Arteries
- There are three main types of blood vessel:
- Arteries- carry blood away from the heart
- Veins- carry blood towards the heart
- Capillaries- involved in the exchange of gas of materials with tissues
- Each vessel has a particular function and is specifically adapted to carry out that function efficiently
- There are also smaller vessels, arterioles that branch off from arteries and venules that branch into veins
Arteries carry blood under pressure
- Key features:
- Carry blood at high pressure away from the heart
- Carry oxygenated blood (except the pulmonary artery)
- Have a narrow lumen
- Have thick muscular walls compared to the size of the lumen
- The strong muscular walls contain elastic fibres to allow them to stretch and spring back
- Blood flows through at a fast speed
- The structure of an artery is adapted to its function in the following ways
- Thick muscular walls withstand the high pressure of blood and maintain the blood pressure as it recoils after the blood has passed through
- A narrow lumen also helps to maintain high pressure
Veins
Veins take blood back to the heart
- Key features:
- Carry blood at low pressure towards the heart
- Have thin walls as the blood is at a lower pressure
- Have a larger lumen than arteries
- Contain valves
- Blood flows through at a slow speed
- The structure of a vein is adapted to its function in the following ways:
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- A large lumen reduces resistance to blood flow under low pressure
- Valves prevent the backflow of blood as it is under low pressure
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Comparing the structure of arteries and veins
Capillaries
Capillaries are small
- Key features:
- Very small - too small to be seen with the naked eye
- Carry blood at low pressure within tissues
- Have permeable walls that are one cell thick
- Supply oxygenated blood and nutrients to tissues
- Take away waste and deoxygenated blood
- Speed of blood flow is slow
- The structure of a capillary is adapted to its function in the following ways:
- Capillaries have walls that are one cell thick (short diffusion distance) so substances can easily diffuse in and out of them
- The walls are ‘leaky’ allowing blood plasma to leak out and form tissue fluid surrounding cells
Structure of a capillary
A network of small capillaries allows for efficient exchange of materials in tissues
Exam Tip
The circulatory system can be viewed as a network of cylindrical tubes. The velocity of blood flow varies inversely with the total cross-sectional area of the blood vessels. As the total cross-sectional area of the vessels increases, the velocity of flow decreases. The slow flow in capillaries is because of the high total cross-sectional area of capillaries and not because capillaries are narrow. The collective cross-sectional area of all the capillaries in the human body is about a 1000 fold greater than the aorta. The slow flow in capillaries favours gas exchange and nutrient supply to cells.
