system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Do veins work like muscles? Does physical stress(exercise) make them bigger and create more of them? How are new veins formed? And what causes new veins to form?
gpt
First, veins are one type of blood vessel, which also includes arteries and capillaries. 

Second, in some ways, yes, but also no. Exercise doesn't make them "stronger" because they're not muscles, but it keeps them unclogged, makes it easier for them to expand and contract, and makes them more efficient.

New blood vessels are formed through a process called angiogenesis. Things that that trigger new blood vessel growth include obviously growth, exercise (more muscle mass requires more blood vessels to supply the extra muscle), gaining weight (more fat requires more blood vessels to supply the extra fat) healing wounds, and cancer, since tumors require a lot of blood flow.