Blood Vessels
Created | Updated Jan 28, 2002
Blood Vessels
(The layers reffered to are the layers of the vessel walls. See below for explanation).
Artery
Blood pressure - High
Inner layer - Present
Middle layer - Thick containing muscle and many elastic fibres.
Outer layer - Present
The thick walls allow the arteries to withstand higher pressures.
The elastic fibres expand as blood is forced along them, and recoil to even out the blood flow.
Smooth sphincter muscles allow the diameter of the artery to change adjusting the blood flow to various organs.
Capillary
Blood pressure - Variable
Inner layer - Present
Middle layer - Abscent
Outer layer - Abscent
The very thin walls consisting of only endothelial cells allow diffusion to occur (See exchange surfaces).
White blood cells can squeeze between cells to perform phagocytosis.
Vein
Blood pressure - Low
Inner layer - Present
Middle layer - Contains muscle and a few elastic fibres.
Outer layer - Present
Semi-lunar valves prevent the blood from flowing back down the veins away from the heart.
The muscle surrounding the veins help to move the blood back to the heart by squeezing the veins.
Inner layer = A layer consisting on one layer of endothelial cells.
Outer layer = A layer of collagen fibres to protect the vessels.
