The three different types of blood vessel

The human body contains about one pint of blood for each stone of weight. An adult male casualty could have 12 pints of blood, whereas a six-year-old child may only have four pints. Blood is used for many purposes, but the most relevant one from the first aid point of view is to transport oxygen round the body from the lungs, and return waste gases to the lungs.

Oxygen is what makes blood red. Well-oxygenated blood is a bright red colour, whereas poorly oxygenated blood is much darker, almost purple. This is why casualties with poor oxygenation may have blueish skin (cyanosis). Blood that has just come from the lungs will therefore be bright red, and blood that is returning to the lungs will be darker.

The blood is carried round the body in vessels which are like a plumbing system. They come in three major types:

• Arteries: These are the motorway of the blood system. They take blood out from the heart into the various parts of the body. Blood from an artery has the full force of the heart behind it. Arterial bleeds can be recognized because they are bright red, often involve a lot of blood and come out in spurts in time with the heartbeat. They can quickly prove fatal.
• Capillaries: Arteries divide into other arteries and finally into capillaries. These are small vessels with extremely thin walls and are where most of the gas exchange between blood and tissues take place. Capillary bleeds are usually minor involving oozing blood and are generally easily controlled and not particularly dangerous.
• Veins: Capillaries join up again to form veins which return blood to the heart. Venous blood is much darker than arterial blood because of the low oxygen content. Severe venous bleeds tend to gush like arterial bleeds although with less force. They can still be dangerous (e.g. varicose vein bleeding.) as a significant amount of blood can be lost.

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