[ukulele]

Because the torso is upright either way, and gravity doesn't care if you're sitting or standing

[coldtea]

Gravity does the pulling, and sure the blocking is caused by the lack of movement and the sitting position that obstructs the free flow of blood.

I guess we could say it's not the gravity itself, and it would be correct, but also like saying it's not the bullet that kills you, it's the hole.