There are five types of blood vessels that work together to transport blood throughout the body. Blood vessels are a key component of the circulatory system and vital to all of the bodies tissues. They allow oxygen and nutrients to be delivered to the tissues, as well as carbon dioxide and waste materials to be taken away.
Arteries
Arteries carry blood away from the heart to the rest of the body. The arterial system is split into two main categories:
Systemic Arteries – carry oxygenated blood from the heart to the rest of the body.
Pulmonary Arteries – carry deoxygenated blood from the heart to the lungs.
The blood pressure in the arteries is higher than other blood vessels (peaking when the heart contracts) and so they have thick walls, to help cope with this. The folded endothelium (the inner lining) also helps the arteries to cope with the high pressure by allowing them to expand.
Arterioles
Arterioles are small diameter vessels that carry blood from the arteries to the capillaries. The exchange of blood between the arteries and the arterioles sees the greatest drop in blood pressure.
They have muscular walls, however this is usually only one or two layers of smooth muscle. The walls are composed of three layers, as shown in the diagram:
Capillaries
The smallest type of blood vessel, the capillaries, have the smallest lumen and are only one cell think. They are the sight of metabolic exchange, substances are swapped between them and the cells.
They form networks of capillary beds that allow an increased surface area for exchange. Diffusion of substances between the capillaries and the cells is rapid due to them being only one cell thick.
Venules
Like arterioles, venules have a small diameter, however their walls are much thinner. They drain blood away from the capillaries and carry it to the veins. They are composed of three thin layers:
Veins
Blood is carried from the body to the heart by the veins. Similar to the arterial system the venous system is also separated into to main categories:
Systemic Veins – carry carry deoxygenated blood from the body to the heart.
Pulmonary Veins – carry oxygenated blood from the lungs to the heart.
Blood pressure is at its lowest in the veins, and so they have mush less elastic and muscular tissue than the arteries.
Most of the blood being carried in the veins is going against gravity, for example blood from the arms and legs has to be carried up to the heart. To prevent the blood from flowing backwards, they contain valves. Constriction of surrounding muscles also helps blood flow through the veins.
5 Comparisons of Arteries and Veins
- Arteries have more elastic and muscular tissue
- Veins have a bigger lumen
- Arteries carry blood at a higher pressure than veins
- Veins contain valves, arteries don’t
- Arteries are usually bright red in colour, veins are usually dark red
KEY DEFINITIONS
Lumen – an opening inside a tubular body.
Endothelium – a single layer of cells lining the surface of blood and lymphatic vessels.
Metabolic Exchange – the transfer of small molecules such as oxygen and carbon dioxide.
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