Blood vessels

Arteries

  • Carry blood away from the heart (always oxygenated apart from the pulmonary artery which goes from the heart to the lungs).
  • Have thick elastic and muscular walls.
  • Have small passageways for blood (internal lumen).
  • Contain blood under high pressure.

The arteries carry blood away from the heart

Veins

  • Carry blood to the heart (always deoxygenated apart from the pulmonary vein which goes from the lungs to the heart).
  • Have thin, less muscular walls.
  • Have larger passageways for blood (internal lumen).
  • Contain blood under low pressure.
  • Have valves to prevent blood flowing backwards.
Cross section through a vein showing the vein valve.Cross-section through a vein

Capillaries

  • Found near every living cell of the body.
  • Microscopic – walls are one cell thick, this allows for the diffusion of substances into the cells from the capillaries and out of the cells into the capillaries.
  • Very low blood pressure.
Diagram showing how blood travels through the capillaries. The direction of the blood is right to left. Labelled from right to left are the artery, arteriole, capillaries, venule and vein.Blood travelling through the capillaries

Structure of blood vessels

The structure of the blood vessels can be easily compared when they are seen in this cross-section (not to scale).

Cross section of an Artery, a Vein and a Capillary. The artery has a Thicker smooth muscle layer, the vein has a Thinner smooth muscle layer. Both have a Tough outer coat, Endothelium and  Lumen.
ArteryVeinCapillary
Type of bloodOxygenatedDeoxygenatedBoth
DirectionAway from the heartTowards the heartFrom arteries to veins
PressureHighLowLow
Size of hole (lumen)SmallLargeVery small
Wall thicknessThickThinOne cell thick
ValvesNoYesNo