One thing that was explained to me was that the kidneys control the amount of fluid in the blood based on blood flow- ie if your dehydrated the blood flow drops and the kidneys remove less water and if you have too much water, the flow increases and the kidneys remove more water. When the Heart is damaged, blood flow drops and your kidneys react as if your dehydrated resulting in the removal of less fluid. The body ends up with excess fluid circulating and it contributes to swelling. I'm sure this is a very simplistic explanation. Diuretics are prescribed to cause the kidneys to remove more fluid which will reduce swelling.