Usually, the 160,000 kilometers of blood vessels in your body work incredibly smoothly. However, the forces of age, weight gain, and gravity can conspire to cause lumpy varicose veins.

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Sources:
https://www.circulationfoundation.org.uk/help-advice/veins/varicose-veins-endovenous-laser-therapy
https://www.fi.edu/heart/blood-vessels
http://www.bbc.co.uk/schools/gcsebitesize/science/21c/keeping_healthy/heartdiseaserev2.shtml
http://www.mayoclinic.org/diseases-conditions/varicose-veins/symptoms-causes/dxc-20178128
http://www.nhs.uk/conditions/Varicose-veins/Pages/Whatarevaricoseveins.aspx
https://www.circulationfoundation.org.uk/help-advice/veins
https://www.womenshealth.gov/publications/our-publications/fact-sheet/varicose-spider-veins.html#E
http://search.proquest.com/openview/056a60d02a64d891a387978bb5ddd6f4/1?pq-origsite=gscholar
http://www.hopkinsmedicine.org/healthlibrary/conditions/cardiovascular_diseases/varicose_veins_85,p08259/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3920493/
https://www.ncbi.nlm.nih.gov/pubmed/8616659