Circulatory system

The Heart,

Blood Vessels,

The blood

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The Heart

Humans have a double circulatory system > two circuits joined together.

1) pumps deoxygenated blood to the lungs to take in oxygen. The blood then returns to the heart.
2) the second pumps oxygenated blood around all of the other organs in the body. The blood gives the oxygen to the body cells and the deoxygenated blood returns to the heart to be pumped to the lungs again.

The heart.

The heart is a pumping organthat ensures blood is constantly flowing around the body.
The walls of the heart are mostly made of muscle tissue.

The heart has valves to ensure the blood flows in the right direction- valves don't allow the blood to flow backwards.

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Blood vessels

The three types of blood vessels are:

  • arteries- carry the blood away from the heart
  • capillaries- involved in the exchange of materials at the tissue
  • veins- carry the blood to the heart


Heart pumps blood out at high pressure, the artery walls are strong and elastic.

The walls are think compared to the size of the lumen (hole down the middle).

Arteries contain thick layers of muscle to add strength, and elastic fibres so they can stretch and spring back to shape.

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The blood

Red blood cells.

RBCs carry oxygen from the lungs to all the cells in the body.

Biconcave shap > large surface area for absorbing oxygen.

No nucleus > more room to carry oxygen.

Contain red pigment called haemoglobin. Haemoglobin combines with oxygen in the lungs to become oxyhaemoglobin.
Oxyhaemoglobin splits into haemoglobin and oxygen to release oxygen to the cells.


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Blood 2

White blood cells.

WBCs defend our bodies against disease.

They change shap to engulf pathogens.

They produce antibodies to fight microorganisms.

They produce antitoxins to neutralise the toxins produced by the pathogens.

They have a nucleus.


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Blood 3


Platelets help clot the blood at the site of a wound.
They are small fragments of cells, they have no nucleus.

The help clot the blood at a wound- this stops blood leaving and microorganisms getting in.

A lack of platelets causes excessive bleeding and bruising.


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Blood 4


Pale liquid that carries substances in the blood.

It carries:

  • red and white blood cells and platelets
  • nutrients like glucose and amino acids
  • CO2
  • urea
  • hormones
  • antibodies and antitoxins
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Heart 2

Blood passes through the 4 chambers in this sequence:

  • right atrium
  • right ventricle
  • left atrium
  • left ventricle

1) blood flows into the two atria from the vena cava and pulmonary vein.

2) the atria contract pushing the blood into the ventricles

3) the ventricles contract forcing the blood into the pulomary artey and the aorta, and out of the heart

4) blood then flows to the organs through arteries and back to the heart through veins

5) the atria fill again and the whole cycle starts again.

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Blood vessels 2


Arteries branch into capillaries.

Capillaries are too small to see.

They carry the blood really close to every cell in the body to exchange substances.

Permeable walls so substances can diffuse in and out. They supply glucose and oxygen, and take away waste like CO2.

Their walls are usually one cell thick, this increases the rate of diffusion as it decreases the distance that substances have to diffuse over.

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Blood vessels 3


Capillaries join up to form veins.

Blood is at lower pressure in the veins than arteries, so the walls are thinner.

Veins have a large lumen, this helps blood flow despite the lower pressure.

Veins have valves to keep the blood flowing in the right direction.

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