system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: How come we don't bleed out during surgery?

I've always wondered how the blood doesn't just keep flowing out of you like say, during knee surgery when they cut you open to expose your knee bones. I've always wondered how this works.
gpt
Blood doesn’t wander wherever it wants the body, it’s largely confined to the circulatory system.  In an ideal world, you just don’t cut any of the blood tubes.

Surgical cutting instruments are *extremely* sharp, which lets them cut through tissue very cleanly without tearing open more capillaries than is absolutely necessary.

Surgeons will avoid cutting through larger veins and arteries whenever possible, and can burn or clamp them closed if safe/necessary to do so.

Sometimes this isn’t possible, and the patient will require blood transfusion(s) during the operation because a major blood line had to be opened.