OxfordAQA International GCSE Biology: Circulation in humans: heart, vessels, blood and transplants (9201)

This guide teaches section 3.2.3, Circulation in humans (statements a to t), from the OxfordAQA International GCSE Biology (9201) specification, Version 5.2, for exams from May/June 2018 onwards. Use the course hub and the printable checklist to track progress, and take a free 10-minute diagnostic to find weak spots.

Other pages for this unit: the revision notes and the practice questions. For the cells, tissues and organs that this topic builds on, see the Organisation study guide.

What this unit covers

Statement	What you must be able to do
3.2.3 a, b, c	Describe the circulatory system, the double circulation and the four chambers
3.2.3 d	Explain the natural pacemaker and what an artificial pacemaker does
3.2.3 e	Describe coronary heart disease and how stents help
3.2.3 f, g	Describe faulty valves, replacement valves and artificial hearts
3.2.3 h, i	Trace blood through the heart; name the main vessels
3.2.3 j, k	Relate artery, vein and capillary structure to function
3.2.3 l to q	Describe plasma, red cells, white cells, platelets and clotting
3.2.3 r, s	Explain antigens, rejection and how rejection is reduced; evaluate devices against transplants
3.2.3 t	Use ABO blood groups and a compatibility table

The circulatory system and the double circulation (a, b, c)

The circulatory system is the heart, the blood vessels and the blood. Its job is transport: it carries substances from where they enter the body, or are made, to the cells that need them, and carries waste from cells to where it leaves the body.

The heart is an organ made largely of muscle tissue. It is a pump, and it sits in a double circulatory system. On every complete trip around the body the blood goes through the heart two times:

• the pulmonary circulation takes blood from the heart to the lungs and back;
• the systemic circulation takes blood from the heart to all the other organs and back.

Two circuits mean blood returning from the lungs is pumped again before it travels to the organs, so it reaches them at higher pressure.

The heart has four chambers. The two upper chambers are the right atrium and left atrium. The two lower chambers are the right ventricle and left ventricle. Valves inside the heart keep blood moving one way. You do not need to know the names of the valves.

Blood flow through the heart and the named vessels (h, i)

The atria receive blood. They contract and force it into the ventricles. The ventricles then contract and force blood out of the heart. The route is:

1. Blood from the body returns in the vena cava and enters the right atrium.
2. It passes into the right ventricle.
3. The right ventricle pumps it out through the pulmonary artery to the lungs.
4. Blood returns from the lungs in the pulmonary vein and enters the left atrium.
5. It passes into the left ventricle.
6. The left ventricle pumps it out through the aorta to the body.

The specification limits the vessels you must know to five: aorta, vena cava, pulmonary artery, pulmonary vein and coronary arteries. The coronary arteries branch from the aorta close to the heart and run over the heart’s surface. They deliver oxygenated blood to the heart muscle itself.

Vessel	Carries blood	Oxygen content
Vena cava	from body organs to right atrium	low
Pulmonary artery	from right ventricle to lungs	low
Pulmonary vein	from lungs to left atrium	high
Aorta	from left ventricle to body	high
Coronary arteries	from aorta to heart muscle	high

Notice that the pulmonary artery carries low-oxygen blood and the pulmonary vein carries high-oxygen blood. Arteries carry blood away from the heart and veins carry it back. Oxygen content does not decide the name.

Arteries, veins and capillaries (j, k)

Arteries carry blood away from the heart. Their walls are thick and contain muscle and elastic fibres, so they withstand high pressure and can stretch and recoil as each surge of blood arrives.

Veins carry blood back towards the heart. Their walls are thinner, and many veins contain valves that prevent back-flow, so blood moves only towards the heart.

Capillaries run through the organs. They are very narrow and have thin walls. Substances the cells need pass out of the blood through the capillary walls, and substances the cells produce pass into the blood in the same way. A thin wall and a narrow tube bring blood close to every cell.

Worked example 1: pulse and blood pumped per minute

A student counts 46 pulse beats in 40 seconds. Each beat of the left ventricle pushes out 64 cm³ of blood. Calculate the heart rate and the volume of blood pumped per minute.

Heart rate = 46 ÷ 40 × 60 = 69 beats per minute.

Volume per minute = 64 × 69 = 4416 cm³, which is 4.416 dm³, so about 4.4 dm³ per minute.

During exercise suppose the rate is 150 beats per minute and each beat pushes out 88 cm³. Volume per minute = 88 × 150 = 13 200 cm³. The increase is 13 200 − 4416 = 8784 cm³, and 8784 ÷ 4416 × 100 = 199 per cent (3 s.f.). More oxygen and glucose are delivered to working muscle.

The pacemaker (d)

A group of cells in the right atrium acts as the heart’s natural pacemaker. It controls the resting heart rate. If the rhythm becomes irregular, a surgeon can fit an artificial pacemaker, an electrical device that corrects irregularities in the heart rate. In an exam, say that the device supplies electrical signals so the heart beats regularly.

Coronary heart disease and stents (e)

In coronary heart disease, layers of fatty material build up inside the coronary arteries and narrow them. Less blood flows through them, so the heart muscle receives less oxygen. Muscle short of oxygen cannot release enough energy to keep contracting properly, which causes pain and can lead to a heart attack.

