How do varicose veins form?
Veins are the vessels that return blood to the heart once it has circulated through the body whereas arteries carry oxygen-rich blood from the heart to the body). Veins have one-way valves that help keep blood flowing in the proper direction. If these valves malfunction, blood can flow backwards and pool in the veins, causing them to stretch.
The illustration on the right shows how a varicose vein forms in a leg. Figure A shows a normal vein with a working valve and normal blood flow. Figure B shows a varicose vein with a deformed valve, abnormal blood flow, and thin, stretched walls. The middle image shows where varicose veins might appear in a leg.