Heart failure can occur on either side of the heart or both. More commonly it is left-sided failure. The left ventricle isn't strong enough to keep up with the body's demands. At first the heart tries to compensate by enlarging. The body also tries to retain fluid to keep blood pressure up. After a while these things are harmful in themselves, ultimately fluid retention with a weak heart muscle causes fluid to back up into the lungs. This is why it is sometimes called congestive heart failure.