Blood Vessels
Biology: Cambridge International A Level, topic 8.1.2
Arteries, Arterioles and Veins
The circulatory system is made up of different blood vessels. Each blood vessel is specialised for its function. These blood vessels include:
Arteries
- Arteries transport blood away from the heart to the organs.
- Artery walls have thick layers of muscle.
- The walls maintain a high pressure so blood can be pumped around the body.
- Elastic fibres in the artery wall allow the arteries to stretch.
- The endothelium is folded which also allows the arteries to stretch.
Arterioles
- When arteries reach an organ, they split into many smaller vessels called arterioles.
- The direction of blood flow can be controlled by contracting the arterioles to restrict blood flow and relaxing the arterioles to allow blood to flow.
Veins
- Veins transport blood back to the heart.
- The lumen of veins is wider than the arteries which allows the blood to flow at low pressure.
- There is a thin muscle wall and elastic tissue in the vein walls.
- Valves are located throughout the veins to ensure blood flows towards the heart.
Venules
- Venules are small blood vessels that collect blood coming out of capillaries.
- Blood flows from the vast network of capillaries into network of fewer but larger venules and will eventually flow into the veins.
- Venules range from around 7 micrometers to 1 millimeter in diameter.
Capillaries
Arterioles branch into very small vessels, called capillaries. Capillaries have thin walls and pass very close to the body cells to allow exchange between blood and the organs.
Walls
- Capillary walls (endothelium) are only one cell thick.
- This means that the diffusion distance of substances (e.g. oxygen) from the bloodstream to the tissues is very short.
- A short diffusion distance allows for efficient exchange.
Location
- Capillaries pass very close to the body cells.
- There is a very short diffusion distance between the capillaries and the body cells.
- Exchange between the capillaries and the body cells is rapid.
Surface area
- Arterioles split into millions of capillaries that can form networks around the body cells (e.g. the network around the alveoli).
- The networks create a large surface area for exchange of substances between the bloodstream and the tissues.
- Networks of capillaries are called capillary beds.
Test yourself on Blood Vessels
Which blood vessel contains valves?
Which of the following statements about blood vessels are true?
Which of the following statements about capillaries are true?
What is the advantage of the thin endothelium of capillaries?