Heart failure is a condition in which the heart can't pump blood effectively to the lungs or the rest of the body. This can be because the person has developed a weakened heart muscle or because the heart muscle has thickened, making it difficult to fill the heart, backing up blood into the lungs.

With heart failure, the weakened heart pumps less blood than usual -- causing the kidneys and adrenal glands to produce chemicals that help the body to hold onto salt and water. In addition, the blood vessels constrict to try to raise the blood pressure. This makes it even harder for the heart to push blood through the arteries on the next beat.

With congestion, the legs and ankles may swell because the body is holding onto salt and water. Fluid may also collect in the lungs and interfere with the ability to breathe, especially when lying down. Left untreated, heart failure worsens and may prevent the heart from pumping enough blood to keep the person alive.  WebMD