Blood Vessels

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Blood Vessels
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Blood vessels are hollow tubes that carry
blood.Interesting Facts

• If you could take all the vessels in a child and
  line them up they would stretch about 60,000
  miles.
• Adult blood vessels could reach about 100,000
  miles.

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Blood vessels carry blood between the heart,
different tissues, and organs of the body Three
types of blood vessels arteries, veins, and
capillaries. Vessels have the ability to expand
to allow more blood to flow through them and to
contract to help control the flow of blood.
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Sequence of blood flowBlood flows from the big
arteries to the smaller arterioles then to the
capillaries. The capillaries connect to the
smallest veins called venules. Blood flows from
the venules to the large veins and back to the
heart.
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Arteries are elastic, muscular tubes that carry
blood from the left ventricle of the heart to the
capillaries. The walls of the arteries are
thicker than other vessels because of the
pressure.
Fact The diameter of the aorta is about the same
as a garden hose.
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Walls of the arteries are three layers thick

• Inner layer Tunica Intima consists of single
  flattened cells called endothelium
• Middle layer Tunica Media thicker part of
  arterial wall
• Outer layer Tunica Externa (or Adventia)
  composed of fibrous tissue that gives strength to
  the wall of the artery.

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Do you remember that we said that arteries had a
lot of elasticity? Here you can clearly see the
internal elastic membrane of an artery.
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Veins

• Veins carry blood from capillaries to the heart.
• Increase in size as they progress toward the
  heart.
• Veins that connect to capillaries are venules.
• Follow a path parallel to arteries.
• More veins than arteries
• Veins at any one moment carry 70 of the blood.

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Veins like arteries have three layers but are not
as thick.
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Located within the veins are valves that allow
blood to flow toward the heart but does not allow
the blood to flow backward. The only vein that
does not have valves is the vena cava. Valves in
the legs sometimes breakdown and allow the blood
to flow backwards. This is called varicose veins.
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Artery-Vein PairArtery is on left, vein on right
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Capillaries

• Smallest working unit in the vascular system
• Connect arterioles to venules
• Only 1 cell thick to allow for nutrient and gas
  exchange

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Longitudinal section of a vein. Note that this
cannot be a capillary since its diameter is more
than 1-cell wide.