Oh man, I feel you on this! Had a similar situation when doing a biomech piece last year. Basically arteries have thicker, more muscular walls because they handle high pressure blood pumping FROM the heart. That’s why you’d draw them with more defined edges. Veins are thinner-walled and carry blood back TO the heart at lower pressure, plus they have those little valves inside. For your anatomical heart piece, arteries should look more robust and veins slightly flatter. I actually found this breakdown super helpful when I was researching for my client: compare veins and arteries . It’s got visual comparisons that made it click for me. Now I can actually explain the design choices instead of just winging it!