The Circulatory System and Heart
Humans have a double circulatory system, in which the right ventricle pumps deoxygenated blood to the lungs and the left ventricle pumps oxygenated blood around the rest of the body, keeping blood pressure high enough to supply the organs quickly. Arteries carry blood away from the heart at high pressure with thick muscular and elastic walls, veins return blood at low pressure with valves and a wide lumen, and capillaries are one cell thick to allow substances to diffuse into and out of the blood.
Method
- Learn the path of the blood as a loop you can recite: vena cava, right atrium, right ventricle, pulmonary artery, lungs, pulmonary vein, left atrium, left ventricle, aorta, body, back to the vena cava.
- Attach a reason to each structural feature rather than listing features: the left ventricle wall is thicker than the right because it pumps blood all the way around the body rather than only to the lungs; valves prevent backflow; the coronary arteries supply the heart muscle itself with oxygenated blood.
- Learn the three vessels as a comparison table in words: artery (thick muscular and elastic wall, small lumen, carries blood away from the heart at high pressure), vein (thinner wall, large lumen, valves, returns blood at low pressure), capillary (one cell thick, allows diffusion of oxygen and glucose out and carbon dioxide in).
- Learn the four components of blood with a function each: red blood cells (no nucleus, biconcave, contain haemoglobin, carry oxygen), white blood cells (defend against pathogens), platelets (fragments that help clotting), plasma (a straw-coloured liquid that transports the cells, carbon dioxide, urea, glucose and hormones).
- For coronary heart disease, learn the cause and then the three treatments with one advantage and one disadvantage each: stents (widen the artery, work quickly, but do not treat the underlying cause), statins (reduce blood cholesterol so fatty deposits build up more slowly, but must be taken long term and can have side effects), and a bypass or transplant (used when damage is severe, but is major surgery with a risk of rejection).
- For rate of blood flow calculations, use rate = volume / time and check the unit asked for, converting minutes to seconds or cubic centimetres to cubic decimetres before dividing.
Worked example
A person's heart pumps 4800 cubic centimetres of blood per minute. The heart rate is 60 beats per minute. Calculate the volume of blood pumped from the left ventricle in one beat, and the volume pumped in one second.
- Volume per beat = total volume per minute / number of beats per minute.
- Substitute: 4800 / 60 = 80 cubic centimetres per beat.
- For the volume per second, divide the volume per minute by 60 seconds: 4800 / 60 = 80.
- State both answers with units: 80 cubic centimetres per beat, and 80 cubic centimetres per second.
- Sense check: the two numbers are equal only because the heart rate happens to be 60 beats per minute, so one beat takes exactly one second.
Practice questions
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Q1Name the blood vessel that carries deoxygenated blood from the heart to the lungs.Show answer
Answer: The pulmonary artery.
Q2Why is the wall of the left ventricle thicker than that of the right ventricle?Show answer
Answer: It has to pump blood at high pressure all the way around the body, whereas the right ventricle only pumps to the lungs.
Q3State the function of the valves in the heart and in veins.Show answer
Answer: They prevent the backflow of blood, keeping it moving in one direction.
Q4Give two adaptations of a red blood cell.Show answer
Answer: It has no nucleus, so there is more room for haemoglobin, and it is a biconcave disc, giving a large surface area for oxygen to diffuse across.
Q5Name the substance in red blood cells that binds oxygen.Show answer
Answer: Haemoglobin, which combines with oxygen to form oxyhaemoglobin in the lungs.
Q6Give one advantage and one disadvantage of a stent.Show answer
Answer: Advantage: it widens the narrowed artery so blood flow to the heart muscle is restored quickly, and it lasts a long time. Disadvantage: it is a surgical procedure with a risk of infection or a blood clot forming, and it does not treat the underlying cause.
Q7Which component of blood transports carbon dioxide, urea and hormones?Show answer
Answer: Plasma.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Explain the advantage of a double circulatory system compared with a single circulatory system.
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A patient has a build-up of fatty material in a coronary artery. Explain how this can lead to a heart attack.
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Compare the use of statins and stents in treating coronary heart disease, and give a reasoned recommendation for a patient with mildly raised cholesterol and no symptoms.
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