Between heart beats, the heart muscle expands as it refills with blood. When it is time for the next heart beat, the muscle contracts, pumping the blood out to the body. 

To accomplish the refilling process, that is, to make the heart muscle expand between beats, a certain amount of pressure is required. Think of the expansion of the heart muscle as the pulling apart of two index fingers wrapped by a rubber band. Pressure must be applied to the band to separate the fingers.

What if we made the band more rigid and less elastic, say by wrapping it around our fingers twice? More pressure would be needed to pull fingers apart. Similarly, if that heart muscle becomes more rigid, the pressure required to expand the muscle as it refills with blood increases. This increase in pressure inside the heart can cause fluid to leak out into the lungs and cause shortness of breath.

What causes the heart muscle to become more rigid? The major causes are older age, high blood pressure, blockages in the heart arteries, being overweight and diabetes. Treatment is difficult, but often involves diuretics to reduce the pressure inside the heart and improve breathing. 

When we think of fluid build up inside the lungs, we usually think of a weak heart that contracts poorly. However, in the case of a stiff heart, the problem is not how the heart empties, but how it fills.