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Topic 9 · 9.1–9.2

Circulation and the heart

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe the circulatory system; describe single and double circulation and explain the advantages of double circulation EXTENDED.
  • Identify the structures of the heart; state that arteries carry blood away and veins return it; identify the valves EXTENDED.
  • State how heart activity is monitored; investigate and explain the effect of physical activity on heart rate.
  • Describe coronary heart disease, its risk factors, and the roles of diet and exercise in reducing it.
  • Explain wall thickness, the septum and how the heart functions EXTENDED.

📚The biology

Circulatory systems

A circulatory system is a system of blood vessels with a pump (the heart) and valves to ensure one-way flow of blood.

Two flow diagrams. Fish, single circulation: the heart pumps deoxygenated blood to the gills; oxygenated blood goes on to the body organs and back to the heart, passing through the heart once. Mammal, double circulation: the right side of the heart pumps blood to the lungs and back to the left side (pulmonary circulation); the left side pumps it to the body organs and back to the right side (systemic circulation).
EXTENDED In a mammal, blood passes through the heart twice for each complete circuit of the body.

EXTENDED A fish has a single circulation: blood passes through the heart once per circuit, from heart to gills to body and back. A mammal has a double circulation: heart → lungs → heart → body → heart. The advantages: blood returning from the lungs is pumped again, so it reaches the body at high pressure and flows quickly, delivering oxygen and glucose fast enough for a high rate of respiration; the lungs receive blood at lower pressure, which does not damage their delicate capillaries.

The heart

A section through the mammalian heart, drawn from the front so the heart's right side is on the left. On the left: vena cava entering the right atrium, the atrioventricular valve, the right ventricle, and the pulmonary artery leaving it. On the right: pulmonary veins entering the left atrium, the atrioventricular valve, the left ventricle with a thick muscular wall, and the aorta leaving it. The septum separates the two ventricles; semilunar valves sit at the bases of the aorta and pulmonary artery. Blue arrows show deoxygenated blood, red arrows oxygenated blood.
Right side: deoxygenated blood to the lungs. Left side: oxygenated blood to the body. The left ventricle wall is the thickest.

Blood is pumped away from the heart in arteries (aorta, pulmonary artery) and returns to it in veins (vena cava, pulmonary vein). The heart muscle itself is supplied with blood by the coronary arteries on its surface.

EXTENDED Features to explain:

  • EXTENDED The left ventricle has a thicker muscle wall than the right, because it pumps blood at high pressure all round the body; the right pumps only to the nearby lungs.
  • The atria have thinner walls than the ventricles: they only pump blood into the ventricles just below.
  • The septum separates oxygenated and deoxygenated blood, so the body receives blood with as much oxygen as possible.
  • Atrioventricular valves (between atria and ventricles) and semilunar valves (at the bases of the aorta and pulmonary artery) stop blood flowing backwards.
Two simple diagrams of one side of the heart. First, the atrium contracts: the atrioventricular valve is open and blood flows into the ventricle, while the semilunar valve in the artery is closed. Second, the ventricle contracts: the atrioventricular valve closes and the semilunar valve opens, so blood is pushed into the artery.
EXTENDED The sounds of the heartbeat (“lub-dub”) are the valves closing.

EXTENDED How the heart works: the atria contract, pushing blood through the open atrioventricular valves into the ventricles; then the ventricles contract, the atrioventricular valves close (so blood cannot return to the atria), and the semilunar valves open as blood is pushed into the arteries. When the ventricles relax, the semilunar valves close, stopping backflow from the arteries.

Monitoring the heart and exercise

Heart activity can be monitored by an ECG (electrocardiogram), by the pulse rate, and by listening to the sounds of the valves closing with a stethoscope. During physical activity the heart rate increases.

EXTENDED During exercise, muscles respire faster, needing more oxygen and glucose and producing more carbon dioxide. The heart beats faster (and more strongly) to deliver oxygen and glucose to the muscles faster and remove carbon dioxide faster.

