Blood Vessels (SQA National 5 Biology): Revision Note

Arteries

• Arteries are part of three different blood vessels found in the circulatory system

• Arteries carry blood away from the heart at high pressure

• Blood is oxygenated in all arteries apart from the pulmonary artery which carries deoxygenated blood

• The walls of arteries are thick and muscular and contain elastic fibres to withstand the high pressure

• Arteries have a narrow lumen to help maintain high pressure

Capillaries

• Capillaries form extensive networks that carry blood through tissues and organs, connecting arteries to veins

• They are the site of exchange between the blood and body cells, allowing oxygen and nutrients to enter cells and carbon dioxide and other wastes to leave

• Adaptations of capillaries maximise the efficiency of exchange in the following ways:

  • Their walls are only one cell thick and contain small gaps that let fluids and dissolved substances pass through easily

  • Capillaries have a very narrow lumen which slows blood flow, giving more time for exchange

  • The large surface area of the capillary networks increases the efficiency of diffusion

• Capillaries carry oxygenated blood from arteries to body cells and deoxygenated blood from the cells to veins

Veins

• Veins carry blood towards the heart at low pressure

• Veins transport deoxygenated blood away from the body

  • The single exception to this is the pulmonary vein, which carries oxygenated blood

• The walls of veins are thin in comparison to arteries

• Veins have a wide lumen

• Valves in veins prevent blood from flowing backward

Comparing the blood vessels