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Arteries

• Usually carry oxygenated blood from the heart to the rest of the body
• Pulmonary artery is the exception as it carries deoxygenated blood from the heart to the lungs.
• Thick walls (with muscle and elastic fibres) to withstand high pressure.
• Muscle and elastic fibres within the walls also allow the artery to expand and recoil with each surge of blood.

Veins

• Usually carry deoxygenated blood from the body back to the heart.
• The lumen is large and reduces friction as the blood moves through.
• The pulmonary vein is the exception as it carries oxygenated blood from the lungs to the heart.
• Blood is moving at a low pressure so the walls are thin.
• Very few muscle and elastic fibres because blood does not surge through veins.
• Valves are present to prevent the backflow of blood.

Capillaries

• Allow the diffusion of substances (e.g. oxygen, carbon dioxide, dissolved food and urea) between the blood and the body’s cells or vice versa.
• Walls are 1 cell thick providing a thin, permeable surface for diffusion.