart condition or an injury should be taking readings rather than exercising.
The dissection
A sheep or pig heart from a butcher, fresh, refrigerated, used the same day. One heart per four learners works, and so does one for the whole class as a demonstration.
Tell the class a week before
Some learners will not want to take part, for religious, ethical or personal reasons. Say so in advance, have the photographs and labelling work ready as a full alternative, and nobody has to explain their reason.
The heart, in order
- Look at the outside. The coronary vessels run across the surface, and they are the ones that feed the heart muscle itself
- Squeeze both sides. The left is noticeably harder and thicker
- Find the vessels. Two large veins entering the right side, one large artery leaving each side. The aorta has the thickest wall of the lot
- Cut down through the right atrium and ventricle and open it out
- Compare the wall thicknesses, then find the valves and the white cords that hold them
- Repeat on the left
The lungs
- Feel them. Spongy, light and elastic
- Blow into the trachea through a hosepipe and watch them inflate. Do not inhale the backflow
- Cut the trachea open and feel the C-shaped cartilage rings that hold it open
- Drop a small piece of lung into water. It floats, because it is mostly air space
Safety, including the parts the textbook leaves out
| Point | What to do |
|---|---|
| Gloves | Everyone who touches the specimen, and nobody with an open cut handles it |
| Blades | Cut away from yourself. Nobody works alone with a blade. Count blades out and count them back in |
| A blunt blade | More dangerous than a sharp one. It needs force, and force slips |
| The backflow | Do not inhale when blowing into the trachea |
| Disposal | Sealed bag, general waste, the same day. Never left in a classroom bin overnight |
| Afterwards | Disinfect the bench and the instruments. Treat it as a kitchen surface, because that is what it is |
| Feeling faint | It happens. Work near a window and keep a chair free |
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| You cannot tell left from right | Common, and the labels are not printed on it | Squeeze both sides. The thick one is the left, and the pointed end is the left |
| The heart is hard to cut | Straight out of the fridge | Twenty minutes at room temperature |
| The lungs will not inflate | The pipe is not sealed in the trachea | Hold it tightly, or use a narrower pipe |
| It all looks like one red mass | Poor light, and no plan | Work through the vessels in order with the photographs beside you |
| A pulse reading makes no sense | They used their thumb | Two fingers, thumb side of the wrist |
| Two learners get very different resting rates | Normal. 60 to 100 bpm is all typical | Use the class pattern, not one learner's number |
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks. Part A is breathing, gaseous exchange, blood vessels and the dissection observations. Part B is the heart rate investigation with the graph
- Marking memorandum, including the three answers that look right and score nothing
Related practicals
- Plant and animal cells under a microscope, Grade 9. Term 1, and the microscope is useful for the lung tissue here
- Food tests for starch and fats, Grade 9. The digestive system practical, and the assessed task in this strand
- The pH scale and indicators, Grade 9
What you need to run it
Instruments from us, a specimen from a butcher, and two things we are still sourcing.
Our dissecting set of instruments is R499 for twenty stainless instruments in a zipped case, with 15 in stock. It lasts a decade and it covers every dissection from here to Grade 12.
You will need to supply two things we do not yet stock: disposable gloves and a dissecting tray. Both are being catalogued, because every dissection needs them and four pages in this library are waiting on them. Until then, a box of gloves from a pharmacy and a plastic tray from a kitchen shop will do the job.
The specimen comes from a butcher on the morning of the lesson. Ask for a whole sheep or pig heart, and a lung set if you want the respiratory half. Fresh, refrigerated, used the same day.
For the lung tissue under magnification, a student microscope shows the air spaces beautifully, and it is the same instrument the Grade 9 cells practical uses.