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Heart, blood and circulation

Follow blood flow and explain adaptations of vessels and blood cells.

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The key idea

The heart pumps blood through a double circulatory system.Arteries, veins and capillaries are adapted to different pressures and exchange roles.

Double circulationBlood returns to the heart between the lungs and body.
Double circulationBlood returns to the heart between the lungs and body.lungsheartbodyheart to lungs to heart to body
Revision notes

The bit that matters

Learn the process in clean chunks. If a sentence explains a cause, make sure you can say the effect too.

1

Structure of the heart

The heart is a muscular organ that pumps blood around the body in a double circulatory system.The right side pumps deoxygenated blood to the lungs, and the left side pumps oxygenated blood to the rest of the body; the left ventricle has a thicker muscular wall because it pumps blood at higher pressure over a greater distance.Valves prevent the backflow of blood, and the coronary arteries supply the heart muscle itself with oxygenated blood.

2

Controlling heart rate

The natural resting heart rate is controlled by a group of cells in the right atrium that act as a pacemaker, producing a small electrical impulse.Artificial pacemakers are electrical devices fitted to correct irregularities in the heart rhythm.Blood is carried into the right atrium by the vena cava and leaves the left ventricle through the aorta.

3

Blood vessels

Arteries carry blood away from the heart at high pressure and have thick, elastic, muscular walls.Veins carry blood back to the heart at low pressure, have thinner walls and a wider lumen, and contain valves to prevent backflow.Capillaries are one cell thick to allow efficient exchange of substances such as oxygen, glucose and carbon dioxide between the blood and the tissues.

4

Blood components

Blood is a tissue made of plasma containing red blood cells, white blood cells and platelets.Red blood cells contain haemoglobin, which binds oxygen to form oxyhaemoglobin, and they have no nucleus to make more room for haemoglobin.White blood cells defend against pathogens, and platelets are cell fragments that help the blood to clot at a wound.

Key terms

Definitions to learn

Double circulatory system

A system in which blood passes through the heart twice for each complete circuit of the body.

Artery

A blood vessel with thick muscular walls that carries blood away from the heart at high pressure.

Vein

A blood vessel with thinner walls and valves that carries blood back to the heart at low pressure.

Capillary

A narrow blood vessel one cell thick where exchange of substances with tissues occurs.

Haemoglobin

The red pigpigment in red blood cells that binds oxygen to form oxyhaemoglobin.

Pacemaker

A group of cells in the right atrium that controls the natural resting heart rate.

Worked example

Explain why capillaries have thin walls and form dense networks.

1

Thin walls create a short diffusion distance.

2

Dense networks create a large surface area.

3

This speeds exchange between blood and tissues.

Final answer

Capillaries allow efficient exchange by diffusion.

Exam habit

Trace blood flow step by step: right atrium → right ventricle → lungs → left atrium → left ventricle → body.Arteries carry blood away from the heart — they do not always carry oxygenated blood.

Watch out

Arteries carry blood away from the heart. They do not always carry oxygenated blood.

Examiner tips

How to score full marks

  • 1Remember 'A for Away' — Arteries carry blood Away from the heart; this also applies to the pulmonary artery even though it carries deoxygenated blood.
  • 2The left ventricle wall is thicker than the right because it pumps to the WHOLE body, not because it pumps more blood.
  • 3When describing red blood cells, link each adaptation (no nucleus, biconcave shape, haemoglobin) to carrying MORE oxygen.
Practice questions

Try these yourself

Open each answer only after you have explained the full biological process.

