Blood vessels

• Elastic fibres= composed of elastin and can stretch and recoil, providing vessel walls with flexibility.
• Smooth muscle= contracts or relaxes, which changes the size of the lumen.
• Collagen= provides structural support to maintain the shape and volume of the vessel.
• Arteries

  =Carry blood away from the heart to tissues of the body.

  • Carry oxygenated blood except the pulmonary artery (from heart to lungs)
  • Under high pressure.
  • Artery walls contain elastic fibres, smooth muscles and collagen.
  • Elastic fibres enable them to withstand the force of blood pumped out and stretch to take larger volumes blood. In between contractions elastic fibres recoil and return to normal. Helps even out surges to give a continuous flow.
  • Lining of artery is smooth so the blood flows easily over it.
• Aterioles

  = Link arteries and capillaries.

  • Have more smooth muscle and less elastin in their walls as they have little pulse surge, but can constrict or dilate to control the flow of blood into individual organs.
  • When the smooth muscle in the arteriole contracts it constricts the vessel and prevents blood flowing into a capillary bed= vasoconstriction.
  • When smooth muscle in the wall of an arteriole relaxes, blood flows through into the capillary bed= vasodilation.
• Capillaries

  = Microscopic blood vessels that link arterioles with venules.

  • Lumen is so small red blood cells have to travel in single file.
  • Substances are exchanged through capillary walls between tissue cells and the blood.
  • Gaps between the endothelial cells that make up the capillary walls in most areas are relatively large. This is where many substances pass out of capillaries into the fluid surrounding the cells.
  • Blood entering capillaries from arterioles is oxygenated, by the time it leaves the capillaries for venules it has less oxygen and more carbon dioxide.
• Capillary adaptations

  • Provide a large surface area for diffusion of substances in and out the blood.
  • Total cross-sectional area of the capillaries is always greater than the arteriole supplying them so rate of blood flow falls. The slow movement of blood through capillaries gives more time for the exchange of materials by diffusion between blood and cells.
  • Walls are a single endothelial cell thick, giving a very thin layer for diffusion.
• Veins

  = Carry blood away from the cells and towards the heart.

  • Deoxygenated blood flows from capillaries into venules into larger veins until it reaches vena cava.
  • Don't have a pulse- surges from heart are lost in narrow capillaries.
  • Hold a large reservoir of blood.
  • Blood pressure is low.
  • Contain valves.
  • Walls contain lots of collagen and relatively little elastic fibres, have a wide lumen and a smooth thin lining so blood flows easily.
• Venules

  Link capillaries with the veins.

  • Have thin walls with a little smooth muscle.
• Adaptations of veins

  • Have one way valves, they are flaps or infoldings of the inner lining of the vein. Prevents blood flowing backwards as blood is pumped at very low pressure.
  • Bger veins run between big, active muscles. When muscles contract they squeeze the veins, forcing the blood towards the heart.
  • Breathing movements of chest act as a pump for blood in veins towards the heart.