Congestive heart failure occurs when the heart fails to work effectively and is unable to supply the body with enough blood. The entire heart may be affected, or in some cases, only one side. When the left side, which receives blood from the lungs, is affected, the heart fails to completely empty blood when it contracts, allowing the blood to back up into the lungs.. When the right side of the heart, which receives blood from the entire body, is affected, congestion spreads throughout the body.