Revision - GCSE Biology - the circulatory system and blood

The circulatory system

  • The heart is a double pump – Arteries take blood away from the heart while veins take blood to the heart. The right (left as you look at the diagram, right in real life) takes deoxygenated blood to the heart, while the left takes oxygenated blood around the body.
  • Arteries carry blood at high pressure
  • As a rule arteries carry oxygenated blood while veins carry deoxygenated blood. However, the pulmonary vein and artery break this rule and carry the opposite type of blood.
  • The arteries split off into thousands of tiny capillaries and take blood to every cell
  • The veins transport the deoxygenated blood at low pressure back to the heart.
Circulatory System

The heart is made to pump by little pulses of electricity from the wall of the right atrium

Blood Vessels

- Artery Vein Capillaries
Pressure High Low Low
Lumen size Small compared to whole size large large compared to whole size, but tiny because capillaries are very small
Blood type Oxygenated Deoxygenated Both
Extra information Walls are strong and elastic There are valves which stop the blood flowing the wrong way Walls are 1 cell thick to allow substances to pass over
Picture Same as artery (labelling)


The heart

Heart diagram

In the heart the valves are to prevent the backflow of blood (See hand drawn diagram for more information). The heart pumps in three stages

  • Blood flows into the two atria
  • The atria gently push the blood into the ventricles (the sphincter muscles which let the blood into the atria from the vena cava and pulmonary vein close to stop blood flowing back out).
  • The ventricle contracts pushing blood around the aorta and pulmonary artery. The valves stop backflow

NB – The left side of the heart has a thick muscular wall because it needs to pump blood around the whole body


Various useful words

  • Bicuspid valve – The valve between the left atrium and left ventricle
  • Tricuspid valve – The valve between the right atrium and ventricle
  • Heart tendons/heart strong – are attached to the valves and stop them flipping inside out
  • Septum – Muscular wall between the right and the left of he heart
  • Semi lunar valves – the valves between the right ventricle and the pulmonary artery


Red blood cell White blood cells Plasma Platelets
They carry oxygen around the body They defend the body against disease Carries everything apart from oxygen Let the blood clot if you are cut
They have a donut shape to maximize surface area. They also contains a lot of haemoglobin which when combines with oxygen is red, and which has a lot of iron.
When it is combined with carbon dioxide it combines with water to form weak carbonic acid.
Around 70% of the carbon dioxide leaks out of the cell into the plasma.
Phagocytes – Eat germs, therefore they are large and change shape.
Lymph cells – Identifies the germs with its huge brain (nucleus) and sends out antibodies.
Is pale and straw coloured, and carries – Blood cells and platelets, nutrients (e.g. glucose or amino acids) Carbon dioxide, Urea, Hormones They are fragments of cells, and therefore contain no nucleus. They stop blood pouring out, and micro-organisms getting in.
Carbon monoxide combines with haemoglobin 300* more readily, and this time it does mix with the haemoglobin to form carboxyhaemoglobin, thus no oxygen can bond and the cell is useless.
They are made by the bone marrow (anaemia = not enough blood or iron).
Germs contain antigens, and lymphocytes identify the germs by the antigens Antibodies and antitoxin made by the white blood cells. -
They have no nucleus for more room They can also release antitoxins They also carry the following chemicals -


Plasma proteins

  • Albumen – Makes the blood viscous
  • Globulin – Is created by the lymph cells for destroying germs, and certain types are needed for blood clotting
  • Fibrinogen – Important for the clotting of blood.

After the platelets have done their bit the fibrinogen is made into a mesh of solid fibres called fibrin. To stop blood clotting in our body we have anticoagulants

(NOTE – Old red blood cells are broken down by the liver after about four months when they become in effective) and the haemoglobin added to bile)

Also See

Here are the other comprehensive GCSE Biology notes by Prometheus: