It receives more blood from your lungs than it can pump out into your body. The blood it can't handle backs up and seeps out of your blood vessels into other tissue, including your lungs. Cells get waterlogged. This causes swelling called edema . This is very scary the first time it happens to you, believe me.
Different people are different, though.
I swell in my face and stomach, then my hands, then my feet and ankles. Other people swell just in their abdomen, which is called ascites . If the fluid just sits there long enough, scar tissue can form. This may become a chronic type of edema called brawny edema . With edema, sometimes if you press your thumb into your swollen ankle, it doesn't spring right back.