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Blood Vessels

Updated on December 11, 2012

Closed Systems

In humans the circulatory system is closed this means that the blood always remains in the vessels. The blood leaves the heart through the arteries, then splits into the arterioles, then the capillaries. The blood then flows back towards the heart, the capillaries gather into venules, the venules meet to form veins and the veins lead back to the heart.


Arteries carry blood away from the heart. Because of this they are under high hydrostatic pressure and are built to with stand this.

Arteries have a thin lumen, to keep the pressure high.

The endothelium is squamous to reduce friction from passing blood.

Arteries have a thick tunica media, this contains smooth muscle to constrict the lumen keeping the pressure high, and elastic fibres which recoil against the smooth muscle to open the lumen.

The tunica externa is made from mostly collagen this is for support against the high hydrostatic pressure.


Veins carry blood back to the heart, because of this the lumen is wide as blood pressure is low.

Veins have thinner layers of elastic tissue, smooth muscle and collagen as the hydrostatic pressure is low.

The main feature of a vein is that they have valves to prevent back flow of blood.


Capillaries consist of only a one cell thick endothelium. They are approximately 7 micrometers thick, the approximate size of a red blood cell. This reduces the diffusion distance as the red blood cell and capillary wall must be close.


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