How veins collapse
If blood is unable to flow smoothly it clots. The lining of arteries, veins and capillaries is perfectly smooth so that the blood can flow without clotting. The reason blood clots when you cut yourself is because as it leaves the blood vessels it stops flowing smoothly.

Introducing a needle (that tears and scratches the vein lining) and then drugs (that can irritate and cause swelling of the vein lining) creates turbulence in the blood flow.

This causes tiny clots to form on the lining of the vein. These tiny clots roughen the lining causing more turbulence - and the formation of more small clots. Gradually the vein fills up with the clots and blocks.

The clots then turn into scar tissue which shrinks, pulling the sides of the vein together, leaving it 'collapsed'.

Veins that are damaged or swollen may partially recover, but collapsed veins never recover. The blood finds another way back to the heart and lungs through smaller or deeper veins further back down the system.

Injection can scratch the inside of the vein and leaves a hole which,
during healing, roughens the lining of the vein.

Clots form in the turbulent blood around the site.

As the vein becomes narrower the turbulence increases
and the clots form more quickly.

Finally, the edges of the vein heal together and the vein collapses
as the scar tissue draws the sides together.

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