Coronary heart disease

The outside of a heart with coronary arteries branching over its surface. One branch has a blockage; the heart muscle beyond it gets no oxygen. Beside it, cross-sections of a healthy artery with a wide lumen, and an artery narrowed by fatty deposits.
Fatty deposits narrow the coronary arteries; a blood clot can then block one completely, so part of the heart muscle is starved of oxygen.

Coronary heart disease (CHD) is the blockage of coronary arteries, so heart muscle receives less oxygen and glucose, causing chest pain or a heart attack. Risk factors: a diet high in saturated fat and salt, lack of exercise, stress, smoking, genetic predisposition, age and sex (more common in males). A balanced diet low in saturated fat reduces fatty deposits; regular exercise strengthens the heart, lowers blood pressure and helps control body mass.

✏️Worked example

A student’s heart rate was recorded before, during and after 8 minutes of exercise. (a) State the resting heart rate. [1] (b) Calculate the percentage increase from resting to the highest rate. [2] (c) State how long after exercise stopped it took to return to the resting rate. [1] (d) EXTENDED Explain why the heart rate increased. [3]
A graph of heart rate against time from 0 to 20 minutes. Resting rate 72 beats per minute until 2 minutes; during exercise from 2 to 10 minutes it rises to 118, 138, 146 and 148; after exercise it falls to 120, 96, 84, 76 and back to 72 at 20 minutes.

(a) 72 beats per minute.

(b) increase = \( 148 - 72 = 76 \); percentage increase = \( \tfrac{76}{72} \times 100 = 106\% \) (105.6%).

(c) Exercise stopped at 10 minutes; the rate was back to 72 at 20 minutes: 10 minutes.

(d) The muscles respire faster to release more energy for contraction. They need more oxygen and glucose, and produce more carbon dioxide, so the heart beats faster to deliver oxygen and glucose to the muscles and remove carbon dioxide more quickly.

Check it. A percentage increase is calculated on the starting value: 76 ÷ 72, not 76 ÷ 148 (which would give 51%).
“The heart pumps faster to make more energy.” The heart does not make energy for the muscles; it delivers the oxygen and glucose the muscles use in respiration.

📝Practise

In the style of the multiple-choice, theory and practical papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Which blood vessel carries blood from the heart to the lungs? A: aorta. B: pulmonary artery. C: pulmonary vein. D: vena cava.
B. Arteries carry blood away from the heart.
2. (Theory.) State three ways in which the activity of the heart can be monitored. [3]
ECG; pulse rate; listening to the sounds of the valves closing (with a stethoscope).
3. (Theory.) State four risk factors for coronary heart disease, and one change to lifestyle that reduces the risk. [3]
Any four: diet high in saturated fat; lack of exercise; stress; smoking; genetic predisposition; age; being male. Change: e.g. eat less saturated fat / exercise regularly / stop smoking.
4. (Practical.) Describe how to measure a person’s pulse rate accurately. [3]
Place two fingers (not the thumb) on the wrist or neck; count the beats for 60 s (or for 15 s and multiply by 4); repeat and calculate a mean.
5. (Theory.) EXTENDED Explain why the wall of the left ventricle is thicker than the wall of the right ventricle. [2]
The left ventricle pumps blood at high pressure all around the body; the right ventricle pumps only to the lungs, which are close, at lower pressure.
6. (Theory.) EXTENDED A baby is born with a hole in the septum between the ventricles. Explain why the baby may lack energy and look slightly blue. [3]
Oxygenated and deoxygenated blood mix, so blood pumped to the body carries less oxygen; less aerobic respiration in the cells means less energy is released.
7. (Theory.) EXTENDED Describe what happens to the valves when the ventricles contract. [2]
The atrioventricular valves close, stopping blood flowing back into the atria; the semilunar valves open, letting blood into the aorta and pulmonary artery.
8. (Theory.) EXTENDED State one advantage of a double circulation over a single circulation. [1]
Blood reaches the body at high pressure, so oxygen is delivered to tissues quickly (and the lungs receive blood at low pressure, protecting their capillaries).

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • British Heart Foundation — how the heart works, with animations
  • Royal Society of Biology — investigating the effect of exercise on pulse rate