The appearance of ‘new’ veins
When veins have been blocked the blood still needs to get back to the heart and has to find another way. Veins are connected at lots of junctions - so this means taking a different route and going up veins that are further back in the system.

But as the bigger veins collapse the ones that get used to re-route the blood become smaller and smaller. When a very small vein has to carry a lot of blood it may swell up but, like a balloon, as well as getting bigger the walls become thinner and much, much weaker.

When a 'new' vein appears it is usually one of these small, weaker veins that has 'blown up'. If used for injection they often burst as soon as they are punctured. Even if they can be used for a while they usually block up within a few hits because the clots that form to stop the bleeding are big enough to stop the flow of blood.

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