Blood Vessels

Arteries: carry blood away from the heart

Capillaries: these are involved in the exchange of materials at the tissues

Veins: carry blood to the heart 

The heart pumps blood at high pressure.

The artery walls are therefore strong and elastic.

• Walls are thick compared to the size of the lumen
• They contain thick layers of muscle to make them strong

- Arteries branch into capillaries

- Capillaries are tiny

- They carry blood really close to every cell in the body, exchanging substances with them

Structure:

• Permeable walls so substances can diffuse in and out
• They supply food & oxygen and take away wastes like carbon dioxide
• The walls are one cell thick, increasing the rate of diffusion by decreasing the distance over which it occurs

Transport blood back to the heart

- Capillaries join up to form veins

Structure:

• Blood pressure is lower in the veins so the cell walls don't need to be as thick as the artery walls
• Larger lumen than arteries to help the blood flow despite the lower pressure
• Valves to keep the blood flowing in the right direction- prevent backflow