IGCSE Biology: the heart, blood vessels and blood

Key ideas board by board

Cambridge 0610 Core students identify the muscular wall, septum, atria, ventricles, one-way valves and coronary arteries; state that arteries carry blood away from the heart and veins return it; know that heart activity is monitored by ECG, pulse rate and listening to the valves closing; investigate the effect of exercise on heart rate; and describe coronary heart disease as blockage of the coronary arteries, with risk factors (diet, lack of exercise, stress, smoking, genetic predisposition, age and sex). Supplement adds single circulation in fish, double circulation in mammals and its advantages, atrioventricular and semilunar valves, the relative thickness of chamber walls, the heart cycle, and the vessels of the liver (hepatic artery, hepatic veins and hepatic portal vein).

Edexcel 4BI1 asks everyone for heart structure and function (2.65), heart rate changes during exercise and with adrenaline (2.66), coronary heart disease risk factors (2.67), how vessel structure relates to function (2.68) and the general circulation including the heart, lungs, liver and kidneys (2.69). Blood composition, plasma, red blood cell adaptations and white blood cells (2.59 to 2.62) are for all; vaccination (2.63B) and clotting (2.64B) are Paper 2 only.

Arteries, veins and capillaries

The pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood, the reverse of most vessels. Define arteries by direction, not by oxygen content.

How the heart works (Cambridge Supplement, Edexcel 2.65)

The atria contract and push blood through the atrioventricular valves into the ventricles. The ventricles then contract: the rising pressure closes the atrioventricular valves (preventing backflow into the atria) and opens the semilunar valves, so blood leaves through the pulmonary artery and aorta. When the ventricles relax, the semilunar valves close to prevent backflow from the arteries.

Wall thickness: the atria only pump blood a short distance into the ventricles, so their walls are thin. The left ventricle wall is thicker than the right because it pumps blood at higher pressure around the whole body, while the right ventricle pumps blood only to the nearby lungs, where lower pressure avoids damaging the delicate capillaries.

The advantage of a double circulation: blood returns to the heart after the lungs to be pumped again, so it reaches the body at high pressure and flows quickly, delivering oxygen and glucose fast enough for an active, warm-blooded animal.

Worked example: heart rate data

A student counts 18 pulse beats in 15 seconds at rest. Heart rate = 18 x (60 / 15) = 18 x 4 = 72 beats per minute. After two minutes of running, she counts 42 beats in 20 seconds: 42 x 3 = 126 beats per minute.

Percentage increase = (126 - 72) / 72 x 100 = 75%. Explanation for the increase: during exercise muscles respire faster and need more oxygen and glucose and must remove more carbon dioxide, so the heart beats faster to deliver blood more quickly. Edexcel also expects adrenaline: it is released in stress or excitement and increases heart rate.

Recovery time (the time to return to the resting rate) is used as a measure of fitness: fitter people usually recover faster. If the question asks for improvements, repeat the measurements and take a mean, and control the type and length of exercise.

Common mistakes

• Saying arteries carry oxygenated blood and veins deoxygenated blood. The pulmonary vessels break this rule; the definition is direction.
• Saying the left ventricle is thicker because it "pumps more blood". Both ventricles pump the same volume; the left pumps at higher pressure over a longer distance.
• Saying capillaries have "thin walls" without "one cell thick" and the link to a short diffusion distance.
• Saying valves "stop blood flowing". They prevent backflow.
• Labelling the heart diagram from your own left and right. The heart is drawn as if you are facing the person, so its left side is on the right of the page.
• Saying platelets "form a scab" or "clot the blood" with no detail. Cambridge Supplement wants fibrinogen converted to fibrin forming a mesh.

Exam technique

Order-of-flow questions are best answered as an ordered list of named chambers, valves and vessels. In "explain" questions about structure, every structural point needs a function: "thick elastic wall, to withstand high pressure", "valves, to prevent backflow". Coronary heart disease questions usually reward a chain: fatty deposits narrow the coronary arteries, less blood reaches the heart muscle, less oxygen for respiration, the muscle cannot contract and may die, causing a heart attack.

Discuss questions on diet and exercise want both sides: what reduces risk (less saturated fat and salt, regular exercise, not smoking) and why (lower blood cholesterol, lower blood pressure, a stronger heart).

How one-to-one lessons help

The heart is a diagram-heavy topic, and the shared whiteboard suits it: a tutor gives a blank heart outline, the student labels and traces the flow, and the tutor stops each slip at once. Lessons then move to exam questions on wall thickness, valves and exercise data, marked against the mark scheme for your child's board and tier.

Self-check

1. Trace blood from the vena cava to the aorta, naming chambers, valves and vessels.
2. Explain why the left ventricle wall is thicker than the right.
3. Compare arteries, veins and capillaries in a table.
4. List the four components of blood and their functions.
5. Calculate heart rate in beats per minute from a count.
6. Explain how coronary heart disease can cause a heart attack, and list risk factors.
7. (Cambridge Supplement) Explain the advantage of a double circulation.