Funny I should click on this, as I have a heart condition known as atrial fibrillation.
Actually there are four pulmonary arteries going into the left atrium. Yes, arteries. Go figure. They come directly from the lungs.
I suspect there are four so we dummies can run a lot of fast 5 k's when we are 60 and in the process blow out the same atrium.
The atrium, as I understand it, acts rather like a supercharger, giving several pulses of blood into the ventricle which then (all loaded up) pushes it out to the body.
I've never paid much attention to how the other half works, but I believe it pumps the blood coming back from the body out to the lungs.
DC