Heart failure means that the heart simply doesn’t function very well. Either it can’t contract or “squeeze”, causing blood to flow through the circulatory system, or it can't relax, allowing the chamber to fill with the blood returning from the lungs. In either case the blood simply backs up and causes problems, most notably in the lungs.

Because the volume and pressure of blood in the lungs is now greater, water in the blood is forced to move from the blood vessels into the air spaces of the lung itself. This is known as "pulmonary edema" and is experienced by the patient as difficulty in breathing and can be seen on a chest X-ray.

The extra volume of blood in the lungs also means that the right ventricle now has to pump against an increased pressure. When the pressure is high enough that it can no longer do this, blood backs up into the rest of the body. The most susceptible points in the body are those farthest away from the heart, such as the feet, where the net force is to move water out of the blood stream and into the tissue. As a result, a failing right ventricle can be noted clinically by swollen feet and ankles.