2. Blood vessels

• • Different components utilised in some blood vessels:

  1. Elastic fibres: composed of elastin + can stretch & recoil = providing vessel walls with flexibility
  2. Smooth muscle: contracts/relaxes = changes lumen size
  3. Collagen: provides structural support to maintain shape & volume of vessel

   
• Ateries & arterioles

  • Carries blood AWAY from heart --> to tissues of body
  • Carry oxygenated blood

  --> except pulmonary artery + umbilical artery which carries deoxygenated blood from fetus to placenta

  • Blood in arteries = under high pressure
• Artery walls have:

  • elastic fibres
  • collagen
  • endothelium
  • smooth muscle
• Elastic fibres in artery walls:

  • Enable them to withstand force of blood pumped out of heart = within limits maintained by collagen, to take in large blood vol
  • In between contractions = elastic fibres recoil + return to original length = helps to even out surges of blood pumped from heart to give a continuous flow
  • However, you can still feel a pulse [surge of blood] when heart contracts = which elastic fibres cant fully eliminate
  • Lining of artery endothelium is smooth = so blood flows over it easily
  • Corrugated endothelium = allows expanding
• Smooth muscle in arteries:

  • Arterioles have more smooth muscle + less elastin than arteries as they have little pulse surge --> can still constrict/dilate to control blood flow
  • When smooth muscle in arteriole contracts = constricts vessel + prevents blood flowing into capillary bed = vasoconstriction
  • When smooth muscle in arteriole relaxes = blood flows in capillary bed = vasodilation
• Capillaries

  • Microscopic blood vessels that form an extensive network
  • Lumen of capillary = 10μm

  --> so small that only 1 RBC [7.5-8μm] can travel through in a single file with other following

  • Gaps between endothelial cells that make up capillary walls = relatively large

  --> where most substances pass out of capillaries into fluid surrounding cells

  --> exception is capillaries in CNS = which have very tight junctions between cells
• Capillaries:

  • In most organs blood entering capillaries is oxygenated and leaving deoxygenated = lungs + placenta are the exception --> as they have deoxygenated blood entering capillaries and oxygenated leaving through venules
  • WBC's combat infection in tissues by squeezing through intercellular junctions of capillary walls
• Ways in which capillaries are adapted for their role:

  1. Provide large SA for diffusion of substances in and out of blood
  2. Total-cross sectional area of capillaries = always greater than arteriole supplying them so rate of blood flow falls allowing more time for exchange of materials by diffusion between blood + cells
  3. Walls 1 endothelial cell thick = thin [short] layer for diffusion
  4. Cells of wall have gaps [pores] which allow blood plasma to leak out + form tissue fluid
• Veins & venules

  • Carries blood away from cells = towards heart + carries deoxygenated blood
  • Pulmonary vein = carries oxygenated blood from lungs to heart
  • Umbilical vein = carries oxygenated blood from placenta to fetus
  • 2 main vessels carrying deoxygenated blood back to heart:

  1. Inferior vena cave: from lower parts of body
  2. Superior vena cava: from head + upper body
• Veins & venules:

  • Veins don’t have a pulse = surges from heart pumping are lost as blood passes through narrow capillaries

  --> hold a large reservoir of blood [60% of your blood vol is in your veins at any given time]

  • BP in veins = low compared to arteries
  • Medium sized veins [majority of venous system] = have valves to prevent backflow of blood
• Vein walls contain:

  • Lots of collagen
  • Little elastic fibres
  • Wide lumen
  • Smooth thin lining [endothelium] so blood flows easily
  • Venules = have thin walls with little to no smooth muscle + several venules join to form a vein
• • Deoxygenated blood needs to go back to heart = however blood is under low pressure so needs to move against gravity
• • 3 man adaptations that enable body to overcome this problem

  1. Majority of veins have one-way valves at intervals [infoldings of veins inner lining]. When blood flows in direction pf heart = valves open so blood --> if blood flows backwards valves close
  2. Many of the bigger veins run between big, active muscles in body. When muscles contract = veins squeeze = blood forced towards heart --> valves prevent backflow when muscles relax
  3. Breathing movement of chest = acts as a pump = pressure changes + squeezing actions move blood in veins of chest & abdomen towards heart