blood vessels

• arteries and arterioles

  artery walls contain elastic fibres, smooth muscle and collagen

  arterioles have more smooth muscle and less elastin in their walls than arteries
• arteries function of structure

  elastic fibres -enable to withstand force of blood pumped out of heart+stretch to take larger blood volume. between heart contractions, elastic fibres recoil +return to original length(helps even out surges of blood pumped from heart=continuous flow)

  endothelium is smooth so flood easily flows over it
• arterioles function of structure

  less elastin than artery as have little pulse surges but can constrict / dilate to control blood flow into individual organs.

  when smooth muscle in arteriole contracts, it constricts vessel+prevents blood flowing into capillary bed(vasoconstriction). when smooth muscle relaxes, blood flows into capillary bed(vasodilation)
• veins and venules

  deoxygenated blood flows from capillaries into venules then veins.

  veins dont have a pulse as surges from the heart are lost as blood passes through capillaries.

  up to 60% blood is in veins at one time. blood pressure is low.
• veins function of structure

  low blood pressure so most veins have one way valves at intervals. walls contain lots of collagen and little elastic fibre, vessels have wide lumen + smooth, thin lining so blood flows easily.
• venules function of structure

  link capillaries and veins.

  very thin walls with a little smooth muscle.
• capillaries

  link arterioles with venules

  lumen so small rbc have to travel through single file.

  substances exchanged through capillary walls between tissue cells and blood.

  gaps between endothelial cells that make up capillary wall are relatively large as is where many substances pass put into surrounding fluid.

  provide large SA for diffusion of substances in/out of blood.
• capillaries function of structure


  walls are single endothelial cell thick giving a very short diffusion distance