Blood Vessels

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Blood Vessels

(The layers reffered to are the layers of the vessel walls. See below for explanation).


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.


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.


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.

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