system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are there four pulmonary veins leading back to the heart from the lungs, but only two pulmonary arteries leading out of the heart?
gpt
Because the pulmonary veins are a lower pressure system, they need a larger collective diameter to allow for reasonable flow and filling of the left atrium within a reasonable time.

There are only two pulmonary arteries because the right ventricle can push blood at a higher pressure and adequately perfuse the lung.