1Describe one adaptation of an artery.
Mark scheme
  1. 1.Link the feature to pressure.
Arteries have thick muscular and elastic walls to withstand high pressure.
2Why do red blood cells lack a nucleus?
Mark scheme
  1. 1.Think about the space available for haemoglobin.
This leaves more space for haemoglobin, increasing oxygen transport.
3Explain one effect of a blocked coronary artery.
Mark scheme
  1. 1.Link blood supply, oxygen and respiration.
Less oxygen reaches heart muscle, reducing aerobic respiration and possibly causing a heart attack.
4Name the blood vessel that carries blood from the heart to the lungs.[1 mark]
Mark scheme
  1. 1.Recall the vessel leaving the right ventricle.
Pulmonary artery (1).
5State the function of platelets in the blood.[2 marks]
Mark scheme
  1. 1.Think about what happens at a wound.
They help the blood to clot (1) (to seal wounds and prevent blood loss / entry of pathogens) (1).
6Explain why capillaries are well adapted for the exchange of substances.[3 marks]
Mark scheme
  1. 1.Link wall thickness and width to diffusion.
Walls are only one cell thick (1) so there is a short diffusion distance for substances to cross (1); they are narrow / form a large network giving a large surface area for exchange (1).
7Describe two ways a red blood cell is adapted to carry oxygen.[4 marks]
Mark scheme
  1. 1.Give the adaptation and link it to oxygen transport.
Contains haemoglobin (1) which binds oxygen to form oxyhaemoglobin (1); has no nucleus (1) so there is more room to carry haemoglobin/oxygen (1); biconcave shape (1) increases surface area for absorbing oxygen (1). (Any two with explanation.)
8A person has a blocked coronary artery. Explain why this can cause a heart attack and how a stent could help.[5 marks]
Mark scheme
  1. 1.Explain what the coronary artery supplies, the effect of the block, then the role of the stent.
The coronary arteries supply the heart muscle with oxygenated blood / glucose (1); a blockage reduces the oxygen supply to the heart muscle (1), so the muscle cells cannot respire enough / aerobically and may die, causing a heart attack (1). A stent is a tube inserted to hold the artery open (1), allowing blood to flow through again so oxygen reaches the heart muscle (1).
9State the correct sequence of chambers and vessels through which blood flows in the pulmonary circuit, starting at the right atrium.[4 marks]
Mark scheme
  1. 1.Trace blood through the right side of the heart to the lungs and back.
Right atrium (1) → right ventricle (1) → pulmonary artery → lungs → pulmonary vein (1) → left atrium (1).
10Explain why the left ventricle has a much thicker muscular wall than the right ventricle.[3 marks]
Mark scheme
  1. 1.Link the destination of blood from each ventricle to the pressure required.
The left ventricle pumps blood to the whole body via the aorta (1); this requires much higher pressure than the right ventricle, which only pumps blood to the nearby lungs (1); a thicker muscular wall contracts more forcefully, generating the necessary higher pressure (1).
11Explain the role of valves in the heart and in veins.[4 marks]
Mark scheme
  1. 1.State what valves do and why this is important in each location.
Valves in the heart prevent backflow of blood between chambers (1) ensuring blood flows in the correct direction during each heartbeat (1); valves in veins prevent backflow as blood travels back to the heart at low pressure (1), allowing blood to flow only towards the heart (1).
12Compare the structures of arteries and veins and link each difference to their functions.[5 marks]
Mark scheme
  1. 1.Give at least two structural differences.
  2. 2.Explain each in terms of the pressure blood is under.
Arteries have thicker, more muscular and elastic walls than veins (1) to withstand and maintain the high pressure of blood leaving the heart (1); veins have a wider lumen than arteries (1) which reduces resistance and helps blood flow back to the heart at low pressure (1); veins have valves to prevent backflow while arteries do not (1).
13A patient is given a heart rate monitor that shows their resting heart rate is 42 beats per minute, which their doctor says is unusually low. Suggest what medical device might be fitted to correct this and explain how it works.[4 marks]
Mark scheme
  1. 1.Identify the device.
  2. 2.Explain its mechanism.
An artificial pacemaker (1); this is an electrical device implanted in the chest (1) that produces regular electrical impulses (1) to stimulate the heart to beat at a normal rate (1).
14Explain how the structure of plasma makes it effective at transporting a wide range of substances around the body.[4 marks]
Mark scheme
  1. 1.Describe what plasma is.
  2. 2.List substances it carries.
Plasma is a liquid (water-based) component of blood (1); being a liquid allows it to carry dissolved substances throughout the body (1); it transports glucose, amino acids, hormones and carbon dioxide (1); as well as urea to the kidneys for excretion (1).
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