blood vessels

• there are three types of blood vessels in humans: arteries, veins and capillaries
• arteries carry blood away from the heart, under pressure
• arterial walls have a thick lumen and contain layers of muscle
• capillaries are very small, and their lumen is only one cell thick (to allow easy diffusion)
• veins take blood back to the heart
• veins carry blood under lower pressure, therefore they have a relatively thin lumen