A stent is a short tube of mesh placed inside a narrowed coronary artery. It is expanded against the artery wall and stays there, holding the artery open so blood can flow again.

Worked example 2: percentage narrowing

A coronary artery normally has an internal diameter of 3.6 mm. Fatty deposits reduce it to 1.5 mm. A stent then opens it to 3.2 mm.

Decrease = 3.6 − 1.5 = 2.1 mm. Percentage decrease = 2.1 ÷ 3.6 × 100 = 58.3 per cent.

After the stent, the increase from the narrowed diameter is 3.2 − 1.5 = 1.7 mm, and 1.7 ÷ 1.5 × 100 = 113 per cent (3 s.f.).

Faulty valves, replacements and artificial hearts (f, g)

Heart valves can become faulty in two main ways:

• the valve tissue stiffens, so the valve cannot open fully and blood is held back;
• the valve develops a leak, so some blood flows backwards.

A faulty valve can be replaced with a biological valve, taken from a human or another mammal, or with a mechanical valve.

An artificial heart is a mechanical pump. It is occasionally used to keep a patient alive while they wait for a heart transplant, or to give the patient’s own heart a rest so it can recover.

Blood (l to q)

Blood is a tissue. The fluid part is plasma, and the white blood cells, platelets and red blood cells are suspended in it.

Plasma transports:

• carbon dioxide, from the organs to the lungs;
• soluble products of digestion, from the small intestine to other organs;
• urea, from the liver to the kidneys.

Red blood cells have no nucleus. They are packed with the red pigment haemoglobin. In the lungs, haemoglobin combines with oxygen to form oxyhaemoglobin. In other organs, oxyhaemoglobin splits into haemoglobin and oxygen, which the cells use. Red cells therefore carry oxygen from the lungs to the organs.

White blood cells have a nucleus. They are part of the body’s defence system against microorganisms.

Platelets are small fragments of cells with no nucleus. They help blood clot at a wound.

Clotting is a series of enzyme-controlled reactions. They change soluble fibrinogen into fibrin, which forms a network of fibres. The network traps blood cells and forms a clot, which seals the wound and reduces blood loss and the entry of microorganisms.

Worked example 3: magnification of a red blood cell

A red blood cell has a real diameter of 7.5 µm. In a drawing it measures 36 mm across. Calculate the magnification.

Convert units: 7.5 µm = 0.0075 mm.

Magnification = image size ÷ real size = 36 ÷ 0.0075 = × 4800.

Antigens, transplants and rejection (r, s)

Antigens are proteins on the surface of cells. Your immune system recognises the antigens on your own cells as “self”.

In an organ transplant, a diseased organ is replaced by a healthy one from a donor. The recipient’s antibodies may attack the antigens on the donor organ, because they do not recognise them as part of the recipient’s body. This is rejection. To reduce it:

• use a donor organ with a tissue type similar to the recipient’s;
• give the recipient drugs that suppress the immune system.

Evaluating devices and transplants

You may be asked to weigh treating organ failure with a mechanical device against a transplant. Use the points below, and always finish with a judgement tied to the patient described.

	Mechanical device	Transplant
Advantages	available without a donor; no tissue-type matching; no rejection	a living organ does the whole job; no moving machinery to fail
Disadvantages	blood clots can form on artificial surfaces; may need anticoagulant drugs; infection risk; needs a power supply (artificial hearts); mechanical failure	shortage of donors and waiting time; rejection risk; immunosuppressant drugs make infection more likely; major surgery

ABO blood groups (t)

There are four main human blood groups: O, A, B and AB. The need for blood typing is simple: if a patient receives red cells carrying an antigen their own antibodies recognise as foreign, those cells are attacked and the transfusion harms the patient.

Group	Antigens on red cells	Antibodies in plasma
A	A	anti-B
B	B	anti-A
AB	A and B	none
O	none	anti-A and anti-B

Compatibility table for red cell donation (yes means the recipient can safely receive that donor’s cells):

Recipient	From O	From A	From B	From AB
O	yes	no	no	no
A	yes	yes	no	no
B	yes	no	yes	no
AB	yes	yes	yes	yes

Group O red cells have no A or B antigens, so no recipient’s antibodies attack them. Group O is therefore the universal donor.

Worked example 4: how many donors will do?

A hospital has 200 registered donors: 88 are group O, 70 group A, 30 group B and 12 group AB. What percentage of them could give red cells to a group B patient?

Compatible donors are group O and group B: 88 + 30 = 118.

118 ÷ 200 × 100 = 59 per cent.

Common errors

• Saying the pulmonary vein carries deoxygenated blood. It carries oxygenated blood.
• Describing a stent as a bypass. A stent props an artery open from inside.
• Saying red cells “carry oxygen” without naming haemoglobin and oxyhaemoglobin.
• Saying platelets turn into fibrin. Platelets help clotting; the enzyme reactions change fibrinogen into fibrin.
• Claiming a transplant removes the need for drugs. Immune-suppressing drugs are needed to reduce rejection.
• Reading the compatibility table the wrong way round. Always say donor and recipient.

Official syllabus

OxfordAQA International GCSE Biology (9201), specification Version 5.2, for exams May/June 2018 onwards, published by OxfordAQA. This guide follows section 3.2.3, statements a to t.