Blood Vessels
Arteries:
- Carry blood away from the heart.
- Under high pressure, so have smooth endothelium to allow blood to flow easily and have elastin to allow them to stretch as pressure increases with pulse surges.
- Connect arteries and capillary beds.
- Have less elastin.
- Muscle allows vasoconstriction and vasodilation to control blood flow into tissues.
Veins:
Venules:
- Carry blood back to heart.
- Veins have little elastin as there is no pulse surge and there is low blood pressure as pressure is lost in capillary bed.
- The contraction of muscle surrounding veins pumps blood back to heart.
- Most large veins have one way valves to stop back-flow.
Venules:
- Connect capillary bed to veins.
Capillaries:
- Microscopic blood vessels that link arterioles and venules.
- Just large enough for 1 red blood cell to pass through.
- Fenestrations (gaps) in endothelium allow substances dissolved in blood plasma to move in and out.
- Gaseous exchange between oxygenated blood and respiring tissues occurs in capillaries.
- Adapted to their role because...
- They have a large SA:V ratio for diffusion.
- They are only 1 cell thick so there is a small diffusion distance.
- Total area of capillary bed > total area of arterioles, so pressure drops causing blood to move slower to allow more time for diffusion.
- They have a large SA:V ratio for diffusion.