The circulatory system

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Blood transports materials around the body and protects against disease. It consists of cells, solutes and liquid. Blood is pumped away from the heart at high pressure in arteries and returns to the heart at low pressure in veins. The various components of the circulatory system have adaptations linked to their functions.


Blood is used to transport materials around the body, and to protect against disease. Blood contains plasma, a liquid that contains dissolved substances, cells and cell fragments. These include:

  • Red blood cells, which transport oxygen
  • White blood cells, which protect against disease
  • Platelets, which help the blood to clot

Blood is made up of plasma, red and white blood cells and platelets (

Red blood cells

The table explains how the structure of a red blood cell is adapted to its function.


Small size lets red blood cells pass through narrow capillaries. Flattened disc shape Provides a large surface area, allowing rapid diffusion of oxygen Contains haemoglobin Haemoglobin absorbs oxygen in the lungs and releases oxygen in the rest of the body Does not contain a nucleus Increases amount of space inside the cell for haemoglobin


Plasma transports dissolved substances around the body, including:

  • Hormones
  • Antibodies
  • Nutrients, such as water, glucose, amino acids, minerals and vitamins
  • Waste substances, such as carbon dioxide and urea

Blood vessel

FunctionAdaptions Artery Carry blood away from heart at high pressure Thick, elastic, muscular walls to withstand pressure and to exert force (pulse). Capillary Allow exchange of materials between blood and tissues Thin permeable walls. Vein Return low pressure blood to heart

Large diameter to offer least flow resistance. Valves to prevent back flow.

The heart

The heart pumps blood in two ways:

  • The right side pumps blood to the lungs
  • The left side pumps blood to the rest of the body

You need to be able to identify these parts, and where they are:

  • Atria (left and right)
  • Ventricles (left and right)
  • Semilunar, tricuspid and bicuspid valves
  • The four main blood vessels of the heart:
    • Vena cava
    • Pulmonary artery
    • Pulmonary vein
    • Aorta

The diagram below shows the main parts of the heart, and how it is able to pump blood.



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