Edexcel IGCSE Biology · Spec 2.65-2.66
Heart Structure, Valves and Exercise
Covers identify the chambers, valves, septum and major vessels, Explain one-way blood flow through the heart and Explain why exercise increases heart rate.
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Heart Structure, Valves and Exercise
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Explained
Inside the heart, and why the blood can only go one way
The heart is two pumps side by side, separated by a wall of muscle called the septum. The right side handles deoxygenated blood and sends it to the lungs. The left side handles oxygenated blood and sends it to the rest of the body.
The septum keeps the two apart. If oxygenated and deoxygenated blood mixed, blood leaving for the body would carry less oxygen than it could, and the whole advantage of a double circulation would be lost.
One point of orientation before anything else: diagrams are drawn as if you were facing the person, so the right side of the heart appears on the left of the page. Getting that the wrong way round turns every subsequent label into an error.
The four chambers
Two atria on top receive blood arriving at the heart. Two ventricles below pump it out.
Atria have thin walls because they only push blood down into the ventricles, a distance of a few centimetres.
Ventricles have thick muscular walls because they pump blood out of the heart. The left ventricle is noticeably thicker than the right, and the reason is distance: the right ventricle pumps only to the lungs, while the left pumps to the whole body and needs to generate a much higher pressure.
That comparison is a favourite question, and the answer is about where the blood is going rather than about the blood itself.
The valves
Valves make the flow one way. Each opens when the pressure behind it is higher and is pushed shut when the pressure in front is higher, so they operate automatically, with no control needed.
The atrioventricular valves sit between each atrium and its ventricle, and they stop blood flowing back into the atria when the ventricles contract. Cords of tissue anchor them so they cannot turn inside out under the pressure.
The semilunar valves sit at the exits from the ventricles, in the aorta and the pulmonary artery, and they stop blood falling back into the ventricles as those relax.
A question about why valves are needed wants the phrase prevents backflow, and then the consequence: blood travels in one direction only, so it keeps circulating rather than sloshing back and forth.
The vessels attached
The vena cava brings deoxygenated blood from the body into the right atrium. The pulmonary artery carries it from the right ventricle to the lungs. The pulmonary vein brings oxygenated blood from the lungs to the left atrium. The aorta carries it from the left ventricle to the body.
The pulmonary vessels are the exception that questions test. The pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood, which is the reverse of every other vessel. The definition of an artery is that it carries blood away from the heart, not that it carries oxygen.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks how something can change the heart rate, for two marks, and lists three points: that adrenaline is released, that it enters the blood or blood plasma or is a hormone, and that it speeds up the heart rate.
Its guidance then does something unusual. It states that the phrase adrenaline is a hormone scores two marks by itself, because the single clause satisfies the first point and the second at once.
That is worth understanding rather than memorising. Marking points are claims, not sentences, and one well constructed sentence can carry two claims. Naming the substance and classifying it in the same breath is more efficient than writing two separate statements, and it is why precise vocabulary pays.
Notice also the third point: speeds up. Not changes, not affects. If the question stem has already told you that the heart rate changes, saying it changes repeats the question back, and the mark is for the direction.
Why exercise raises the heart rate
Muscles working harder respire faster, so they need more oxygen and more glucose delivered, and they produce more carbon dioxide and more heat that must be carried away.
A faster heart rate moves more blood per minute, so all four of those needs are met at once. That is the chain to write: more respiration, so more oxygen and glucose required and more carbon dioxide produced, so the blood must circulate faster.
Adrenaline is what raises the rate, released in anticipation of activity as well as during it, which is why the heart speeds up before a race rather than after it has started.
Spec 2.65-2.66
What you need to know
- Identify the chambers, valves, septum and major vessels
- Explain one-way blood flow through the heart
- Explain why exercise increases heart rate
Active recall
Quick check
Answer each question before opening the answer.
What is the function of the septum?
It prevents oxygenated and deoxygenated blood from mixing
Which vessel leaves the right ventricle?
The pulmonary artery
Why does heart rate increase during exercise?
To increase oxygen and glucose delivery and remove carbon dioxide and heat faster
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