Blood vessels and their functions

• Arteries carry blood away from the heart into smaller vessels
• Arterioles are smaller than arteries and controls flow to the capillaries
• Capillaries are tiny vessels linking arterioles to veins
• Veins carry blood from capillaries back to the heart
• Arteries have a thick muscle layer to allow the constriction and dilation of smaller arteries in order to control the volume of blood flowing through
• Arteries have a thick elastic layer to keep blood pressure high, allowing it to stretch at each heartbeat and spring back when the heart relaxes
• Arteries have a thick wall to prevent the vessels bursting under pressure
• Aortic semi-lunar valves prevents the backflow of blood to the left ventricle
• Arterioles have thick muscle layers to control the constriction of the lumen to control its movement into the capillaries
• Arterioles have a thinner elastic layer compared to arteries because the blood pressure is lower
• Veins have a thin muscle layer because they can't control the flow of blood
• Veins have a thin elastic layer because the low pressure will not cause them to burst and pressure is too low to create a recoil action
• Veins have thin walls because the pressure is too low for risk of bursting, and it allows them to be flattened easily
• Valves are present in the veins to ensure blood doesn't flow backwards
• Arteries

  • walls are thick and contain collagen
  • wall has elastic tissue so can stretch
  • smooth muscle can contract and restrict
  • endothelium can be folded and unfolded
• Veins

  • thinner wall
  • extended lumen not circular
  • less elastic tissue and muscle
  • pocket valves for low pressure
• Capillaries are one cell thick in order to have a short diffusion pathway
• There are lots of capillaries in order to have a large surface area for exchange
• Capillaries have a narrow diameter and lumen so that red blood cells are flattened against the side of the capillary reducing diffusion distance
• There are spaces between the endothelial cells of the capillaries to allow white blood cells to escape and deal with infections