system
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human
How do surgeons connect small veins when reattaching limbs?

How do surgeons know when they have stitched together enough blood supply when reattaching limbs? I imagine suturing arteries are “easier” because they are generally much larger than veins but I wonder at what point do the surgeons just decide some blood vessels are too small to reconnect and just leave it up to the body to heal. I have never taken an anatomy class so I’m not well informed on the human body.
gpt
If the two ends cannot be stitched together, or repaired with bovine pericardium grafts then usually major arteries and veins are bypassed with vein grafts from the patient. Small arteries and veins are usually not reattached in any surgery because because over time the can form collaterals and re anastomose (angiogenesis). The principal concern are the main arteries and veins, the flexor and extensor tendons or muscles, the bones and repairing the nerves.