Why does fluid build up around the lungs in congestive heart failure (chf)?

Inadequate pump: The heart is actually 2 pumps. The right side pumps blood to the lungs. This is generally low pressure system so unless there are other reasons the right ventricle tends to function pretty effectively. The left side is the workhorse pump. The left ventricle is a muscle that can pump a large amount of resistance. When it fails the blood backs up into the lungs because the two pumps are in series.

Problem is the heart: In chf, the heart cannot effectively pump blood from the lungs to the systemic circulation. So, the fluid cannot move forward and it thus remains backed up in the lungs. The fluid stays in the lungs because the heart is unable to do its job of pumping that from the lungs to the rest of the body.

Pump failure: When a pump like the heart is failing, fluid backs up behind it. In the body that's where the lungs are. Fluid cannot go through the heart fast enough and so blood accumulates in the lungs causing them to be congested with too much fluid. Air spaces have fluid in them instead of air making the performance of the lungs impaired.