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Biology 12. Circulatory System – Part 2. Types of Blood Vessels. There are 5 main types of blood vessels Arteries Arterioles Capillaries Venules Veins. The aorta, the largest artery in the body is almost the diameter of a garden hose

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Biology 12

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biology 12

Biology 12

Circulatory System – Part 2

types of blood vessels
Types of Blood Vessels
  • There are 5 main types of blood vessels
    • Arteries
    • Arterioles
    • Capillaries
    • Venules
    • Veins

The aorta, the largest artery in the body is almost the diameter of a garden hose

  • The aorta in a blue whale is so big, you could swim down it
  • Capillaries, on the other hand, are so small that it takes ten of them to equal the thickness of a human hair
  • One pound of excess fat adds approximately 200 miles of extra capillaries. This increases BP!
  • If you stretched your veins out, and hooked them all together, they would go around the world 2 times!!
  • Function
    • Transport blood away from the heart
  • Structure
    • Thick, elastic walls
  • Location
    • Usually deep, along bones
    • This protects them from injury and temperature loss
  • Notes
    • Walls can expand
    • Arteries have very high blood pressure
    • Expansion is the “pulse” we feel
  • Function
    • Control blood flow into capillaries
  • Structure
    • Smaller in diameter than arteries, thinner walls
    • Have pre-capillary sphincters
  • Location
    • Leading towards all capillaries
  • Notes
    • Blood Pressure > Osmotic Pressure
    • Regulate blood pressure with pre-capillary sphincter muscles
      • Can dilate or constrict to constrict to increase or decrease blood flow to a particular capillary bed
  • Function
    • Connect arteries to veins
  • Structure
    • Very thin walls (1 cell thick)
  • Location
    • Everywhere; within a few cells of each other
    • Site of “capillary-fluid exchange”



capillary fluid exchange
Capillary-Fluid Exchange

Arteriole Side

  • Blood pressure at arteriole side = 40 mmHg
  • Osmotic pressure = 25 mmHg
  • Net blood pressure (15 mmHg) forces water out of the blood into the interstitial fluid
  • Water carries with it the O2 and nutrients
  • Because there is more O2 and nutrients in interstitial fluid, it diffuses into body cells
capillary fluid exchange2
Capillary-Fluid Exchange
  • The large things (ex. RBC, WBC, platelets, blood proteins) stay in the capillary because they are too big to leave
  • Because most of the water has left, the blood becomes very hyperosmotic (concentrated)
  • The venule side of the capillary is therefore under great osmotic pressure to draw water back into the blood
capillary fluid exchange3
Capillary-Fluid Exchange

Venule Side

  • Osmotic pressure at venule side = 25 mmHg
  • Blood pressure = 10 mmHg
  • Blood very concentrated (has little water)
  • Net osmotic pressure (15 mmHg) forces water back into the blood
  • Water carries with it CO2 and metabolic wastes (urea)
  • These are carried to the kidneys and other excretory organs to be removed
  • Function
    • Drain blood from capillaries
  • Structure
    • Thinner walls than veins
  • Location
    • Often near the surface
  • Notes
    • Join to form veins
    • Osmotic Pressure > Blood Pressure
    • The end result is no change in blood volume (no volume is lost in the exchange)
  • Function
    • Transport blood towards the heart
  • Structure
    • Inelastic walls, contain one way valves
  • Location
    • Often near to surface
  • Notes
    • Blood pressure & velocity is much lower than in arteries
    • Valves prevent blood from flowing backwards
    • Surrounded by skeletal muscle, “squeezes” blood along
how does it all fit together
How Does It All Fit Together?


  • Carry blood away from the heart
  • Elastic


  • Very thin tubes
  • Connect arteries to veins
  • Can close down or open up to regulate blood flow
  • Gas exchange


  • Bring blood towards the heart
  • Have valves to stop blood from moving backwards