Blood vessels

• Arteries carry very high pressure blood from the heart to the organs in the body. Like this...

  
• Arteries have very thick muscular walls. This allows them to withstand the very high pressure of the blood. Blood travels through the arteries in surges every time the heart beats. Elastic fibres stretch when the surge of blood passes through and then recoil in between surges, which keeps the blood moving. Like this...

  
• Blood travels to the organs and arteries, but once its in the organs the blood flows through the pillories.
• Capillaries (part 1):

  When the blood passes through capillaries, substances such as glucose and oxygen diffuse from the blood to the cells. Like this...

  
• Capillaries (part 2): Cardon dioxide diffuses from the cells back to the blood.

  
• Capillaries have very thin walls, so the diffusion pathway is very short. This allows substances to diffuse rapidly between the blood and the body cells. Like this...

  
• Once the blood has passed through the organs, it now makes its way back to the heart in veins. But there is a problem, because the blood is now travelling slowly and at low pressure. That means that it could stop or eve go backwards. Like this...

  
• Structure of veins:

  Veins have a thin wall. The blood pressure is low so the wall does not need to be thick. Many veins contain valves. The job of valves is to stop blood flowing backwards. When the blood is flowing in the correct direction, then the valves open to allow the blood to flow through. Like this...

  
• Structure of veins (part 2):

  When the blood starts to flow backwards the valves shut. Like this...

  
• • There are three main types of blood vessel:
  • Arteries
  • carry blood away from the heart
  • Veins
  • carry blood towards the heart
  • Capillaries
  • involved in the exchange of gas of materials with tissues
  • Each vessel has a particular function and is specifically adapted to carry out that function efficiently.
  • There are also smaller vessels, arterioles that branch off from arteries and venules that branch into veins.
• Arteries carry blood under pressure (part 1):

  • Key features:
  • Carry blood at high pressure away from the heart.
  • Carry oxygenated blood (except the pulmonary artery).
  • Have a narrow lumen.
  • Have thick muscular walls compared to the size of the lumen.
• Arteries carry blood under pressure (part 2):

  • The strong muscular walls contain elastic fibres to allow them to stretch and spring back.
  • Blood flows through at a fast speed.
  • The structure of an artery is adapted to its function in the following ways.
  • Thick muscular walls withstand the high pressure of blood and maintain the blood pressure as it recoils after the blood has passed through.
  • A narrow lumen also helps to maintain high pressure.
• Veins take blood back to the heart:

  • Key features:
  • Carry blood at low pressure towards the heart.
  • Have thin walls as the blood is at a lower pressure.
  • Have a larger lumen than arteries.
  • Contain valves.
  • Blood flows through at a slow speed.
  •  The structure of a vein is adapted to its function in the following ways:
  • A large lumen reduces resistance to blood flow under low pressure.
  • Valves prevent the backflow of blood as it is under low pressure.
• Comparing the structure of arteries and veins (part 1):

  
• Comparing the structure of arteries and veins (part 2):

  
• Capillaries (part 2):

  • Key features:
  • Very small - too small to be seen with the naked eye.
  • Carry blood at low pressure within tissues.
  • Have permeable walls that are one cell thick.
  • Supply oxygenated blood and nutrients to tissues.
  • Take away waste and deoxygenated blood.
  • Speed of blood flow is slow.
• Capillaries (part 2):

  • The structure of a capillary is adapted to its function in the following ways:
  • Capillaries have walls that are one cell thick (short diffusion distance) so substances can easily diffuse in and out of them.
  • The walls are ‘leaky’ allowing blood plasma to leak out and form tissue fluid surrounding cells.
• Structure of a capillary: