The circulatory system in humans – WJECBlood vessels

Arteries

• Carry blood away from the heart (always oxygenated apart from the pulmonary artery which goes from the heart to the lungs).
• Have thick elastic and muscular walls.
• Have small passageways for blood (internal lumen).
• Contain blood under high pressure.

Veins

• Carry blood to the heart (always deoxygenated apart from the pulmonary vein which goes from the lungs to the heart).
• Have thin, less muscular walls.
• Have larger passageways for blood (internal lumen).
• Contain blood under low pressure.
• Have valves to prevent blood flowing backwards.

Capillaries

• Found near every living cell of the body.
• Microscopic – walls are one cell thick, this allows for the diffusion of substances into the cells from the capillaries and out of the cells into the capillaries.
• Very low blood pressure.

Structure of blood vessels

The structure of the blood vessels can be easily compared when they are seen in this cross-section (not to scale).