Veins
Veins carry blood back to the heart and lungs at reduced pressure and they need some help. They get this from the movement of muscles squeezing them and forcing the blood along. They never have a pulse - anything with a pulse is an artery.

To stop the blood getting squeezed both ways there are small valves that flap shut, preventing the blood flowing backwards.

You must inject with the flow of blood otherwise you force fluid against the valves, causing extra vein damage, swelling and clotting.

Finding a vein
Veins start off as the tiniest capillaries carrying blood back to the heart, which merge, forming bigger and bigger veins as they progress. The outer (peripheral) veins merge with deeper, bigger veins. On the arm, one of the main junctions is at the inside of the elbow - which is the least dangerous place to inject and is where most people start injecting.

The more a needle rubs on the inside of a vein the more damage it will do. Bigger needles do more damage than small ones. Small veins get damaged more easily than big ones.

You can increase the blood flow in veins - and make them bigger - by:

• Relaxing
• Warming yourself up
• Flexing your muscles
• Gripping your upper arm
• Warming up the limb by bathing it in warm water, but don’t inject heroin or other sedatives in the bath: if you fall unconscious you could drown.

If this doesn’t work you can slow the blood flow out of your arm and fill the vein by using a tourniquet - which must be one that can be released quickly and easily without causing movement that can disturb the needle. Tourniquets that are too tight don’t work because they stop the blood getting in as well.

If you do use a tourniquet it must be released before you start injecting, otherwise the blood carrying the drugs won’t be able to get past and the vein may tear at the injection site and some of what you inject will escape from the vein, causing swelling at the injection site: a miss. Never share touniquets - they may get contaminated with blood when people are taking them off.

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